Powdery mildew on trees

Growing your own veggies and herbs is always a challenge, but nothing gets me more frustrated than when I have to battle powdery mildew.

It’s one of the most common diseases that will afflict your plants, especially if you’ve been gardening for a while. It’s caused by many different types of fungus that cause white, powder-like spots to appear on the leaves and stems of your plants.

If you leave it unchecked, it can quickly affect the health of your plants, reducing their vegetative growth and inhibiting their ability to fruit or flower.

Fortunately, you can treat it in a variety of ways, which I’ll cover below.

First: What Is Powdery Mildew?

Powdery mildew completely covering this leaf. It will spread quickly to the rest of the plant if left unchecked. Photo by Pollinator

When people refer to “powdery mildew”, they could mean many different types of fungus-related plant diseases, However, the one that is suspected to have caused the rest of the other fungus’ to develop is Erysiphe cichoracearum, a fungus that primarily attacks squash plants.

Regardless of the specific type of fungus that afflicts your plants, they all act in a similar way. The fungus will spread over the vegetation of your plants and prevent photosynthesis and the plant’s ability to utilize nutrients. Without catching it early, the damage may be too far along to stop, and you will have to remove the plant from your garden completely.

How To Identify Powdery Mildew

The most obvious sign of this disease is the distinct powdery, white dust that covers a plant’s leaves. These start out small, but grow in size as the disease progresses.

The difference between downy mildew and powdery mildew. source

It can be easy to confuse powdery mildew with a similar disease, downy mildew. To tell which type of mildew you have, look at where it appears on the plant. Downy mildew only grows on the underside of plant leaves, whereas powdery mildew will grow anywhere. Downy mildew also lacks the powder-like appearance:

Symptoms of Powdery Mildew

As the disease progresses, the small white spots begin to form a root-like structure that enters the plant’s leaves and saps them of nutrition. Leaves will become starved and begin to turn yellow.

If the disease continues to progress, leaves will turn brown and die. This is a bad enough problem on its own, but the dying leaves also open your plant up to sun damage, a malformation of buds and fruit, as well as potentially failing to fruit altogether.

How To Prevent and Treat Powdery Mildew

Complete prevention of powdery mildew outbreaks can be difficult. The spores of the fungus are carried through the air on gusts of wind. The spores then fall on plant surfaces and begin to reproduce if the conditions are right. In moist conditions, powdery mildew may not be a problem. However, other types of plant diseases can grow in damp conditions.

The best ways to prevent powdery mildew are:

  • Choose plants that are resistant to powdery mildew
  • Avoid planting vulnerable varieties in the shade
  • Manage aphid problems, as they can carry the spores into your garden
  • Provide moisture to leaves on a regular basis
  • Remove dried or diseased plant matter immediately upon seeing it
  • Use a variety of home or professional treatments if your plants have a serious mildew problem

Best Products to Treat Powdery Mildew

A variety of commercial products are available to cure powdery mildew. Many of these are fungicides that contain copper to kill off the powdery mildew spores.

However, a number of other treatments can provide good results at lower cost, such as:

Common baking soda that you keep in your kitchen for cooking and baking can be used to prevent the spread of powdery mildew in your garden. Just dilute one tablespoon of baking soda in one gallon of water.

Add 1/3 teaspoon of dish-washing liquid to help the solution stick to the leaf surfaces of the plant. Do not save leftover solution. Make a new solution each time you provide treatment for the plants.

Potassium bicarbonate is a powdery compound that has a number of uses in food processing, in medicinal products and for wine-making. This compound can also be used in solution as a treatment for powdery mildew problems on garden plants.

The advantage of using potassium bicarbonate is that the compound is effective against powdery mildew that is already established, instead of a preventative measure.

Common mouthwash that you have in your medicine cabinet or on your sink can be used to treat powdery mildew in your garden. The mouthwash should be ethanol-based and should be mixed one part mouthwash to three parts water.

Because mouthwash is formulated to kill germs, it can be used as a powdery mildew spray to eliminate the spores that will continue to reproduce and damage your plants.

Common household vinegar can also be diluted and used a powdery mildew treatment. Mix four tablespoons of vinegar in one gallon of water and spray onto the plants every three days. The solution can be used safely for gardens with edible fruits & vegetables.

However, vinegar is an acidic substance and repeated use can negatively impact the condition of your plants.

Sulfur and sulfur spray have been used for many years to prevent and eliminate molds on plants. It can be used to treat powdery mildew and a variety of other plant diseases. It can be found at your local garden center or plant nursery.

Some products include both sulfur and lime, which is thought to be even more effective against powdery mildew. However, these compounds can burn delicate plant tissue, so use them only as directed and space applications a sufficient amount of time apart to avoid harming the plants.

Milk has been recommended for powdery mildew for generations, but only now is the science behind it being investigated. A study found that a 10% solution of milk was as effective against the mold as other methods of treatment. Milk is an inexpensive and completely organic way to fight this plant disease and can be used safely in any planting area.

Powdery mildew spores require a hot, dry environment in order for the spores to spread and proliferate. When conditions are moist, the spores cannot multiply. Keeping your garden plants slightly moistened will help to prevent the spread of powdery mildew spores that are carried on the wind.

**Note: Many other plant diseases thrive in a damp environment, so using water is not a long-term solution.

Neem oil is an extract made from the fruit and seeds of the neem tree, which is native to India. It has been used as an insecticide and anti-fungal for thousands of years, and it is still useful today as an organic compound to eliminate garden pests.

Neem oil is used to remove powdery mildew by reducing the ability of the spores to reproduce. It’s often recommended as a garden spray against plant insects, but it is also effective as a powdery mildew spray.

However, it may be more effective as a preventative than as a treatment when the problem has already occurred.

Although powdery mildew can be a stubborn and frustrating problem, good gardening practices can reduce the likelihood of developing this troublesome plant disease.

Try a few of the treatments above to see which works best for your garden.

The Green Thumbs Behind This Article:
Kevin Espiritu
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How to Get Rid of White Fuzzy Fungus on Plants (Powdery Mildew)

We’re used to tree leaves changing color throughout the year, but dusty white usually isn’t in their color palette.

If you see white stuff on plant leaves, it’s probably powdery mildew. True to its name, this fungus covers plant leaves and stems with what looks like powdered sugar.

Learn more about what powdery mildew is, how it develops, and what you can do to prevent it.

Powdery Mildew 101: Life Cycle, Symptoms, Treatment and Prevention

I see white stuff on plant leaves or stems. Is it powdery mildew? What are the symptoms?

Look for leaves that have some irregular white spots or look like they’ve been dipped in powdered sugar. If you see that, it’s probably powdery mildew.

Confirm by checking the temperature. It thrives in dry, warm days in the 60s and 70s, followed by cool, humid nights.

What trees and shrubs get powdery mildew the most?

This list could have hundreds of different trees and shrubs on it! Below are a few that are commonly affected:

  • Azaleas
  • Crape myrtle
  • Dogwoods
  • Euonymus (Wintercreeper)
  • Hydrangeas
  • Lilacs
  • London plane trees
  • Ninebark
  • Oaks
  • Roses
  • Rhododendrons

What’s the life cycle of powdery mildew?

The fungus overwinters on plant buds, stems or fallen leaves. Then, spring’s 60-degree temperatures activate the fungus, which causes spore production during damp nights.

During the day, the wind spreads the spores, which often land on nearby trees and plants. This is when you’ll see the first signs of infection.

This process usually takes a week or less and will repeat as long as mild days and humid nights continue.

How can I treat or control powdery mildew? Is there a powdery mildew fungicide?

One good thing about powdery mildew is that it’s not a big threat. Dusty white leaves are about as bad as it gets.

Still, you want to help your tree look fresh again. Start by cutting off the affected leaves to reduce the chances of further infection. Then, if treatment is warranted, consider using a fungicide or horticultural oil just as symptoms appear. Continue applying every week or two until temperatures reach 80 degrees during the day and nighttime humidity drops.

Can I use vinegar to treat powdery mildew?

In place of a fungicide, some home remedies are said to curb powdery mildew symptoms. DIY treatment isn’t always a surefire solution, but it’s worth a shot! Research suggests this. Mix 1 ½ tablespoons of baking soda, 1 tablespoon vinegar and 1 teaspoon of dish soap into 1 gallon of water.

What are some tips for powdery mildew prevention?

Powdery mildew thrives in specific conditions, so take those away, and you reduce its chances.

While you can’t dial down your local temperature or humidity level, you can cut down on the amount of moisture in your tree.

Here’s how:

  1. Avoid wetting the tree’s leaves too late in the day.
  2. Prune the tree to increase air circulation and access to sunlight.
  3. Before winter, remove infected limbs and twigs to cut down on next year’s spore spread.
  4. If replanting or replacing the tree is practical, consider moving the plant to a better spot. Lots of sunshine and space between trees are a must!
  5. If you’re planting something new, look for a species or cultivar that’s resistant to powdery mildew.

Powdery Mildew Fungus On Trees – How To Treat Powdery Mildew On Trees

Powdery mildew is an easy disease to recognize. On trees with powdery mildew, you’ll see a white or gray powdery growth on the leaves. It’s usually not lethal in trees, but it can disfigure fruit trees and limit their productivity. You can prevent powdery mildew fungus on trees by using proper cultural practices but treating powdery mildew on trees is also possible. Read on if you want to learn how to treat trees with powdery mildew.

Powdery Mildew Fungus on Trees

Powdery mildew attacks many plants, and trees with powdery mildew are no exception. Trees can be infected by different fungi. Most powdery mildew fungus on trees release overwintering spores when conditions are moist.

Moist conditions are also necessary for the spores to germinate and infect a tree. Once a tree is infected, however, the fungus grows well without humidity.

Preventing and Treating Powdery Mildew on Trees

Trees with powdery mildew are not usually damaged seriously by the fungus, but fruit trees are the exception. The disease attacks new buds, shoots and flowers on fruit trees, distorting new growth.

On apple trees, as well as apricot, nectarine, and peach trees, you’ll see web-like scars on the immature fruit of infected trees. A rough corky spot develops at the point of infection.

If you are wondering how to treat powdery mildew on trees, you’ll do best to give the trees excellent care to prevent the infection in the first place. Prevent powdery mildew fungus on trees by planting them in sunny sites, trimming inner branches to increase air circulation, and limiting fertilizer.

Treating powdery mildew on trees begins by vigilance. Keep your eye on your fruit trees as the new shoots develop in the springtime, looking for symptoms of powdery mildew. If you see deformed, puckered leaves, it’s time to get out the pruners. Disinfect the cutting edges, then prune out and discard the diseased portions of the plant immediately.

At the same time, apply fungicides to protect the remaining leaves on the fruit tree. You’ll need to repeat the fungicide applications according to label instructions to protect the trees over the entire season.

Key to Diseases of Oaks in the Landscape

Bulletin 1286 View PDF picture_as_pdf

Key to Oak Diseases

What is an “identification key?” It is a road map of sorts that aids in the identification of a disease or an insect specimen observed in the field. It is based on a series of questions that have as answers choices between two or more alternative pathways. The choices describe symptoms or morphological characteristics thought to be most distinctive of a particular insect or disease. In order to use these keys, you must first decide or make an educated guess as to whether the damage or symptom is caused by an INSECT or DISEASE. –From “Florida Tomato Scouting Guide”, University of Florida.

The following key will help you identify common diseases of oak trees.

1. FOLIAGE AND/OR TWIGS AFFECTED go to… 2
1. TRUNK OR BRANCES AFFECTED go to… 10
1. ROOTS OR ROOT COLLAR AFFECTED go to… 13
2. Leaf necrosis (dead spots or blights) present go to… 3
2. Necrosis absent; leaves affected but remain green go to… 6
3. Tiny to sprawling brown or gray spots along leaf margins and/or veins; no leaf blistering or puckering associated with spots go to… 4
3. Tan, crusty spots associated with leaf puckering; seen in the fall (Oak Leaf Blister) view info
3. Marginal, tip or interveinal leaf scorching; affectes all leaves on a branch go to… 5
4. Spots seen in the spring; leaves are tattered or distorted; twigs killed (Anthracnose) view info
4. Sprawling spots seen in the fall; often with yellow halo (Tubakia Leaf Spot) view info
5. Scorching from leaf tip toward petiole; yellow to red line often precedes advancing scorch symptom on leaf (Bacterial Leaf Scorch) view info
5. Yellow, brown or dull green scorching from leaf tip toward petiole or beginning at leaf petiole; affected branch often wilts (Oak Wilt) view info
6. Leaves and/or twigs with mycelium on upper or lower surface go to… 7
6. Light green, puckered areas in the spring; areas become necrotic in fall (Oak Leaf Blister) view info
6. Small, yellow pin-point spots on upper leaf surface; brown, hair-like projections on leaf underside (Fusiform Rust) view info
7. Upper or lower surfaces of leaf with white, powdery growth; pale green spots may also be present when growth is absent (Powdery Mildew) view info
7. Black, sooty or powdery growth typically on upper leaf surface (Sooty Mold) view info
10. Branches or trunk galled or swollen (Crown Gall) view info
10. Branches show dieback; cankers on trunk; tree in decline go to… 11
10. Branches show flagging, wilting; foliage yellows or browns go to… 12
11. Black, crusty, small spots or vertical stripes on trunk or branches (Hypoxylon Canker) view info
11. Conks or mushrooms protrude from trunk (Heart or Canker Rot) view info
11. Sour- or yeasty-smelling sap oozes from trunk; trunk stains dark (Bacterial Wetwood) view info
12. Vascular discoloration present (Oak Wilt) view info
12. Vascular discoloration not present (Bacterial Leaf Scorch) view info
13. Established tree in slow decline, white or thread-like material under bark (Armillaria Root Rot) view info
13. Conks or mushrooms growing at root collar (Heart Rot or Wood Decay) view info
13. Tree is newly transplanted; leaves turn brown, bark on lower stem may peel away (Root Rot) view info

Oak Leaf Blister

Taphrina caerulescens

Symptoms

Oak leaves begin to show chlorotic, blister-like areas on the upper surface that can be as large as one half inch in diameter (Fig.1 & Fig.2). The lower surface has gray depressions that correspond to the raised blisters. As the disease progresses, the blisters turn brown and the leaf will curl as the blisters coalesce. Premature leaf drop also may occur. Trees are not severely damaged, but the appearance of the tree may be unsightly. All oak species are susceptible to this disease.

Figure 1. Chlorotic blister-like lesions. Figure 2. Puckered lesions of oak leaf blister.

Favorable Conditions

Leaves become infected when buds are beginning to open in the spring. The pathogen can survive winter on plant twigs and bud scales. The disease develops during wet, humid, mild conditions in the spring. Spores causing oak leaf blister are spread by wind and rain.

Control

Rake fallen leaves and debris and discard to reduce disease inoculum. No chemical control is necessary.

Anthracnose

Apiognomonia quercinia

Symptoms vary with host, weather and time of infection. Shoot blight is one of the first symptoms seen in spring. Blighting causes leaves and shoots to brown and shrivel. Young leaves become cupped or distorted with necrotic lesions. Large lesions often follow leaf veins or are delimited by leaf veins (Fig. 3). Old lesions are papery and gray to white in color (Fig. 4). On the underside of an infected leaf, tiny brown fungal fruiting bodies may be visible on or near major veins. Premature leaf drop is common. Mature leaves are fairly resistant and the symptoms are simple necrotic spots. If infection is severe, branch cankers and twig dieback can occur during winter and early spring. The symptoms usually first appear on lower branches and then spread upward.

Figure 3. Anthracnose lesions following a leaf vein. Figure 4. Distorted leaves with papery lesions.

Favorable Conditions

Anthracnose fungi overwinter in twigs and plant debris. If winters are mild, the pathogen is active causing cankers and dieback. Spores are spread by wind and rain during the spring and infect new shoots. This disease can have multiple cycles per year if the weather is moderately warm and wet.

Rake and destroy fallen leaves. Prune lower branches to increase air circulation and ensure proper tree fertility and irrigation. Chemical controls are rarely recommended unless trees are newly established.

Tubakia Leaf Spot (Actinopelte Leaf Spot)

Tubakia dryina

The symptoms caused by Tubakia are brown or reddish brown blotches on the leaves. Premature leaf drop and twig cankers are common if the trees are severely infected. Spots are well-defined on young leaves and enlarge to necrotic blotches on older leaves (Fig. 5 & Fig. 6). Small fungal fruiting bodies can be seen within the lesions. Lesions may cause leaves to collapse if they are on veins and restrict water movement.

Figure 5. Newly developed circular lesions caused by Tubakia. Figure 6. Brown lesions on older leaves of Tubakia leaf spot.

The disease overwinters on twigs and plant debris. It favors wet, humid conditions and warm temperatures. Primary spore dissemination is by wind and rain. Red oaks are more susceptible than white oaks.

Rake and dispose of fallen leaves. Prune trees to increase air circulation and ensure proper fertility and irrigation.

Bacterial Leaf Scorch

Xylella fastidiosa

Symptoms of bacterial leaf scorch are described as marginal leaf burn and are very similar to drought stress symptoms (Fig. 7). In addition to marginal leaf burn, there is a defined reddish or yellow border separating the necrosis from green tissue. Symptoms are more noticeable in late summer after hot, dry conditions, but symptoms can be expressed all year. Symptoms first appear on one branch or section of branches and on the oldest leaves. Each year symptoms will reoccur and progress to other parts of the tree (Fig. 8).

Figure 7. Marginal leaf burn due to bacterial leaf scorch. Figure 8. Branch dieback due to bacterial leaf scorch.

The disease is spread by leafhoppers, spittlebugs and through root contact with neighboring trees. Since the pathogen is harbored within insects, warm temperatures and high populations of leafhoppers and spittlebugs are conducive for bacterial leaf scorch.

Remove severely infected trees and replant using resistant species. Control weeds (to minimize insect populations) and ensure proper tree fertility and irrigation to maintain health and vigor.

Oak Wilt

Ceratocystis fagacearum

Oak wilt is rare in Georgia. The disease has not been positively identified at the UGA Plant Disease Clinic. Oak wilt is first observed near the top of the tree. Browning and bronzing of the leaves from the margins toward the petiole are the first symptoms of oak wilt. Eventually the leaves will drop prematurely and the tree will die. White oaks are moderately resistant to oak wilt. Red oaks often die within four weeks of the first symptoms. It may take years before other oaks die. The most reliable way to diagnose oak wilt on live oak is to observe leaf veins (Fig. 9). Veins are chlorotic and eventually turn brown. Oak wilt on red oaks simply wilts young leaves and they turn pale green and brown. Mature leaves may have water soaking lesions or turn bronze on the leaf margins (Fig. 10). Only individual branches or a portion of the canopy show symptoms. The fungus may cause brown streaking of sapwood (Fig. 11). Red oaks may have fungal mats that break through the bark (Fig. 12).

Figure 9. Chlorotic veins on live oak. Figure 10. Bronzing of leaf margins on red oak.
Figure 11. Brown streaking in sapwood. Figure 12. Cracked bark due to fungal mats.

The disease is spread by oak bark beetles and root grafts. It is most active during moderate temperatures. Red oaks are more susceptible than white oaks. The pathogen cannot tolerate temperatures above 90 degrees F ; so often, the pathogen dies in leaves and twigs. However, it may remain in the trunk and in the roots. Oaks are most susceptible in spring as new wood is forming.

Remove infected trees. Trenching may be used to prevent root grafting. Trunk injections using a systemic fungicide have had some effect in reducing oak wilt spread within newly infected trees; however, this method is not recommended for trees in the southeastern United States because incidence is limited and of little value once disease is identified.

Fusiform Rust

Cronartium quercuum

Lower leaf surfaces often have inconspicuous yellow to orange pustules and black hair-like clusters (Fig 13 & Fig. 14). However, the orange pustules can become very noticeable in summer and fall as rust infection continues on oak leaves. Leaves may have yellow spots or necrotic blotches and develop premature leaf drop. Symptoms on oak are not severe, but it is the alternate host to a more devastating disease in pines.

Figure 13. Yellow pustules. Figure 14. Black hair-like clusters.

Fusiform rust is most damaging in slash and loblolly pines. During the spring, pine galls produce spores that are wind-blown to young oak leaves. The pustules on the bottom of oak leaves then release spores in late spring that are carried by the wind to branch tips and tender, green pine needles. Most pine infections occur in April and early May. One spore stage (urediniospores) produced on oak leaves can continue to re-infect new oak leaves throughout the summer during warm, moist, humid conditions. This stage of the disease does not affect pines.

Disease is usually insignificant to oaks, except in tree nurseries where the orange pustules can make leaves unsightly. Fungicides can reduce rust development on young newly planted oaks, but control is directed mostly towards saving pines.

Powdery Mildew

Erysiphe spp.

Figure 15. White, powdery growth on the upper side of an oak leaf.

Powdery mildew is a white, powdery growth on the upper side of leaves (Fig. 15). Tiny black fruiting bodies are usually present in late summer or fall. Infection may also occur in the buds and shoots causing a witches’ broom as in the case of live oaks. Symptoms appear to be superficial.

Powdery mildew survives the winter on fallen leaves and in the spring, wind and water disseminate spores to new leaves. Cool, moist weather conditions in the spring are ideal for infection. Powdery mildew prefers warm days around 80 degrees F and cool nights of 60 degrees F. Temperatures above 90 degrees F will reduce powdery mildew growth. Powdery mildew also requires high humidity but dry leaves. This pathogen usually attacks a crowded canopy with poor air circulation.

Irrigate early in the morning, improve air circulation and remove fallen leaves. Witches’ brooming may be pruned from the tree. Fungicides are generally not recommended or needed as the disease is not lethal to the tree.

Sooty Mold

Various fungi

Figure 16. Black crust of sooty mold on foliage.

Sooty mold is a crust-like or powdery black growth on the foliage and stems of infested plants (Fig. 16). Sooty molds grow superficially and do not penetrate the leaves. Sooty mold fungi grow mainly on the excrement of sap-sucking insects such as aphids, scales, and whiteflies. Sooty molds do not attack the plant and it is mainly a cosmetic problem. Occasionally, severe build-ups can inhibit photosynthesis and reduce plant vigor. Sooty molds may persist long after the insects have left.

Sooty molds are present where there is a high insect population. Spores are dispersed by water.

Reduce insect population. No control is directed toward the sooty mold as its growth will stop when the insect population is controlled.

Crown Gall

Agrobacterium tumefaciens

Roots, stems and crowns form large, rough galls (Fig. 17). Plants with large galls are often stunted and are the first to suffer from environmental stresses. Damage is greatest when galls encircle the root crown. Mature trees may survive, but severely stressed or diseased trees may be killed by secondary pathogens. Galls are initially spongy, becoming hard with age (Fig. 18). Dead surface tissue will decay and slough-off.

Figure 17. An extreme case of crown gall. Figure 18. Mature gall that is hard and rough.

A crown gall bacterium is dispersed in soil and water and enters through wounds on roots and crowns. Once inside, the bacterium transfers some of its genetic material into the trees cells. These cells form tumors and grow at a rapid rate. The bacteria may survive in the soil for two years and galls develop in temperatures near 72 degrees F. Temperatures above 86 degrees F stops normal cells from transforming into tumor cells but does not prevent galls after transformation.

Avoid introducing crown gall bacterium into an area. Use resistant varieties and avoid injuring tree roots and trunks. Control insects that feed on stems and roots. Disinfect tools when pruning infected trees.

Hypoxylon Canker

Hypoxylon spp.

Yellowing and wilting of leaves often related to physiological stress may be the first symptom of this disease. Fungal mats (stroma) will develop beneath the bark of infected trees. Bark will begin to slough-off due to pressure from the stroma beneath it. Stroma exposed by sloughing bark is hard, tan to silver gray on the outside, and black within (Fig.19 & Fig.20). There may be small patches to elongated strips. Old stroma eventually loses the gray color and appears black. The sapwood becomes tan to light brown and has a definite black border.

Figure 19. Gray and black patches. Figure 20. Silver gray patches.

Hypoxylon cankers are opportunists that attack trees weaken by other factors such as heat, drought, wound, root injury or other diseases. Oak species most commonly infected are black, blackjack, laurel, live, post, and white oaks. Hypoxylon can be present as a latent colonist in healthy trees and this may account for the rapid invasion of stressed trees. The fungus is favored by warm temperatures of 60 to 100 degrees F but the optimal temperature is near 86 degrees F. Spores are primarily wind dispersed.

Keep trees vigorous with proper fertility and irrigation. Prevent wounds from mechanical, weather and insect injury. Remove infected limbs or trees once they become hazardous to property or people, and to reduce disease spread to adjacent trees.

Heart or Canker Rot

Inonotus spp.

Figure 21. Smooth, shelf-like conk.

The first symptom of this disease is a white rot of the heartwood and the eventual death of the cambium and sapwood. The decay removes lignin and the wood becomes spongy. The basidiocarps or conks of Ionotus andersonni exhibits dull brown peg like structures that may exceed 20 inches in length on the outside of the tree. Conks of I. hispidus form in the summer and early autumn. The conks are smooth and shelf-like and are fully grown within one to two weeks (Fig. 21). The top of the fungus is yellow to rusty red and the porous bottom is rust color. After three weeks, they dry and fall to the ground. Conks of I. dryadeus form at the base of infected trees among root flares. Conks are initially white or light-colored and turn black and crusty with age. Infected trees show symptoms of general tree decline including branch dieback, loss of leaves and yellowing or browning of leaves in summer. Trees weakened by drought stress, wounding or other injuries are most susceptible.

Figure 22. Cracked bark due to fungal mats.

Oaks in the red and black group are most often infected. Branch stubs within 16 feet of the ground are the most common infection sites, but entry also occurs through other injuries (Fig.22). Optimal temperature for growth is 95 degrees F. Old conks or remnants may continue to fruit for up to five years.

Avoid unnecessary injury or stress to trees. Remove hazardous trees to protect property and people. Time pruning of infected branches to minimize exposure of susceptible tissues and when spores are not disseminated (late winter or spring).

Bacterial Wetwood

A sour odor is often associated with wetwood as water-soaked wood with large numbers of dead bacteria begin to break down. The build-up of bacterial populations within the tree causes fermentation resulting in internal gas pressure of up to 60 pounds per square inch. Foliage sometimes wilts and branches may dieback. However, most of the time, wetwood is a minor problem that leaves a vertical streak on the bark where pressurized liquid escaped out of wounds (Fig. 23 & Fig. 24). Many times, secondary fungi and bacteria infect the surface liquid and create a slimy texture on the bark.

Figure 23. Staining of bark. Figure 24. Bark stain due to wetwood.

Bacteria that cause wetwood tolerate low oxygen and are often found in soils and on plant surfaces. Bacteria enter through assorted wounds above and below the soil line. The bacteria may lay dormant during the greatest periods of growth and become active in mature or older tissues.

There are no known controls for bacterial wetwood. A 10 percent bleach solution may be used cosmetically to clean stains off the bark.

Armillaria Root Rot

Armillaria spp.

There are several general symptoms that accompany Armillaria root disease, including crown dieback, growth reduction, premature leaf drop or death of the tree. Because these fungi commonly inhabit roots, their detection is difficult unless characteristic mushrooms are produced at the base of the tree. Removing the bark will expose the characteristic, white mycelial rhizomorphs that grow between the wood and the bark (Fig. 25). Short-lived mushrooms may be found growing in clusters around the bases of infected trees. They are honey brown to reddish in color (Fig. 26). The fungus breaks down lignin and cellulose causing the wood to become spongy.

Figure 25. White rhizomorphs of Armillaria. Figure 26. Honey-colored Armillaria mushrooms.

Spread occurs when rhizomorphs contact uninfected roots. Rhizomorphs can grow for distances of up to 10 feet and penetrate the roots by a combination of mechanical pressure and enzymatic action. Mushrooms are produced in late summer or autumn, and are most abundant during moist periods.

Vigorously growing trees often confine the fungi to localized lesions and limit their spread up the roots by secreting resin and rapidly forming callus tissues. But when infected trees are in a weakened condition, Armillaria spreads rapidly through the roots.

Because these fungi are indigenous to many areas and live on a wide variety of plants and woody material, their eradication is not feasible. Management should be directed toward increasing tree vigor through proper irrigation and fertility.

Root Rot

Phytophthora spp.

Above-ground symptoms vary, but generally include reduced tree vigor and growth, yellowing or chlorosis of leaves and eventual collapse or death of the tree. Infected trees may decline slowly over one or more years, or they may collapse and die rapidly after resuming growth in the spring. Rapid death of trees usually occurs following excessively wet periods. On trees that decline slowly, leaves will yellow or brown while leaves on healthy trees remain green. To observe below ground symptoms, you need to remove several inches of soil around the base of the declining tree. A diagnostic reddish-brown discoloration of the inner bark and wood can be observed after cutting away the outer bark layer (Fig. 27). Similar symptoms can be found on roots, but it is generally difficult to see root symptoms without removing the tree (Fig. 28).

Figure 27. Reddish brown wood discoloration. Figure 28. Brown discoloration of roots.

Phytophthora root and crown rots are caused by several Phytophthora species. While some species are much more destructive than others, all species require extremely wet or saturated soils in order to infect and cause significant damage. These fungi over-winter and persist in soil as mycelium in infected wood and can remain viable in the soil for years. When soils are wet, the fungus germinates and spreads to susceptible plant tissue where they infect. They may also spread to the soil surface and move over longer distances in runoff water. Trees appear to be most susceptible during spring and autumn, which are also the times of year when soil temperatures are most conducive to fungus growth and zoospore production. Fungal activity is low in the winter when trees are dormant.

Control of Phytophthora root rot is most successful using an integrated program of cultural practices including good soil drainage and proper irrigation. Avoid planting trees too deep as this also contributes to disease development and decline.

Forestry Images — www.forestryimages.org

Purdue University — www.ppdl.perdue.edu/PPDL

Texas A&M University — http://plantclinic.tamu.edu

University of Minnesota — http://pdc.umn.edu

University of Natural Resources and Applied Life Sciences

West Virginia University — http://www.davis.wvu.edu

Status and Revision History
Published on Aug 15, 2005
Published on Feb 10, 2009
Published on May 05, 2009
Published with Full Review on Jun 22, 2012

What can I do about this mold on my trees and shrubs?

You are correct in your observation that it is a mold growing on the trunks of your trees. But even more correct is that it is a lichen. Lichens are a combination of two distinct “plants” that have taken up living together to get benefits from each other. The first part of the symbiotic relationship is the blue-green algae that pair up with a fungus (mold) that can’t produce its own food stuffs. The blue-green algae produce sugars and starches and amino acids that the fungus uses to grow, and the fungus provides a substrate for the algae to attach to and grow and prosper.
The lichens on the trunks are more a diagnostic indicator of a thin canopy. There has to be enough filtered light for the lichens to grow and prosper. On a dense canopy tree, you won’t see much in the way of lichen growing on the trunks. Your problem with the dead trees in your yard stems from most likely a drought condition from the years past. Trees will respond to drought not in the same year that you experience the drought but in a couple of years after that. The roots will slowly die as will the top and then reach the point of no return. You may think we have had a lot of rain lately, but the tree with compromised roots will not be able to take advantage of the situation and will suddenly die – mainly because the roots have all died a couple of years ago.
You also might investigate the base of the center tree which looks like the grass has been mowed up to the trunk and the bark has been scraped. Trees, especially young ones, can’t stand to have any bark ruined at their base because they are so thin barked and vulnerable. You also might have had someone use a weed whacker at the base which will ruin the bark and stop the flow of water and nutrients up and down the trunk.
There is no need to remove the lichens from the trunks as they are just sitting there doing their own thing. The thing to do is give the trees you have left proper care to keep them growing and prospering. Plant the trees at the proper depth. Site them in a good location. Keep the grass away from the trunk at the base of the tree. Give them water when we don’t have rain for a week – especially young just planted trees.
You can also use a search engine using the term “lichen.edu” to find more information. Lichens grow on rocks and monuments in the cemetery. Also, they are a food source for reindeer in the Arctic. They were also used for dye for clothing in past centuries.
You can also ask a certified arborist to take a look at your trees. They are trained in the care of trees of all types and can advise you on what you can do to care for them.

Powdery mildew of apples

Note Number: AG0161
Published: December 1999
Updated: August 2010

Powdery mildew of apples, caused by the fungus Podosphaera leucotricha, affects leaves, buds, shoots and fruits, and forms a dense white fungal growth (mycelium) on the host tissue. The disease stunts the growth of trees and is found wherever apples are grown. It has also been reported on pear and quince although damage on these hosts is rarely seen in Australia.

Symptoms

Figure 1. Dormant shoot infected with powdery mildew

Dormant shoots that were heavily infected in the previous growing season are covered with dense white mycelium, and the terminal bud is pinched and shrivelled (Figure 1). Other shoots may look normal but the fungal mycelium may over-winter in otherwise healthy-looking fruit or leaf buds.

Primary mildew infection results from the growth of infected, over-wintered leaf or fruit buds. These buds may be killed, or they may grow abnormally; leaves become narrow, brittle, curled and covered with a white powdery layer, while flowers may be stunted and fail to develop (Figure 2). Secondary mildew infections may appear as a powdery mottling on either side of the leaves.Early fruit infection causes a web-like russet on the skin that may be difficult to distinguish from early spray damage. Less commonly, fruit may be distorted and partly covered with a white powdery coating of spores (Figure 3).

Economic importance

Figure 2. Primary mildew of apple blossom and leaves resulting from bud infection in the previous season

Powdery mildew can be a problem in all Victorian apple districts where adequate control measures are neglected. Highly susceptible cultivars include Jonathan, Bonza, Jonagold, Elstar, Pink Lady, Lady William and Gala. Powdery mildew is favoured by dry conditions, unlike apple scab, which is worst in districts of high rainfall.

Losses attributed to powdery mildew are hard to quantify. They include poor tree growth, dead buds, reduced leaf area, reduced shoot growth and lowered quality of fruit. Heavy infection that continues over several seasons may reduce yields by up to 80 per cent.

Disease cycle

The fungus overwinters in fruit and leaf buds, and the first powdery mildew symptoms occur on flower trusses and shoots that emerge from infected buds in spring. Spores from these infected leaves give rise to secondary mildew infections on the leaves of shoots which are rapidly extending throughout spring and early summer. Spores from all sources infect newly-formed buds in leaf axils. Fruit buds on spurs are susceptible to infection by powdery mildew for about one month, between pink bud and petal fall. Leaf buds are susceptible to infection for about one month after they appear in the axils of leaves on extending laterals. However, the terminal buds on laterals can be infected throughout the period of lateral growth. Infected buds are generally not killed but provide the main source of primary infection in the following season.

Control measures

Figure 3. Fruit infected with powdery mildew showing the white powdery coating of spores

Powdery mildew may be controlled by removing infected buds, by modifying the environment so that it is less favourable to infection and by spraying to protect buds from infection. However, there are several features of powdery mildew that make control difficult. Powdery mildew develops in the absence of rain, and at temperatures above 20°C. Rain is harmful to mycelium and suppresses production of spores. The time between germination of spores on leaf surfaces and infection and production of fresh spores is short, so there is a rapid recovery after any reduction in the number of spores. Plenty of spores are usually available for infection.

The emergence of new leaves on rapidly-growing laterals makes complete protection with fungicides difficult. Also, the buds in leaf axils are protected by leaves, and are difficult to reach with fungicides. Control measures must be applied thoroughly to be effective.

Removal of infected buds

The terminal bud is most commonly infected with powdery mildew, and therefore can be an abundant source of spores for infecting new leaves and buds. Remove infected terminal buds during pruning where practicable.

Modification of the environment

The humidity at the surfaces of leaves is an important factor influencing germination of spores. Maximum germination occurs at high relative humidity, but not in free water. High relative humidity occurs at leaf surfaces when the air is calm, but it is reduced by air turbulence. Trees with open canopies have more air turbulence and therefore less powdery mildew than in trees that have dense foliage. Similarly, wind breaks should be managed in such a way that turbulence is reduced enough to prevent fruit damage, but not so much as to result in completely calm conditions that favour mildew development.

Monitoring

Monitoring for mildew during winter and the growing season will assist with decisions about mildew control. The incidence of mildewed terminal buds in winter can be used to help plan mildew control strategies for the coming spring. Shoots with infected terminal buds can be assessed immediately after flowering using a minimum of 10 trees per block. For assessment of secondary mildew infection, label 15 trees per ha and assess 10 extension shoots per tree. Inspect the top 5 unfolded leaves per shoot for mildew, and record the incidence of mildewed shoots. Assess the same trees at intervals during the growing season. If mildew levels increase over time then control measures may need to be improved.

Control by spraying

There is no adequate substitute for a series of spring and summer sprays to prevent infection of fruit and leaf buds. Detailed work on powdery mildew at East Malling in the U.K. has shown that infection of fruit buds occurs before petal fall, and that infection of terminal buds can occur until lateral extension is complete. Thus, frequent spraying from pink bud until the end of lateral extension is needed to protect fruit, leaf and terminal buds. Frequency of spraying is more important than the dosage of fungicide, and two weeks should be the maximum period between sprays if control in orchards with a history of mildew is to be effective. High volume sprays are generally more reliable for mildew control, although recent work has shown that low-volume sprays can be equally effective.
A range of fungicides is registered for the control of powdery mildew, and your choice will depend on costs and any experience with phytotoxicity.

Contacts/services available from DEPI

For effective pest and disease control, correct diagnosis is essential. Phone Crop Health Services on (03) 9032 7515 or fax (03) 9032 7604.

Acknowledgements

This Information Note was published in December 1999.
It was reviewed by W.S. Washington, Plant Standards in August 2010.

ISSN 1329-8062

Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
Melbourne, Victoria

This publication is copyright. No part may be reproduced by any process except in accordance with the provisions of the Copyright Act 1968.

The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication

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