White powder on grapes

Symptoms Management Options Causes More info

Lorraine Berkett & Morgan Cromwell, University of Vermont

All green tissues of the grapevine are susceptible to powdery mildew (Erysiphe necator (Schw.) Burr.) infection. The disease appears as a whitish-gray powdery coating on the leaves or fruit caused by fungal mycelium and conidia on the surface of the plant. On leaves, initial symptoms appear as chlorotic spots on the upper leaf surface that soon become whitish lesions. Late in the season, small black round structures (chasmothecia) begin to appear on the white powdery lesions. On shoots, infected areas have the appearance of brown/black diffuse patches; on dormant canes, these patches are reddish brown. Severe leaf infections can cause distortion, drying, and premature drop. Infected berries can become covered with the fungus, may turn dark brown, shrivel, and split, and/or may not ripen properly. Berry infection may lead to further infection by spoilage microorganisms that reduce the quality of wine, even if the powdery mildew infection is mild.

Powdery mildew lesions. Photos by Lorraine Berkett, University of Vermont.

Cultural Management Options

Disease development is strongly favored by high humidity and cloudy weather, in addition to relatively warm temperatures. Therefore, pruning and training practices that promote an open canopy with good air circulation and light penetration can reduce the favorable conditions for powdery mildew development. Selection of an open planting site with rows in the direction of the prevailing wind and direct sunlight will also promote air circulation and decrease shading. These cultural methods may not only decrease the incidence and severity of the disease, but also promote thorough fungicide spray coverage.

Chemical Management Options

For effective management of powdery mildew, fungicide sprays may be needed early in the growing season for highly susceptible cultivars and when disease pressure from the previous year is high. Fungal spores (ascospores) are released from overwintering sites (chasmothecia) on the bark of the vine, from bud break until shortly after bloom when it is rainy and temperatures are above 50ºF. However, additional spores (conidia) produced in lesions continue to be spread throughout the season by wind. The most critical time to manage fruit infection with fungicides is immediately before bloom through two to four weeks after bloom. It is important to remember powdery mildew can be a problem during dry growing seasons because fungal spores (conidia) can be disseminated by wind and cause infections with high humidity alone and do not require rain for release or infection. Chemical management of powdery mildew should take into consideration disease resistance to fungicides. Refer to a grape pest management guide (see Recommended Links) for fungicide specifics and efficacy information.

Causes

Powdery mildew is caused by a fungus, Erysiphe necator (Schw.) Burr., which is native to eastern North America. The fungus overwinters as tiny black fruiting bodies (chasmothecia) in bark crevices on the grapevine. Spores (ascospores) from overwintering sites are initially released with 0.1 inch of rain when temperatures are 50ºF. These spores are carried by the wind and cause primary infections. Primary infections develop into lesions which produce another type of spore (conidia) that, within six to eight days, result in the powdery or dusty appearance of the lesions. These conidia cause secondary infections throughout the remainder of the growing season and result in the exponential spread of the disease throughout a vineyard. Infection can occur during temperatures of 59ºF to 90ºF, but temperatures from 68ºF to 77ºF are ideal. High relative humidity is conducive for conidia production and rainfall actually negatively impacts spore production (which is the opposite from other grape diseases caused by fungi). Overwintering structures (chasmothecia) are formed in the fall and are washed into the crevices of the bark on the vine trunk where they will remain and release ascospores during the start of the next growing season.

Recommended Resources

Video: Grape Powdery Mildew in Pinot Noir Vines, Oregon State University

Grapevine Powdery Mildew, Cornell University

Powdery Mildew on Grape, Ohio State University

Grape Powdery Mildew, University of California

New York and Pennsylvania Pest Management Guidelines for Grapes

Midwest Small Fruit and Grape Spray Guide

Grape Diseases and Management Guides, Washington State University

Powdery Mildew in Eastern Washington Commercial Grape Production, Washington State University

Powdery Mildew in Western Washington Commercial Grape Production, Washington State University

Field Guide for Integrated Pest Management in Pacific Northwest Vineyards, Washington State University

Video: Grape Powdery Mildew, University of Kentucky

Reviewed by Stephen Jordan, University of Wisconsin-Madison and Damon Smith, Oklahoma State University

Grape (Vitis spp.)-Powdery Mildew

Cause Erysiphe necator (formerly Uncinula necator), a fungal disease common to all areas of the Pacific Northwest. The disease tends to be more severe on the west side of the Cascade Range but is a chronic problem in arid districts where over-the-canopy irrigation is used for early-season frost protection or watering. There is low tolerance to this disease on the fruit as it can impart negative tastes later in the wine. An infection of 3% to 5% of the berries results in an oily, viscous mouth feel in the wine. Vitis vinifera (European) cultivars are susceptible to powdery mildew while V. labrusca (American) cultivars are more resistant. Hybrids between the two species have various levels of resistance. Other hosts include Boston ivy, Virginia creeper, and Ampelopsis (porcelain berry). The fungus may overwinter as a group of thin threads called hyphae, inside the vine’s dormant buds and/or as small black bodies (chasmothecia) on the exfoliating bark of the vine.

Buds on new shoots can be infected 4 to 6 weeks after shoots start growing but not after bud scales become suberized. These new infected buds remain quiescent until the next growing season. The fungus infects developing buds during the growing season. Shortly after budbreak, the fungus becomes active and covers the emergent shoot with a large white mass of threads or mycelium (“flag shoots”). Flag shoots have rarely been observed in commercial western Oregon or eastern Washington vineyards.

Chasmothecia on the exfoliating bark release sexual spores during rainy weather above 50°F from budbreak through bloom. This weather also favors infection that results in individual powdery spots, called colonies, on the surface of leaves growing close to the bark.

Many asexual spores (conidia) are produced on the surface of powdery mildew colonies. Under optimal conditions of mild temperatures and high humidity, a single spore can germinate, infect the plant, and produce a new colony and a new crop of spores in 3 days. Temperatures over 85°F and/or sunlight (ultraviolet) inhibit conidia germination. Nights where temperatures fall below 40°F can damage colonies, increase the time new ones form, and reduce their size. Free water from rain and/or irrigation can wash conidia off a colony, burst conidia, or result in poor or abnormal germination of conidia.

Grape berries are highly susceptible from the time calyptras (hoods) fall off to shortly after bloom when berries are about pea size (BBCH 73 to 75). Susceptibility of the fruit drops rapidly after that time. Grapes do not get new infections on fruit after 8% Brix but can still have sporulation up to 15% Brix. Leaves and canes, however, can be infected up to and past harvest.

Symptoms Powdery mildew can attack all aboveground plant parts. In early stages, whitish or grayish patches are on leaves and, if severe, ultimately cover both surfaces. Colonies are more easily detected in full sunlight with the sun over your shoulder. Later in the season, the colony darkens and is peppered with minute black dots (chasmothecia). On fruit, the fungus at first may look grayish or whitish but later has a brownish, russeted appearance. Infected fruit cracks and drops from the cluster. Even blossoms sometimes can be infected, causing them to dry up or fail to set fruit. When green shoots and canes are infected, the affected tissues appear dark brown to black in feathery patches. Patches later appear reddish brown on the surface of dormant canes.

Flag shoots are difficult to detect. Some young shoots may be covered with a large white mass of threads or mycelium. Others may have only a hint of thin threads on the shoot. Shoots generally are delayed in bud break and appear stunted and somewhat yellowed compared to healthy shoots. Can be confused with normal trichome density of young shoots of some cultivars.

Cultural control Activities and tactics that increase sunlight exposure to leaves and fruit reduce powdery mildew severity.

  • Prevent excess vigor through proper selection of rootstocks, training systems, irrigation, and fertility for the vineyard site.
  • Practice timely sucker control.
  • Keep canes cut back close to the top wire of vertical trellises for better spray coverage.
  • Removing leaves to manage bunch rot also helps fungicide cover the clusters, which helps control powdery mildew.
  • Manage sources of powdery mildew outside of the vineyard such as susceptible plants found in decorative gardens near tasting rooms or gazebos or volunteer vines along fence rows or riparian areas.

Chemical control Begin applications at 6 inches shoot growth and continue at regular intervals through the growing season. Strongest materials and shortest intervals should be used from prebloom (BBCH 57) through bloom and continued for 2 weeks after bloom. Adjusting application timing to coincide with early bloom (BBCH 61 to 65) has resulted in more consistent control of powdery mildew in western Oregon. A postharvest application may help control late-season infections in some years. Thorough coverage of all actively growing tissue is essential for good control. Do not extend application intervals in the Willamette Valley past 2 weeks.

Research in California has shown that sulfur sprays at budbreak initiate a release of ascospores and have some utility in overall disease management. Although this may occur in the more arid areas of the Pacific Northwest, it may not have the same utility in wetter areas west of the Cascade Range.

Resistance to many different fungicide groups has been documented for grape powdery mildew worldwide. Resistance to Group 3 and 11 fungicides has been documented in Oregon and Washington. To prevent developing resistant fungi, alternate or tank-mix materials from different groups with different modes of action. Also, limit applications from any specific group to two (2) or fewer sprays. New York recommends to tank-mix Group 3 or Group 11 fungicides with sulfur or other products with a different mode of action. Several forecasting programs are available to help time applications (see below).

  • Abound at 10 to 15.5 fl oz/A. Do not apply within 14 days of harvest or with silicone-based surfactants. Sprayers used to apply Abound should not be used on apples. Group 11 fungicide. 4-hr reentry.
  • Aprovia at 8.6 to 10.5 fl oz/A plus an adjuvant. Do not use within 21 days of harvest. Group 7 fungicide. 12-hr reentry.
  • Bicarbonate-based products. Might supplement a normal program when powdery mildew is first observed. Do not mix with acidifying agents. Only use early season since thorough coverage is essential and timed when disease pressure is low. O
    • Kaligreen (82% potassium bicarbonate) at 2.5 to 5 lb/A. 4-hr reentry.
    • MilStop (85% potassium bicarbonate) at 2.5 to 5 lb/A. Oregon and Washington only. 1-hr reentry.
    • Monterey Bi-Carb Old Fashioned Fungicide at 4 teaspoons/2 gal water. H
  • Cinnerate at 13 to 35 fl oz/100 gal water. 4-hr reentry.
  • Copper formulations are registered but provide only moderate control alone. They are not generally recommended. O
  • Eagle 20 EW at 6 to 10 fl oz/A for home vineyard or landscape use. Do not apply within 14 days of harvest. Group 3 fungicide. 24-hr reentry.
  • Endura at 4.5 oz/A. Do not use within 14 days of harvest. Do not use for powdery mildew if you plan to use it for bunch rot. Group 7 fungicide. 12-hr reentry.
  • Flint 50 WG at 1.5 to 2 oz/A. Do not use on ‘Concord’ grapes. Do not use within 14 days of harvest. Group 11 fungicide. 12-hr reentry.
  • Fracture (BLAD) at 20.5 to 24.4 fl oz/A. Reapply if rain occurs within 12 hours of original application. Do not use within one day of harvest. Group BM01 fungicide. 4-hr reentry.
  • Gatten at 6.4 fl oz/A. Do not use within 14 days of harvest. Group U13 fungicide. 12-hr reentry.
  • Horticultural Mineral Oils. Generally effective from 1% to 2% volume to volume. Necrotic foliage may result if applied within 10 days of any sulfur application. Do not tank-mix with copper-based products when fruit is present. Do not use during freezing temperatures, above 90°F, or when plants are under heat or moisture stress. Do not use when foliage is wet because good coverage is essential. Do not use before bloom as it will have a negative impact on predatory mites. 4-hr reentry. O
    • JMS Stylet Oil at 1 to 2 gal/100 gal water. Brix reductions have been observed in several locations when product is used all season.
    • SuffOil-X at 1 to 2 gal/100 gal water.
    • Trilogy at 1% of spray volume but do not exceed 2.5 gal/A. Do not use after bunch closure. Poor to fair control as a stand-alone product.
  • Kenja 400 SC at 20 to 22 fl oz/A. Do not use within 14 days of harvest. Group 7 fungicide. 12-hr reentry.
  • Mettle 125 ME at 3 to 5 fl oz/A. Do not apply within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
  • M-Pede at 1 to 2 gal/100 gal water. Good coverage is essential. Do not use within 3 days of applying sulfur or past verasion. Do not mix with hard water. 12-hr reentry. O
  • Oso SC at 3.75 to 13 fl oz/A. Do not use within 7 days of harvest. Group 19 fungicide. 4-hr reentry.
  • Ph-D WDG at 6.2 oz/A plus an adjuvant. May be applied on the day of harvest. Group 19 fungicide. 4-hr reentry.
  • Prev-Am Ultra at 50 fl oz/100 gal water. Do not use within 14 days of a sulfur application, above 90°F, or when plants are under heat or moisture stress. 12-hr reentry.
  • Procure 480 SC at 4 to 8 fl oz/A. Do not use within 7 days of harvest. Group 3 fungicide. 12-hr reentry.
  • Prolivo 300 SC at 4 to 5 fl oz/A. Can be used day of harvest. Group 50 fungicide. 4-hr reentry.
  • Quintec at 4 to 6.6 fl oz/A. A surfactant is not required when used alone but a nonionic surfactant is preferred if needed for tank-mixes. Do not apply within 21 days of harvest. Group 13 fungicide. 12-hr reentry.
  • Rally 40 WSP at 3 to 5 oz/A. Do not apply within 14 days of harvest. Group 3 fungicide. 24-hr reentry.
  • Regalia at 1 to 4 quarts/A plus another fungicide. Use on 7-day intervals. May be used day of harvest. Group P5 fungicide. 4-hr reentry. O
  • Rex lime sulfur (28%) at 0.75 to 1 gal/100 gal water. 48-hr reentry. O
  • Sovran at 3.2 to 4.8 oz/A. Rotate with other fungicides that have different modes of action. Do not use within 14 days of harvest. Do not use organosilicate surfactants. Some sweet cherries, such as ‘Van’, may be injured if accidentally sprayed. Group 11 fungicide. 12-hr reentry.
  • Spectracide Immunox at 1.25 fl oz/gal water. Do not use within 2 weeks of harvest or more than five (5) times per season. H
  • Sulfur products. Do not extend intervals beyond 14 days. Sulfur can burn foliage when applied above 85°F. The temperature relationship is correlated with increases in the daily maximum, within a few days after application. Grapes in California can withstand sulfur applications (at lower rates) above 85°F if there is no major short-term changes in the daily maximum. May injure labrusca types like ‘Concord’. Do not use within two weeks of an oil spray. Hydrogen sulfide in the wine may increase if used within 35 days of white-wine harvest or within 50 days of red-wine harvest. Wettable powder formulations are less likely to result in hydrogen sulfite in the wine than micronized formulations. Addition of a spreader sticker adjuvant may be helpful. Group M2 fungicides. 24-hr reentry. O
    • Cosavet-DF (80% sulfur) at 2 to 5 lb/A.
    • Kumulus DF (80% sulfur) at 2 to 10 lb/A.
    • Microthiol Disperss (80% sulfur) at 3 to 10 lb/A. Do not use a spreader-sticker. Use at 7- to 14-day intervals.
    • Safer Garden Fungicide (12% sulfur) at 2 fl oz/gal water. Thoroughly sprayed over the entire plant. Do not use within 21 days of harvest. H
  • Tebuconazole-based fungicides are registered. Do not apply within 14 days of harvest. Group 3 fungicides. 12-hr reentry.
    • Orius 20 AQ at 8.6 oz/A.
    • Tebucon 45 DF at 4 oz/A.
  • TopGuard at 8 to 10 fl oz/A. Do not use within 14 days of harvest. Group 3 fungicide. 12-hr to 5-day reentry depending on activity.
  • Topsin M WSB at 1 to 1.5 lb/A plus another fungicide. Do not use within 14 days of harvest. Resistant fungi make this product ineffective and may be prevalent in many vineyards. Group 1 fungicide. 2-day reentry.
  • Torino at 3.4 oz/A. Do not use within 3 days of harvest. Group U6 fungicide. 4-hr reentry.
  • Trionic 4 SC at 4 to 8 fl oz/A. Do not use within 7 days of harvest. Group 3 fungicide. 12-hr reentry.
  • Vacciplant at 14 to 60 fl oz/A plus an effective fungicide. Can be used day of harvest. Unknown efficacy in the PNW. Group P4 fungicide. 4-hr reentry.
  • Vivando at 10.3 to 15.4 fl oz/A. Do not use with horticultural oils, or use within 14 days of harvest. Group 50 fungicide. 12-hr reentry.

Combination Fungicides

  • Aprovia Top at 8.5 to 13.5 fl oz/A. Do not apply within 21 days of harvest. Group 3 + 7 fungicide. 12-hr reentry.
  • Inspire Super at 16 to 20 fl oz/A. Do not use within 14 days of harvest or on ‘Concord’. Group 3 + 9 fungicide. 12-hr reentry.
  • Luna Experience at 6 to 8.6 fl oz/A. Do not use on ‘Concord’ or within 14 days of harvest. Group 3 + 7 fungicide. 12-hr to 5-day reentry depending on activity.
  • Luna Sensation at 4 to 7.6 fl oz/A. Do not use for powdery mildew control if already used for bunch rot control. Do not use on ‘Concord’ or within 14 days of harvest. Group 7 + 11 fungicide. 12-hr reentry.
  • Merivon at 4 to 5.5 fl oz/A. Do not mix with any other materials, use within 14 days of harvest, or on cultivars such as Concord or Niagra. Group 7+11 fungicide. 12-hr reentry.
  • Miravis Prime at 9.2 to 13.4 fl oz/A. Do not use for powdery mildew control if already used for bunch rot control. Do not use within 14 days of harvest. Group 7 + 12 fungicide. 12-hr reentry.
  • Pristine at 8 to 12.5 oz/A. Do not use within 14 days of harvest, or more than five (5) times/year. Do not use on labrusca type grapes such as ‘Concord’ due to possible foliar injury. Group 7 + 11 fungicide. 12-hr or 5-day reentry based on activity.
  • Quadris Top at 12 to 14 fl oz/A. Do not apply within 14 days of harvest or on ‘Concord’ grapes. Group 3 + 11 fungicide. 12-hr reentry.
  • Topguard EQ at 5 to 6 fl oz/A. Do not use with silicone surfactants or within 14 days of harvest. Sprayers should not be used on apples. Group 3 + 11 fungicide. 12-hr reentry.
  • Unicorn DF at 1.75 to 2.5 lb/A plus a non-ionic surfactant. Includes sulfur in the formulation. Group M1 + 3 fungicide. 24-hr reentry.

Notes If you are trying to bring an abandoned vineyard back into production, spraying lime sulfur during the dormant season or micronized sulfur at 100% budbreak may help bring powdery mildew under control with a normal, season-long spray program.

Although OxiDate is registered, it will not control this disease due to its short residual.

Although Revus Top is registered, it contains a chemical for control of downy mildew, a disease not found on grape in the PNW and thus is not recommended for use.

Some registered products offer only suppression of this disease and thus are not recommended for use. These products include Intuity.

Forecasting Several forecasting programs are available for scheduling fungicide applications. The standard Oregon phenology-based program begins applications at 6 to 8 inches shoot growth and continues at regular intervals based on grapevine development. The Gubler-Thomas (UC-Davis) program uses leaf wetness and temperature early in the year to predict ascospore infection periods and only temperature during the summer to predict conidial infection periods. The New York (Gadoury) program is based on rainfall and temperature. The Kast (Oi Diag) program incorporates relative humidity along with temperature and rainfall. All programs have been effective at timing fungicides and controlling powdery mildew in western Oregon. Additional models have been developed in Canada (Carisse) and Italy (Caffi). An ascospore release model for western Oregon incorporates leaf wetness, temperature, precipitation, and relative humidity but was only 66% accurate. The use of rose bushes at the end of rows is based only on anecdotal accounts and has no basis in the scientific literature.

Biological control Short intervals such as 7 days between applications is recommended especially during bloom. Generally more effective when disease-pressure is low. More effective when combined with leaf removal practices and/or integrated with synthetic fungicides at bloom.

  • Actinovate AG (Streptomyces lydicus strain WYEC 108) at 3 to 12 oz/A plus a spreader-sticker. California trials averaged 50% control in 5 trials. 1-hr reentry. O
  • Bayer Advanced Natria Disease Control RTU (Bacillus subtilis strain QST 713) is registered for the home garden. Active ingredient is a small protein. Ineffective as a standalone treatment based on tests in western Oregon. H O
  • DoubleNickel 55 (Bacillus amyloliquefaciens strain D747) at 0.25 to 3 lb/A. Poor control in western Oregon. Group 44 fungicide. 4-hr reentry. O
  • LifeGard WG (Bacillus mycoides isolate J) at 4.5 oz/100 gal water. Unknown efficacy. 4-hr reentry. O
  • Prevont (Bacillus subtilis strain IAB/BS03) at 10 to 30 fl oz/100 gal water. Unknown efficacy. Preharvest interval not specified. 4-hr reentry. O
  • Serenade ASO (Bacillus subtilis strain QST 713) 2 to 4 quart/A. Active ingredient is a small protein. Serenade Garden Disease Control is available for home use. Ineffective as a standalone treatment based on tests in western Oregon. 4-hr reentry. H O
  • Sonata (Bacillus pumilis strain QST 2808) at 2 to 4 quarts/A plus a spreader-sticker. May be applied up to and including the day of harvest. 4-hr reentry. O

Thiessen, L.D, Neill, T.M., and Mahaffee, W.F. 2018. Assessment of Erysiphe necator ascospore release models for use in the Mediterranean climate of western Oregon. Plant Disease 102:1500-1508.

Powdery Mildew of Grape

Powdery mildew is an important disease of grapes worldwide. The disease generally is considered less economically important in Ohio than black rot or downy mildew. However, uncontrolled, the disease can be devastating on susceptible varieties under the proper environmental conditions. Unlike black rot and downy mildew, the powdery mildew fungus does not require free water on the plant tissue surface to infect. Powdery mildew can result in reduced vine growth, yield, fruit quality, and winter hardiness. Varieties of Vitis vinifera and its hybrids generally are much more susceptible than American varieties.

Figure 1. Powdery mildew colonies on upper leaf surface of infected grape leaf.

The powdery mildew fungus can infect all green tissues of the vine. Small, white or grayish-white patches of fungal growth appear on the upper or lower leaf surface (Figure 1). These patches usually enlarge until the entire upper leaf surface has a powdery, white to gray coating (Figure 2). The patches may remain limited throughout most of the season. Severely affected leaves may curl upward during hot, dry weather. Expanding leaves that are infected may become distorted and stunted.

On young shoots, infections are more likely to be limited, and they appear as dark-brown to black patches that remain as dark patches on the surface of dormant canes.

If blossom clusters are affected, the flowers may wither and drop without setting fruit. Infections on cluster stems often go unnoticed, but can be very damaging. Infected cluster stems may wither and dry up, resulting in berry drop (shelling). Affected berries may have patches of fungal growth on the surface similar to those on the leaves, or the entire berry may be covered with the white, powdery growth. Infected berries often are misshapen or have rusty spots on the surface. Severely affected fruit often split open (Figure 3). When berries of purple or red cultivars are infected as they begin to ripen, they fail to color properly and have a blotchy appearance at harvest. Berries are susceptible to infection from early bloom through three to four weeks after bloom.

Late in the season, many black specks may develop on the surface of infected areas. These are the sexual fruiting bodies (cleistothecia) of the fungus.

Causal Organism and Disease Cycle

Figure 2. Grape leaf severely infected with powdery mildew. Note the white, powdery layer of fungal growth on upper leaf surface.

Figure 3. Berry cluster infected with powdery mildew.

Powdery mildew is caused by the fungus Uncinula necator. It was previously thought that the powdery mildew fungus overwintered inside dormant buds of the grapevine. Recent research in New York has shown that almost all overwintering inoculum in the northeastern United States (Ohio) comes from cleistothecia, which are fungal fruiting bodies that overwinter primarily in bark crevices on the grapevine. In the spring, airborne spores (ascospores) released from the cleistothecia are the primary inoculum for powdery mildew infections. Ascospore discharge from cleistothecia is initiated if 0.1 inch of rain occurs with an average temperature of 50 degrees F. Most mature ascospores are discharged within four to eight hours. Ascospores are carried by wind. They germinate on any green surface on the developing vine, and enter the plant resulting in primary infections. The fungus produces another type of spore (conidia) over the infected area after six to eight days. The conidia and fungus mycelia on which they are formed give the powdery or dusty appearance to infected plant parts. The conidia serve as “secondary inoculum” for powdery mildew infection throughout the remainder of the growing season. It is important to note that a primary infection caused by one ascospore can result in the production of hundreds of thousands of conidia, each of which is capable of causing secondary infections that spread the disease.

Temperatures of 68 to 77 degrees F are optimal for infection and disease development, although infection can occur from 59 to 90 degrees F. Temperatures above 95 degrees F inhibit germination of conidia and above 104 degrees F they are killed. High relative humidity is conducive to production of conidia. Atmospheric moisture in the 40 to 100 percent relative humidity range is sufficient for germination of conidia and infection. Free moisture, especially rainfall, is detrimental to survival of conidia. This is in direct contrast to most other grape diseases such as black rot and downy mildew that require free water on the plant surface before the fungus spores can germinate and infect. Low, diffuse light seems to favor powdery mildew development. Under optimal conditions, the time from infection to production of conidia is about seven days. It is important to remember that powdery mildew can be a serious problem in drier growing seasons when it is too dry for other diseases such as black rot or downy mildew to develop.

Cleistothecia are formed on the surface of infected plant parts in late fall. Many of them are washed into bark crevices on the vine trunk

Control

Select an open planting site with direct sunlight. Plant rows in the direction of the prevailing wind in order to promote good air circulation and faster drying of foliage and fruit. Prune and train vines properly in such as way as to reduce shading and increase air circulation.

Varieties differ greatly in their susceptibility to powdery mildew. Cabernet Franc, Cabernet Sauvigon, Chancellor, Chardonnay, Chelois, Gewurztraminer, Merlot, Pinot blanc, Pinot noir, Riesling, Rosette, Rougeon, Sauvignon blanc, Seyval, Vidal 256 and Vignoles are all highly susceptible.

On susceptible varieties, control is based on the use of properly timed applications of effective fungicides. Early season (prebloom through bloom) control of primary infections caused by ascospores must be emphasized. For the most current fungicide recommendations and spray schedules, commercial growers are referred to Bulletin 506, Midwest Fruit Pest Management Guide, and backyard growers are referred to Bulletin 780, Controlling Diseases and Insects in Home Fruit Plantings. These publications can be obtained from your county Extension office or the CFAES Publications online bookstore at estore.osu-extension.org.

Figure 4. Disease cycle of grape powdery mildew. We wish to thank the New York State Agricultural Experiment Station for use of this figure. It was taken from Grape IPM Disease Identification Sheet No. 2.

This fact sheet was originally published in 2008.

Powdery Mildew (Erysiphe necator): Powdery mildew is
a fungal disease that that can infect all green parts of grapevines. It
overwinters as chasmothecia in cracks in the bark of vines. In spring, the
chasmothecia release spores with rain events ≥ 0.1″ when temperatures are ≥
50°F. The spores are blown to nearby tissue where they germinate and
initiate a primary infection. The optimal temperature for primary infection is
77°F, but infections can occur over a wide range of temperatures and free
water is not required beyond the initial rainfall to trigger spore release.
However, powdery mildew infections are favored by high humidity.

Powdery mildew colonies grow and sporulate most rapidly at temperatures of 73
to 86°F and the period of time required for infection and subsequent
sporulation can be as short as 5 to 6 days when temperatures remain in this
range. Cooler temperatures increase the latent period significantly, and
powdery mildew development is inhibited at temperatures ≥ 90°F. However,
the interior of a grapevine canopy is often cooler than the ambient air due to
transpirational cooling and shading. Powdery mildew infections can occur
through the state, but Blanc Du Bois, Black Spanish and some other hybrid
cultivars have a high level of resistance to powdery mildew and typically do
not require protection from powdery mildew during the growing season.

Powdery mildew leaf infections may vary in appearance based on the age of the
foliage when infected, age of the fungal colony and source of infection.
Colonies are first noticed as white circular area with a silvery gray to brown
tinge. On leaves, young colonies may be most visible by looking across the
leaf blade. As colonies sporulate they take on a white appearance, but
eventually become gray with age. Dead leaf tissue may be observed in older
colonized areas.

Early leaf infections may occur on the underside of leaves near trunks or
cordons where the overwintering chasmothecia were likely located. These
infections may go unnoticed, but often form chlorotic spots on the surface of
the leaves that resemble downy mildew. Later infections occur on the surface
of leaves as circular or irregular shaped colonies. Stem infections take on a
similar appearance as leaf infections, but turn dark in color as the season
progress. These colonies are eventually killed as periderm forms in late
season leaving dark colored blotches. Berry infections may be visible as a
whitish coating that covers a portion or the entire berry. Berry infections
that occur before bloom up to a week after berry set often cause the epidermis
of the berry to stop growing during expansion resulting in splitting of the
skin. These cracks can serve as a point of entry for spoilage organisms. The
most critical period for powdery mildew control in fruit is two weeks before
bloom through 30 days after bloom. Maintaining control of foliar infections
throughout the growing season is also important in order to maintain canopy
health for fruit ripening and for post-harvest recovery.

Because humidity plays an important role in powdery mildew infections,
vineyards near bodies of water and areas of the vineyard with poor airflow may
be at greater risk of infection. Improving airflow through the canopy with
proper vineyard design and canopy management can reducing humidity within the
canopy and enhance control of powdery mildew by reducing fruit and foliage
drying times after rainfall, increasing fungicide spray penetration, and
facilitating leaf and cluster exposure to sunlight (ultraviolet light is
lethal to powdery mildew).

Shoots

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Grape Powdery Mildew

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Powdery mildew, caused by the fungus Erysiphe necator (syn. Uncinula necator), is one of the most prevalent and easily recognized plant diseases afflicting grape vines in New Mexico. It appears as a dusty white-gray or greenish-white coating on leaf surfaces or other above-ground plant parts. The disease is most commonly observed on the upper surfaces of leaves (Figure 1), but can also affect the lower leaf surface, young stems, buds (Figure 2), flowers, canes, and young fruit. Severely infected leaves may exhibit mottling or deformity, including leaf curling and withering. Infected fruit turn grayish-white at first and ultimately exhibit a brown russeted appearance. Infected fruit may crack, shrivel, or drop from clusters (Figure 3).

Figure 1. Infected leaf (Yuan-Min Shen, Taichung District Agricultural Research and Extension Station, Bugwood.org).

Figure 2. Infected flower bud (University of Georgia Plant Pathology Archive, University of Georgia, Bugwood.org).

Figure 3. Infected berries (University of Georgia Plant Pathology Archive, University of Georgia, Bugwood.org).

Life Cycle

The powdery mildew fungus overwinters as hyphae inside dormant buds, or as chasmothecia (spore-bearing structures) in bark or on canes, leftover fruit, and leaves on the ground. When hyphae from dormant buds serve as the primary inoculum, the new tissue is infected when the bud breaks dormancy. When chasmothecia provide the primary inoculum, plants are infected in the spring when ascospores (sexual spores) are released from the overwintering structures. Ascospores shoot up into the air currents and are wind-blown to susceptible plants, where new infections begin. During the growing season, the fungus produces conidia (asexual spores) that increase the severity of the disease on infected plants and may spread the fungus from one plant to another.

Conditions for Disease

In New Mexico, powdery mildew is favored by warm temperatures (43-95°F, with optimum temperatures of 68-80°F) and high humidity (40-99% relative humidity). Low light also favors disease development. For this reason, powdery mildew infections are often found in dense canopies where low light conditions and low air circulation prevail.

Management

Planting locations with good airflow are preferable; canopies at these locations will dry faster. There are also several different management practices that can help reduce or prevent powdery mildew. Such practices increase light penetration and reduce relative humidity in the plant canopy. Do not crowd the plants together when planting or training vines. A high canopy designed with air ventilation in mind will be preferable to a canopy that has low ventilation and high leaf density. Airflow and ventilation will discourage mildew growth. Selectively pruning overcrowded plantings and removing leaves are recommended cultural practices to increase light penetration and the circulation of air; this also decreases relative humidity infection. Do not compost infected plant debris. Avoid nitrogen fertilizer applications in the late summer to limit the production of succulent tissue. Water early in the morning to let the tissue and soil dry as quickly as possible. Avoid overhead watering to reduce relative humidity.

Fungicides may be used for managing powdery mildew. For best results, fungicide treatments should begin before the overwintering fungus can infect new growth. The first few treatments are the most important and should be applied at appropriate intervals, starting at bud break or early shoot growth. A powdery mildew index (PMI) model may be used to determine appropriate treatment intervals because frequency will depend upon weather conditions and choice of fungicide. For more information on calculating PMI, please see the University of California’s Agriculture and Natural Resources statewide integrated pest management program at www.ipm.ucdavis.edu. Mildew fungicides are commonly divided into different groups. These groups are classified by their mode of action: amino acids and protein synthesis, glucan synthesis, mitosis and cell division, respiration, signal transduction (quinolines), sterol inhibitor, multi-site activity, biologicals, unknown mode of action, host plant defense induction, and products with mixed modes of action.

See Table 1 for fungicides currently registered for use on grapes to help manage powdery mildew in New Mexico. This table lists fungicides by modes of action. Rotating fungicides with different modes of action is important in resistance management (delaying or preventing the development of fungicide resistance in pathogens).

Table 1. Fungicide Use on Grapes Against Powdery Mildew in New Mexico

Mode of Action Common Name Trade Name
Amino acids and protein synthesis cyprodinil Vangard WG
Glucan synthesis polyoxin D zinc salt Ph-D WDG
Mitosis and cell division thiophanate-methyl T-Methyl 70W WSB
T-Methyl E-AG 70 WSB
Thiophanate-Methyl 85 WDG
Topsin M 70 WDG

Respiration Carboxamides

boscalid Endura
Respiration Strobilurin (QoI) azoxystrobin Abound
Amistar
trifloxystrobin Flint
kresoxim-methyl Sovran
Signal transduction (quinolines) quinoxyfen Quintec
Sterol inhibitor fenarimol Rubigan E.C.
tebuconazole Amtide Tebuconazole 45WDG
Elite 45 DF
Orius 45 DF
Orius R 20 AQ
myclobutanil Eagle 20EW
Myclobutanil 40
Nova 40W
Rally 40W
Spectracide Immunox
triflumizole Procure 480SC
Procure 50WS
Multi-site activity copper ammonium complex Liqui-Cop
copper hydroxide Champ DP
Champ Formula 2
Champ WG
Kocide 101
Kocide 2000
Kocide 3000
Kocide 4.5 LF
Kocide DF
Nu-Cop 3L
Nu-Cop HB
Nu-Cop 50 DF
copper salts of fatty acids Camelot Brand Fungicide
Tenn-Cop 5E
copper soap Copper Soap Liquid Fungicide
Cueva Copper Soap
Liquid Copper Soap
copper sulfate Copper Sulfate Crystals
Crystal Blue Copper Sulfate Crystals
Cuprofix Disperss
Cuprofix MZ Disperss
Cuprofix Ultra 40 Disperss
Phyton 27
mancozeb + copper hydroxide Mankocide
sulfur Kumulus DF
Sulfur Dust
Liquid Sulfur Six
Microthiol Disperss
Sulfur 6L
THAT Flowable Sulfur
Thiolux Jet
potassium salts of fatty acids + sulfur 3-in-1 sprays
Biologicals (mode of action unknown) Streptomyces lydicus Actinovate
Bacillus pumilus Ballard Plus
Sonata
Bacillus subtilis Serenade ASO
Serenade Max
Unknown mode of action hydrogen dioxide Oxidate
StorOx
potassium bicarbonate Amicarb 100
Milstop Foliar Fungicide
petrolium oil BioCover MLT
Brandt Saf-T-Side
Purespray Green
Suffoil-X
neem oil 70% Neem Oil
Bonide Bon-Neem
Concern FTE
Green Light Neem Concentrate
Triact 70
Triology
phosphonates Fosphite
Fungi-phite
Rampart
potassium salts of fatty acids M-Pede
plant extracts Regalia SC
Host plant defense induction Harpin protein Messenger

Products with Mixed Modes of Action

Common Name Trade Name
Strobilurin + sterol inhibitor trifloxystrobin + tebuconazole Adament 50 WG
Multi-site activity + sterol inhibitor mancozeb + myclobutanil Clevis
Strobilurin + carboxamide pyraclostrobin + boscalid Pristine

Disclaimer

The recommendations in this publication are provided only as a guide. The authors and New Mexico State University assume no liability resulting from their use. Brand names appearing in publications are for product identification purposes only. No endorsement is intended, nor is criticism implied of similar products not mentioned. Persons using such products assume responsibility for their use in accordance with current label directions of the manufacturer.

Please be aware that pesticide labels and registration can change at any time; by law, it is the applicator’s responsibility to use pesticides ONLY according to the directions on the current label. Use pesticides selectively and carefully and follow recommended procedures for the safe storage and disposal of surplus pesticides and containers.

To find more resources for your business, home, or family, visit the College of Agricultural, Consumer and Environmental Sciences on the World Wide Web at aces.nmsu.edu.

Contents of publications may be freely reproduced for educational purposes. All other rights reserved. For permission to use publications for other purposes, contact [email protected] or the authors listed on the publication.

New Mexico State University is an equal opportunity/affirmative action employer and educator. NMSU and the U.S. Department of Agriculture cooperating.

Printed and electronically distributed June 2010, Las Cruces, NM.

You’re famished for a bit of cheese, but you notice a blue spot on the chunk of cheddar in your fridge. Is it OK to slice off the bad part and eat the rest?

Is it OK to eat moldy food?

March 30, 201501:16

For many foods it actually is OK to just cut away the mold and eat the rest, said Leslie Bonci, director of sports nutrition at the Center for Sports Medicine at the University of Pittsburgh Medical Center.

But, she noted, “some molds can be quite toxic to the body. You can develop respiratory symptoms, gastrointestinal symptoms and sometimes allergies. Some are very dangerous.”

And while you might think that mold on blue cheese, which gets its color from a certain kind of beneficial mold, might be OK, it isn’t if the mold is any color other than blue, Bonci’s advice is to cut around it.

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Hard foods that are safe, if you pare away the bad spots:

  • Carrots
  • Firm cheeses
  • Pears

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“It’s harder for the mold to penetrate these foods,” Bonci says.

With softer foods, including soft cheeses, “you can’t assume when you’ve cut away the moldy part that you’ve completely gotten rid of it.”

So, if you’ve got some grapes and there’s mold on a couple of them, throw the bunch away.

Bonci’s list of foods that are OK to eat once you’ve removed the mold:

Mold on hard fruit/veggies: Cut about ½ inch around the mold to get rid of it.

Hard cheese: Cut about ½-1 inch around mold, rewrap cheese with new covering.

Hard salami/dry-cured ham: OK to use, mold adds flavor to the salami, can scrub the mold off the coating of the ham.

Gorgonzola/Bleu cheese: Cut out the moldy spot.

Once you’ve cut away the bad part and eaten your fill, don’t put the food back into the package you took it out of, Bonci says. There could be traces of mold left behind that will contaminate the cheese.

You should also clean the entire vegetable bin if you’ve found a piece with mold on it.

Not OK, even if there’s just a bit of mold:

  • Brie, Camembert
  • Hot dogs
  • Bacon
  • Casseroles
  • Leftovers
  • Pasta
  • Jams/jellies
  • Yogurt/sour cream
  • Lunch meat
  • Cooked meats
  • Soft fruits, veggies and even mold on orange rinds
  • Bread or baked goods
  • Sliced, shredded or cubed cheese
  • Nuts and nut butters

And don’t just depend on your eyes, Bonci says. Bologna doesn’t have to have gray fuzz on it to be toxic, for example. By the time the furry growth is seen on the surface, deep “roots” may have penetrated the food.

“If anything tastes musty, that’s a pretty good indicator that there is mold in there,” she added.

Finally, according to the USDA, you can minimize mold growth by:

  • Using leftovers within 3 to 4 days.
  • Cleaning your refrigerator every few months with 1 tablespoon baking soda dissolved in a quart of water.
  • Scrubbing visible mold using 3 teaspoons of bleach in a quart of water.

What Are Food Molds – Are Molds Dangerous?

Some of the following information is from the United States Department of Agriculture, Food Safety and Inspection Service (FSIS). Photo courtesy of the U.S. Environmental Protection Agency (EPA).

What are food molds?

Molds are microscopic fungi that live on plant or animal matter.Mold grows from tiny spores that float around in the air. When some of these spores fall onto a piece of damp food, they grow into mold.

Food mold feeds itself by producing chemicals that make the food break down and start to rot. As the bread rots, the mold grows. There are thousands of different kinds of molds. One mold that grows on lemons looks like a blue-green powder. A mold that grows on strawberries is a grayish-white fuzz. A common mold that grows on bread looks like white cottony fuzz at first. If you watch that mold for a few days, it will turn black. The tiny black dots are its spores, which can grow to produce more mold.

No one knows how many species of fungi exist, but estimates range from tens of thousands to perhaps 300,000 or more. Most are filamentous (thread like) organisms and the production of spores is characteristic of fungi in general. These spores can be transported by air, water, or insects.

Mold on BreadMold on Tomatoes

Are Molds Dangerous?

Yes, some molds cause allergic reactions and respiratory problems. And a few molds, in the right conditions, produce mycotoxins, poisonous substances that can make you sick.

What Are Mycotoxins? Mycotoxins are poisonous substances produced by certain molds found primarily in grain and nut crops, but are also known to be on celery, grape juice, apples, and other produce. There are many of them and scientists are continually discovering new ones. The Food and Agriculture Organization (FAO) of the United Nations estimates that 25% of the world’s food crops are affected by mycotoxins, of which the most notorious are aflatoxins.

What is Aflatoxin? Aflatoxin is a cancer-causing poison produced by certain fungi in or on foods and feeds, especially in field corn and peanuts. They are probably the best known and most intensively researched mycotoxins in the world. Aflatoxins have been associated with various diseases, such as aflatoxicosis in livestock, domestic animals, and humans throughout the world. Many countries try to limit exposure to aflatoxin by regulating and monitoring its presence on commodities intended for use as food and feed. The prevention of aflatoxin is one of the most challenging toxicology issues of present time.

Cheese Molds: An exception is mold on hard cheese, as some cheeses are eaten only after they become moldy! Blue cheese gets its flavor from the veins of blue-green mold in it. When a blue cheese is formed into a wheel, holes are poked through it with thin skewers. Air gets into these holes, and a very special kind of mold grows there as the cheese ripens. If mold develops, cut away one (1) inch on each side of the cheese (throw away) and use the remainder as soon as possible.

According to the Mayo Clinic, some moldy cheeses are safe to eat after the mold has been sliced off, while others are toxic.

Hard and semisoft cheese, such as parmesan, Swiss, romano and cheddar, you can cut away the moldy part and eat the rest of the cheese. Cut off at least 1-inch around and below the moldy spot.

With soft cheeses, such as brie, chevre (goat cheese), blue cheese, and ricotta, the mold that grows on these cheeses cannot be safely removed so they should be discarded. One reason is that the molds can more easily penetrate into the heart of soft cheeses than they can into harder cheeses. This causes spoilage from within that cannot be scraped away. The same goes for any cheese that has been shredded, crumbled or sliced. If mold is found on soft cheese (i.e. cottage cheese, cream cheese) the entire package should be discarded. Mold on soft cheeses are toxic.

Are Molds Only on the Surface of Food?

No – you only see part of the mold on the surface of food – gray fur on forgotten bologna, fuzzy green dots on bread, white dust on Cheddar, coin-size velvety circles on fruits, and furry growth on the surface of jellies. When a food shows heavy mold growth, “root” threads have invaded it deeply. In dangerous molds, poisonous substances are often contained in and around these threads. In some cases, toxins may have spread throughout the food.

Why Can Mold Grow in the Refrigerator?

While most molds prefer warmer temperatures, they can grow at refrigerator temperatures, too. Molds also tolerate salt and sugar better than most other food invaders. Therefore, molds can grow in refrigerated jams and jelly and on cured, salty meats (ham, bacon, salami, and bologna).

Cleanliness is vital in controlling mold, because mold spores from contaminated food can build up in your refrigerator, dishcloths and other cleaning utensils.

Clean the refrigerator or pantry at the spot where the food was stored. Check nearby items the moldy food might have touched. Mold spreads quickly in fruits and vegetables.

Clean the inside of the refrigerator every few months with 1 tablespoon of baking soda dissolved in a quart of water. Rinse with clear water and dry. Scrub visible mold (usually black) on rubber casings using 3 teaspoons of bleach in a quart of water.

Keep dishcloths, towels, sponges and mops clean and fresh. A musty smell means they’re spreading mold around. Discard items you can not clean or launder.

Keep the humidity level in the house as low as practical – below 40 percent, if possible.

How Can You Protect Food from Mold?

When serving food, keep it covered to prevent exposure to mold spores in the air. Use plastic wrap to cover foods you want to stay moist (fresh or cut fruits and vegetables, and green and mixed salads).

Empty opened cans of perishable foods into clean storage containers and refrigerate them promptly.

Don’t leave any perishables out of the refrigerator more than 2 hours.

Use leftovers within 3 to 4 days so mold does not have a chance to grow.

How Should You Handle Food with Mold On It?

Buying small amounts and using food quickly can help prevent mold growth. But when you see moldy food:

Do not sniff the moldy item. This can cause respiratory trouble.

If food is covered with mold, discard it. Put it into a small paper bag or wrap it in plastic and dispose in a covered trash can that children and animals can not get into.

Clean the refrigerator or pantry at the spot where the food was stored.

Check nearby items the moldy food might have touched. Mold spreads quickly in fruits and vegetables.

Learn more about mold on foods: http://www.madsci.org/FAQs/micro/molds.html

Molds on Food

Luncheon meats, bacon, or hot dogs – Discard
Foods with high moisture content can be contaminated below the surface. Moldy foods may also have bacteria growing along with the mold.

Hard salami and dry-cured country hams – Use
Scrub mold off surface. It is normal for these shelf-stable products to have surface mold.

Cooked leftover meat and poultry – Discard
Foods with high moisture content can be contaminated below the surface. Moldy foods may also have bacteria growing along with the mold.

Cooked casseroles – Discard
Foods with high moisture content can be contaminated below the surface. Moldy foods may also have bacteria growing along with the mold.

Cooked grain and pasta – Discard
Foods with high moisture content can be contaminated below the surface. Moldy foods may also have bacteria growing along with the mold.

Hard cheese (not cheese where mold is part of the processing) – Use
Cut off at least 1 inch around and below the mold spot (keep the knife out of the mold itself so it will not cross-contaminate other parts of the cheese). After trimming off the mold, re-cover the cheese in fresh wrap. Mold generally cannot penetrate deep into the product.

Cheese made with mold (such as Roquefort, blue, Gorgonzola, Stilton, Brie, Camembert) – Discard
Soft cheeses such as Brie and Camembert if they contain molds that are not a part of the manufacturing process. If surface mold is on hard cheeses such as Gorgonzola and Stilton, cut off mold at least 1 inch around and below the mold spot and handle like hard cheese (above). Molds that are not a part of the manufacturing process can be dangerous.

Soft cheese (such as cottage, cream cheese, Neufchatel, chevre, Bel Paese, etc.) – Discard
Foods with high moisture content can be contaminated below the surface. Shredded, sliced, or crumbled cheese can be contaminated by the cutting instrument. Moldy soft cheese can also have bacteria growing along with the mold.

Yogurt and sour cream – Discard
Foods with high moisture content can be contaminated below the surface. Moldy foods may also have bacteria growing along with the mold.

Jams and jellies – Discard
The mold could be producing a mycotoxin. Microbiologists recommend against scooping out the mold and using the remaining condiment.

Fruits and vegetables, firm (such as cabbage, bell peppers, carrots, etc.) – Use
Cut off at least 1 inch around and below the mold spot (keep the knife out of the mold itself so it will not cross-contaminate other parts of the produce). Small mold spots can be cut off fruits and vegetables with low moisture content. It is difficult for mold to penetrate dense foods.

Fruits and vegetables, soft (such as cucumbers, peaches, tomatoes, etc.) – Discard
Fruits and vegetables with high moisture content can be contaminated below the surface.

Bread and baked goods – Discard
Porous foods can be contaminated below the surface.

Peanut butter, legumes and nuts – Discard
Foods processed without preservatives are at high risk for mold.

Are Any Food Molds Beneficial?

Yes, molds are used to make certain kinds of cheeses and can be on the surface of cheese or be developed internally. Blue veined cheese such as Roquefort, blue, Gorgonzola, and Stilton are created by the introduction of P. roqueforti or Penicillium roqueforti spores. Cheeses such as Brie and Camembert have white surface molds. Other cheeses have both an internal and a surface mold. The molds used to manufacture these cheeses are safe to eat.

Sources:
From The University of Illinois, College of Agricultural, Consumer and Environmental Sciences, Horticulture Solutions Series.

Science Explorer, published by Owl Books, Henry Holt & Company, New York, 1996 & 1997.

Food Safety Focus, USDA’s Meat and Poultry Hotline.

LSU AgCenter, Louisiana

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