White on zucchini leaves

Cucumber, Squash, Melon & Other Cucurbit Diseases

Prevention & Treatment: This fungus can survive in the soil for many years. Planting resistant varieties (Table 1) is critical in preventing this disease. Careful water management is also important in minimizing root stress. There are no chemical treatments available for control.

Viruses

Cucumber mosaic virus (CMV) on squash.
Division of Plant Industry Archive, Florida Department of Agriculture and Consumer Services, www.insectimages.org

There are several common viruses that can affect cucurbits, including Cucumber mosaic virus (CMV) and Watermelon mosaic virus (WMV). Infected plants may be stunted or have leaves that are mottled, crinkled, or a light green color. Fruits may be irregular in shape, mottled or warty. Various insects transmit these viruses.

Prevention & Treatment: There are no chemicals available to kill viruses. Chemical control of the insects that spread the viruses may minimize the disease. This control method is difficult, because infection occurs immediately after an insect feeds, and insects migrate freely between plants. A good control strategy is to maintain healthy and vigorous plants, plant recommended varieties and monitor your garden for any unusual symptoms as they occur. Keep the area clear of weeds that can harbor insects. Choosing separate areas for early and late plantings may help to reduce virus severity in the late plantings.

Blossom-End Rot

Blossom-end rot appears as a dark-colored dry rot on the end of the fruit where the flower was. The problem is caused by a lack of calcium in the developing fruit. It is an indication that calcium is lacking in the soil or that the plant does not have the ability to take up enough calcium. When growth is rapid, not enough calcium may be delivered to the blossom end of the developing fruit.

Blossom end rot on watermelon.
Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org

Prevention & Treatment: Help prevent blossom-end rot by having your soil tested through your local county Extension office before planting, and lime according to recommendations, usually to pH 6.5. Always maintain an adequate supply of moisture, especially during fruit growth. Mulch plants to prevent rapid drying of the soil and water plants during extended dry periods. Apply 1 to 2 pounds of gypsum per 100 square feet as a supplement to liming on calcium deficient soil. Lime and/or gypsum should be applied before planting.

Do not over-fertilize plants with excessive nitrogen or potassium. Excess amounts of these nutrients reduce the uptake of calcium in the plant. When plants are dark green, extra fertilizer should not be applied.

Irrigate with 1 inch of irrigation water per week if there is inadequate rainfall. Cultivation near crops should be shallow to avoid root injury. Removing fruit with symptoms is recommended.

Table 1. Some Disease-Resistant Varieties for South Carolina.

Cantaloupe
Ambrosia Resistant to some powdery mildews
Mission Tolerant to downy and powdery mildew
Cordele Resistant to some powdery mildews and Fusarium wilt
Earlidew Resistant to Fusarium wilt
Cucumber, slicing
Ashley Resistant to some powdery mildews and downy mildew
Burpless Resistant to some powdery mildews
Poinsett8 76 Resistant to downy mildew, powdery mildew, angular leaf spot, anthracnose and scab
Supersett Resistant to downy mildew
Dasher II Resistant to powdery mildew, angular leaf spot, anthracnose, scab and cucumber mosaic virus
Cucumber, pickling
Regal Resistant to downy mildew, powdery mildew, angular leaf spot, anthracnose, scab and cucumber mosaic virus
Squash, summer
Multipik This yellow-fruited variety does not show greening of fruit caused by viruses (CMV, WMV)
Watermelon
Charleston Gray
Crimson Sweet
Jubilee II
Starbrite
Tiger Baby
All of these varieties have some resistance to anthracnose and Fusarium wilt.

Table 2. Preventative Fungicide Treatments for Cucurbit Diseases.

Vegetable Disease Fungicide
Cantaloupe Downy mildew chlorothalonil or mancozeb1 or copper fungicide4
Powdery mildew sulfur2 or chlorothalonil or horticultural oil + baking soda3
Gummy stem blight chlorothalonil or mancozeb1
Alternaria leaf spot chlorothalonil or mancozeb1
Anthracnose chlorothalonil or mancozeb1
Bacterial wilt no chemical control
Cucumber Anthracnose chlorothalonil or mancozeb1
Downy mildew chlorothalonil or mancozeb1 or copper fungicide4
Powdery mildew sulfur2 or chlorothalonil or horticultural oil + baking soda3
Gummy stem blight chlorothalonil or mancozeb1
Alternaria leaf spot chlorothalonil or mancozeb1 or copper fungicide4
Scab mancozeb1 or chlorothalonil
Cercospora leaf spot chlorothalonil or mancozeb1
Squash Anthracnose chlorothalonil or mancozeb1 or copper fungicide4
Downy mildew chlorothalonil or mancozeb1 or copper fungicide4
Powdery mildew sulfur2 or chlorothalonil or horticultural oil + baking soda3
Gummy stem blight chlorothalonil
Watermelon Bacterial leaf spots or fruit blotch mancozeb1 plus copper fungicide4
Anthracnose mancozeb1 or chlorothalonil5
Gummy stem blight
& black rot (of fruit)
mancozeb1 or chlorothalonil5
Cercospora leaf spot mancozeb1 or chlorothalonil5
Powdery mildew sulfur2 or chlorothalonil5 or mancozeb1
Downy mildew mancozeb1 or copper fungicide4
All Vegetables Postharvest rots Sodium hypochlorite (5.25% commercial bleach). Mix 1 teaspoon of bleach in 1 gallon of water. Dip fruit into solution and rinse. Replenish bleach periodically as needed.
1 Wait 5 days after spraying before harvest.
2 Do not apply sulfur when temperatures are above 85 °F. Sulfur should be used on muskmelons very carefully because some varieties will be damaged by this chemical.
3 3 tablespoons of horticultural oil in a gallon of water and add 3 tablespoons of baking soda. Never apply an oil spray within 2 weeks of a sulfur spray, and do not apply oils when temperatures are above 90 °F or to drought-stressed plants.
4 Fixed copper does not control downy mildew nearly as well as the other fungicides listed. Caution is advised, as copper can be phytotoxic to cucurbits, if applied at temperatures above 90 °F.
5Spray at first appearance and then at 7-14 day intervals. Avoid late-season application after plants have reached full maturity and fruit have begun to size, or cover the watermelon fruit to prevent contact with fungicide, or use mancozeb. Do not mix chlorothalonil with a copper fungicide in the sprayer.

Table 3. Examples of Fungicides Labeled for use in Home Vegetable Gardens on Cucurbits.

Fungicide Examples of Brands & Products
Chlorothalonil Ortho MAX Garden Disease Control (29.6%)
GardenTech Daconil Fungicide Concentrate (29.6%)
Bonide Fungonil Concentrate (29.6%); & RTU1
Hi-Yield Vegetable, Flower, Fruit & Ornamental Fungicide (12.5%)
Southern Ag Liquid Ornamental & Vegetable Fungicide (12.5%)
Tiger Brand Daconil (12.5%)
Ferti-lome Broad Spectrum Landscape & Garden Fungicide (12.5%); & RTU1
Copper fungicides Bonide Copper Fungicide Spray or Dust (wettable powder with copper sulfate)
Southern Ag Liquid Copper Fungicide Concentrate
Monterey Liqui-Cop Copper Fungicide Garden Spray Concentrate; & RTS2
Bonide Liquid Copper Concentrate (a copper soap)
Camelot O Fungicide/ Bactericide Concentrate (a copper soap)
Natural Guard Copper Soap Liquid Fungicide Conc.; & RTU1
Bonide Liquid Copper Fungicide RTU1 (a copper soap)
Horticultural oil Bonide All Seasons Spray Oil Concentrate; & RTU1
Ferti-lome Horticultural Oil Spray Concentrate; & RTS2
Monterey Horticultural Oil Concentrate; & RTS2
Southern Ag Parafine Horticultural Oil
Summit Year Round Spray Oil Concentrate
Mancozeb Bonide Mancozeb Flowable with Zinc
Southern Ag Dithane M-45 Concentrate
Sulfur Bonide Sulfur Plant Fungicide (dust or spray)
Ferti-lome Dusting Sulfur (also wettable for spray)
Hi-Yield Wettable Dusting Sulfur
Safer Brand Garden Fungicide Concentrate
Southern Ag Wettable or Dusting Sulfur
1 RTU=Ready to Use (a small pre-mixed spray bottle)
2 RTS=Ready to Spray (a hose-end spray bottle)

GARDEN + GARDENING + GARDENING TIPS & ADVICE

Summer Squash – Diseases, Pests and Problems

Basic Information

Problem: Squash Bug
Affected Area: Entire Plant
Description: Plants show brown blotches or stunted and distorted growth. Bug is dark to grayish-brown above and paler color underneath.
Control: Plants show brown blotches or stunted and distorted growth. Bug is dark to grayish-brown above and paler color underneath. Control: Pick bugs by hand. They will hide under boards, etc., placed near the plants. * Spray with Rotenone, Pyrethrum or Diazinon. Remove debris so bugs cannot overwinter. * Pesticide use and recommendations for various areas are constantly changing. Check with your County agent for current recommendations.

Problem: Aphids
Affected Area: Leaf and Stem
Description: Small Insects found on new stems and the underside of the leaf. Usually green. They suck fluids from the plant leaving a honey dew substance behind. Leaves turn pale yellow.
Control: Insecticidal soaps or a strong stream of water. Ladybug beetles are natural predators. * Thiodan or Diazinon ? more than one application may be required. A layer of aluminum foil under plants reflects light to underside of leaves and may deter aphids. * Pesticide use and recommendations for various areas are constantly changing. Check with your County agent for current recommendations.

Problem: Blossom-end rot
Affected Area: Fruit
Description: Blossom end of squash is dark and leathery. Hot weather increases susceptibility.
Control: Supply even moisture to plant

Problem: Curly top
Affected Area: Leaf
Description: Leaves curl, plant dies.
Control: Destroy infected plants. Rotate location. Avoid stressing plant by watering evenly and providing adequate fertilizer. Clean up debris in fall.

Problem: Powdery Mildew
Affected Area: Leaf
Description: White powdery substance on leaves.
Control: Use surface or underground watering methods to avoid wetting leaves. Plant resistant varieties. Rotate planting location from year to year.

Problem: Mosaic virus
Affected Area: Leaf and Fruit
Description: Deformed (bumps) and discolored fruit and leaves.
Control: Plant resistant varieties. Control insects. Rotate location.

Problem: Verticillium Wilt
Affected Area: Entire plant
Description: Part of plant turns brown, wilts, and dies. May recover briefly recover briefly, but eventually dies. Brown streaking in vascular tissue.
Control: Destroy infected plants. Rotate the planting location. Clean up debris in fall.

Squash (Cucurbita spp.)-Virus Diseases

See:

Squash (Cucurbita spp.)-Curly Top

Cause The three most common virus diseases in Oregon are zucchini yellows caused by the Zucchini yellows mosaic virus (ZYMV), watermelon mosaic caused by Watermelon mosaic virus 2 (WMV2), and curly top caused by the Beet curly top virus (BCTV). The first two are potyviruses that are spread by aphids and by mechanical means such as equipment and pickers’ hands and clothing. Aphids can move these viruses from weed hosts over relatively long distances. Curly top is restricted to production areas east of the Cascade Range. Cucumber mosaic virus (CMV) also can be a problem; it is spread by aphids and cucumber beetles. CMV overwinters in wild cucumber seed and in wild perennial milkweed, ground cherry, and matrimonial vine.

These viral diseases first become noticeable in mid to late July, often following cutting of grass seed and alfalfa fields, which may displace aphid populations from these fields. Fewer infections tend to occur during wet summers. Infections generally begin at the edges of fields and move inward.

Symptoms Generally, it is difficult to tell ZYMV and WMV2 apart based only on symptoms. Serological testing is the best way to distinguish between them. Curly top; however, is easy to differentiate from the two others.

ZYMV-a prominent yellow and green mosaic, necrosis, and a distortion called “shoestring” are on zucchini leaves. Early infection may result in no fruit set. Later fruit are severely distorted, small and green, and have yellow outgrowths.

WMV2-winter squash leaves range from faint green to a severe yellow mottle. Leaves are malformed, puckered, and blistered. Veins sometimes extend beyond the normal leaf margin. Plants may become bushy with shortened internodes. Fruit can look healthy or can develop knobby overgrowths that distort its shape. Summer squash symptoms are similar, but internode lengths become extended, producing a leggy plant.

BCTV-plants appear stunted at first, then turn yellow and eventually die. Fruit set or fruit development stops when the plant becomes infected.

CMV-leaves are markedly mottled with yellow and green blotches and wrinkled; edges cup down. Early-season infection dwarfs the plant; later infections show the typical mosaic symptoms only on late-season growth following infection. Fruit develops raised, wart-like bumps with pale, whitish-green areas between.

Cultural control

  • Control weeds in the area around squash plantings.
  • Isolate squash plantings from one another if possible.
  • Clean equipment before moving into a healthy field. Try to allow pickers to get clean clothes before entering healthy fields.
  • There is resistance in some varieties to combinations of these viruses:

White spots on zucchini leaves

Hi-
Most likely this is powdery mildew on the leaves. Does it look like someone sprinkled baby powder on the leaves? This disease occurs in very humid, moist and warm environments where there is little or no air circulation.
If it is indeed powdery mildew, my first thoughts go to the location of the plant (wide open and sunny), distance between it and other plants (air circulation) and watering practices (water early in the morning, only at the root zone and only using slow, deep watering so as to feed the roots, but to also minimize moisture in the surrounding environment.) If you do not have these, try these practices first. You can also remove a few leaves if these practices do not help. Removing a few leaves will also help improve air circulation.
Here are some great photos of all of the diseases:

Besides the cultural practices I mention above (also detailed in this factsheet below), the only other option is fungicides. Details are here:
http://umaine.edu/ipm/ipddl/publications/5085e/
Good luck!

A reader has asked about powdery mildew on zucchini plants and fungus-eating ladybirds:
Hi. Wonder if you can sort this.
1. Most fungi need moisture and organic material. This seems to be supported by my zucchinis which seem to get worse powdery mildew when I get water on the leaves. I have read that they like dry weather. Is there evidence for either opinion?
2. Some people say that the ladybirds that feed on this mildew spread it by carrying spores, others reckon they are a controller, eating the fungus down. What is the evidence please for either of these? Many thanks, Barb


Powdery mildew is likely to occur on stressed plants in humid weather when temperatures are between 11-28° C. and, once established will continue to affect the plants even if weather becomes dry. Avoid wetting leaves whenever possible Barb. However, because they like low-humidity weather, it doesn’t mean that they are drought tolerant. Zucchini and some other members of the cucurbit family (melons and squash) produce a lot of foliage and need plenty of water and fertiliser. An efficient way to water this group of plants without wetting the leaves is to put a large drink container (with the base and cap removed) neck downwards near the roots so that all the water goes directly to the root area where it is needed, (see photo). Keep topping up the container until it empties slowly.
The yellowish ladybirds with 26 or 28 spots are the only pests of the ladybird family. They eat the leaves of stressed plants of cucurbits. The beneficial fungus-eating ladybird and larvae can be clearly distinguished from the pest in the photos below. From the far left is the ‘Fungus-eating ladybird’, then the leaf-eating ’26 spot Ladybird’ that damages plants. Next is the larva of the ‘Fungus-eating Ladybird’, which also eats fungus and, last of all is the prickly larva of the ’26 spot Ladybird.

Rather than blame the fungus-eating ladybird for spreading the disease, gardeners should check that their plants have sufficient water and nutrients to avoid stress, and the soil pH is suitable for them to absorb what they need for healthy, disease-resistant growth. Also see Powdery Mildew for treatment of this disease.

Zucchini growing problems: 10 common issues and how to overcome them

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Zucchini and other soft-skinned summer squashes are usually pretty easy to grow. But, gardeners do sometimes face struggles with these productive crops. Perhaps your vines stopped producing in mid summer? Or the fruits were small or deformed? Or maybe your plants simply died before producing any fruits? If you found yourself asking why zucchini growing problems struck your garden, this solution guide is for you.

Top 10 zucchini growing problems

Here are ten reasons why you may have faced zucchini growing problems in the past, and tips for making sure these issues don’t happen again.

Zucchini problem 1: Improper variety selection.

Not all zucchini varieties perform the same. Some are more productive than others, and some are more disease- and pest-resistant. First and foremost, when selecting zucchini varieties for your garden, be sure to seek out disease and pest resistance whenever possible. Varieties with a high level of natural resistance often perform better and produce longer. ‘Tigress’, ‘Green Machine’, ‘Burpee Golden Glory’, and ‘Yellow Fin’ are great choices.

Limiting zucchini growing problems starts with selecting the right varieties.

Zucchini problem 2: Squash vine borers.

One of the biggest zucchini growing problems is a pest known as the squash vine borer. Adult vine borers are day-flying moths that are black and red with dark wings. They’re fast flyers, so gardeners don’t often spot them. The damage caused by their larvae, however, is difficult to miss. Squash vine borer larvae feed inside the main stem of the plant, hollowing it out and eventually causing plant death. You’ll see crumbly, sawdust-like waste collected below a small hole at the base of the plant. To prevent squash vine borers, protect the lower portion of the stem with a wrap of aluminum foil (more on this technique here), or cover the plants with floating row cover until they come into bloom to keep the female moths away from egg-laying sites.

Adult squash vine borers are day-flying moths that look like large wasps.

Zucchini problem 3: Poor pollination.

Zucchini and other squash are insect pollinated, meaning a bee, beetle, or other pollinator is needed to move the pollen from a separate male flower over to a female flower. If there aren’t enough pollinators present, puny or deformed fruits are the result. If your zucchini are mal-formed and stubby on the blossom end, poor pollination is the most pressing of your zucchini growing problems. To improve pollination rates, plant lots of flowering herbs and annuals in and around your zucchini patch. You can also hand-pollinate the vines by using a paintbrush or your fingertip to transfer pollen from the male flowers to the females (more on how to hand pollinate here). Another option is to plant a parthenocarpic variety that doesn’t require pollination to set fruit, such as ‘Easypick Gold’, ‘Partenon’, or ‘Cavili’.

Zucchinis rely on insect pollinators to move the pollen from male to female flowers.

Zucchini problem 4: Powdery mildew.

Powdery mildew is among the most pervasive fungal diseases when it comes to vine crops like zucchini. This pathogen makes the leaves appear to be covered in a talcum powder-like coating. Though it’s primarily an aesthetic issue, severe cases can lead to reduced photosynthesis and reduced production. To overcome powdery mildew, space plants properly – give each one plenty of room so air can circulate and dry off wet foliage. Plant only resistant varieties, such as ‘Anton’, ‘Dunja’, ‘Astia’, and ‘Emerald Delight’, to help combat powdery mildew which is one of the most tenacious zucchini growing problems. Organic fungicides based on potassium bicarbonate (such as GreenCure and BiCarb) are effective as preventatives, as are those based on Bacillus subtilis (such as Serenade).

Powdery mildew is a difficult fungal disease that often strikes zucchini plants.

Zucchini problem 5: Squash bugs.

When it comes to insects that attack squash, none are more difficult to control than squash bugs. These shield-shaped, brown insects suck out plant juices with their needle-like mouthpart, causing stippling, yellowing, and browning of the leaves.

Squash bugs are first seen as clusters of bronze, football-shaped eggs followed by gray nymphs that feed in groups.

The best way to manage squash bugs is to head to the garden every day and inspect the top and bottom of your zucchini leaves for clusters of bronze-colored, football-shaped eggs. Squash bugs are resistant to most pesticides, but very young nymphs can be controlled with applications of insecticidal soap or horticultural oil. Our Guide to Vegetable Garden Pests has more info on this troublesome insect.

Watch this video to see a cool trick for getting rid of squash bugs organically – using duct tape!

Zucchini problem 6: Poor soil.

Zucchini doesn’t require excessively nutrient-rich soil, but it does perform best in soils that are high in organic matter with a soil pH around 6.5. If your pH is too far off that target mark, the plants may fail to produce quality fruit because the soil pH affects the availability of many different nutrients (more on soil pH here). You can also prevent many zucchini growing problems related to the soil by limiting the amount of nitrogen you add to your garden. Excessive nitrogen produces a lot of green leaves, often at the expense of good fruit production. Use only balanced, organic fertilizers on your zucchini patch and test your soil every few years to ensure it’s healthy and well-balanced.

Give zucchini plants plenty of room to grow and make sure they’re planted in soil that’s rich in organic matter.

Zucchini problem 7: Lack of water.

Zucchini growing problems can also stem from irregular soil moisture levels. If plants are allowed to dry out between waterings, fruit production can be negatively impacted. Drought stress is never good for vegetable crops, and zucchinis require consistent, even soil moisture throughout the growing season. If Mother Nature doesn’t supply your garden with at least one inch of water per week, it’s your job to add supplemental irrigation to prevent any possible issues. A 2-3 inch thick layer of mulch helps stabilize soil moisture levels and can reduce the need to irrigate during the hot summer months. You’ll find more information on proper mulching techniques here.

Mulch zucchini well to keep the soil evenly moist. This zucchini patch is mulched with newspaper topped with shredded leaves.

Zucchini problem 8: Blossom end rot.

Zucchini can also be affected by blossom end rot, just like tomatoes and peppers. This physiological disorder causes the blossom end of the fruit to rot into a dark, sunken canker. It’s caused by a calcium deficiency, but it’s the result of inconsistent watering. Calcium can only come into a plant as it absorbs water in through its roots. When there’s no water in the soil to absorb, the plant can’t access calcium either and blossom end rot is the result. To prevent blossom end rot from striking your zucchini, make sure the plants receive ample, consistent applications of water throughout the growing season. Adding more calcium will not solve the problem.

Zucchini problem 9: Bacterial wilt.

Though this pathogen tends to be more problematic on cucumbers, it sometimes strikes zucchini as well. Sadly, this is one of those zucchini growing problems that’s the kiss-of-death when it strikes. Spread by the cucumber beetle, bacterial wilt causes otherwise healthy plants to wilt and die without prior warning. To combat potential problems, keep cucumber beetles in check by trapping them on yellow sticky cards fastened to stakes just above the tops of the plants.

Growing healthy, productive zucchini happens when you provide plants with everything they need.

Zucchini problem 10: Not enough sun.

Though it isn’t the worst of the zucchini growing problems you might face, lack of sun can definitely affect plant health and production. Zucchini plants need a minimum of six to eight hours of full sun per day. Lower light levels can result in long, lanky plants with pale green foliage and reduced yields. Poor pollination can also be a side effect of light levels that are too low because pollinators tend to prefer foraging in sunnier areas, particularly on cooler days. Select a full-sun site when planting your zucchinis.

Zucchini plants require six to eight hours of full sun per day to perform their best.

Zucchini growing problems don’t have to decimate your crop

Though zucchini growing problems may strike your garden from time to time, with these management tips, you can manage the issues organically and enjoy bushels of delicious zucchini all season long.

For more on growing healthy zucchini, check out these related posts:

Cucumber plant problems
Guide to Vegetable Garden Pests
A Handy Guide to Harvesting Vegetables
Types of Landscape Mulch
A Compost Guide

What challenges have you faced with your zucchini crops and how did you overcome them?

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