This is the eighth part of a series on fall tree diseases and disorders. This article examines powdery mildew and sooty mold.
During fall, the environment undergoes a drastic shift, with cool temperatures prevailing, and inclement weather often abundant. In this climate, many plant diseases flourish. The following describes some of the most common plant diseases to occur in fall, and how they affect their hosts.
Powdery Mildew (Podosphaera xanthii)
Powdery mildew is a fungal disease that affects a wide range of plants. It is caused by numerous fungi in the order Erysiphales, of which Podosphaera xanthii is the most frequently cited. Powdery mildew is one of the most recognizable plant diseases. It forms white, powdery patches on the leaves of infected plants. Powdery mildew seldom causes significant damage to host plants, though severe infections can reduce their ornamental value.
Powdery mildew infects thousands of plant species. The fungi that cause powdery mildew generally target specific hosts, and only infect plants from the same genus or family. Only a few species of powdery mildew fungi are capable of initiating infections in multiple plants. Many plants are susceptible to several different species of powdery mildew fungi. Trees and shrubs that are vulnerable to infection include apple, ash, azalea, barberry, birch, buckeye, catalpa, cotoneaster, crabapple, dogwood, elm, euonymus, hawthorn, ironwood, lilac, linden, magnolia, maple, oak, serviceberry, spirea, rhododendron, rose, and viburnum. Powdery mildew infections also occur on many different flowers, including African violets, begonias, English ivy, lilacs, kalanchoe, photinia, roses, snapdragons, and zinnias. Powdery mildew infections are common on many crops. Beans, cucumbers, pumpkins, tomatoes, squash, and zucchinia are especially prone to infection.
Symptoms of Infection
Powdery mildew infections are distinctive. Infected plants display white powdery patches on the leaves and stems. Lower leaves tend to be the most affected. As the disease progresses, the patches enlarge. An abundance of asexual spores are formed, and the mildew may appear on the rest of the plant. The first visible symptoms are raised blister-like areas on young leaves that cause them to curl, exposing the lower leaf surface. Infected leaves become covered with a white to gray powdery growth, usually on the upper surfaces. On plants that produce flowers, flower buds may become laden with mildew, and never blossom. The leaves of severely infected plants turn brown, and drop prematurely. Powdery mildew prefers young, succulent growth. Mature leaves are generally not affected.
- When planting, select resistant plants, and place them in sunny locations.
- Prune plants to promote sun exposure, and improve air circulation.
- Remove diseased foliage from infected plants. Disinfect pruning tools between each cut. Use a solution comprised of nine parts water and one part bleach.
- Maintain plant vigor through sound cultural practices. Ensure that plants are sufficiently watered, especially during extended periods of drought. Apply a layer of organic mulch around the base of plants to improve soil quality, moderate soil temperature, and retain soil moisture. Use a slow-release, organic fertilizer to feed crops. Avoid excessive nitrogen applications.
- Gently wash foliage occasionally to disrupt the spore-releasing cycle.
- Neem oil and PM wash can be applied on plants that are grown indoors. These will help prevent fungal infections.
- Chemical treatment of powdery mildew is possible. Fungicides are effective at combating the disease. Treat plants with a silicon solution. Silicon assists the plant’s cells in defending against fungal attacks by degrading fungal haustoria. Potassium bicarbonate and baking soda may also assist in treating powdery mildew infections.
- On select plants, such as summer squash, pumpkins, grapes, and roses, milk is an effective organic treatment. Milk is often applied to infected plants by home gardeners. Milk should be diluted with water, and sprayed on susceptible plants as a preventative measure, or at the first sign of infection. Repeated weekly applications can help control or eliminate the disease.
Sooty mold is a collective term applied to several species of dark fungi. The fungi grow on honeydew excreted by insects, or exudates from leaves of certain plants. Sooty mold growths are composed of fungal complexes consisting of ascomycetes, and fungi imperfecti. Some of the common genera include Aureobasidium, Antennamariella, Limacinula, and Capnodium.
Honeydew is a sweet, sticky liquid excreted by certain insects as they ingest large quantities of sap from a plant. These insects include aphids, leafhoppers, mealybuds, psyllids, soft scales, and whiteflies. Both the immature and adult stages of these insects feed by extracting sap from plants. They assimilate some of the nutrients contained in the sap, excreting the rest as honeydew. Insect honeydews contain sugars, amino acids, proteins, minerals and vitamins, all of which contribute to fungal growth.
Wherever honeydew is placed, sooty molds can become established. Honeydew can be found on leaves, branches, fruits, vegetables, concrete, sidewalks, and outdoor fixtures. It is commonly observed on the leaves of ornamental plants such as azaleas, lilacs, gardenias, camellias, and laurels. It may also be found on plants located under pecan, maple or hickory trees. Occasionally, citrus plants may exude sticky secretions, upon which sooty mold may form.
Symptoms of Infection
Sooty molds grow on surfaces where honeydew deposits accumulate. Coatings of sooty mold reduce or block sunlight penetration, making photosynthesis less efficient. Without adequate sunlight, plant growth is stunted, causing leaves to wither and drop prematurely. Sooty mold growths can also develop on outdoor structures and furniture. Large masses of sooty mold create an unseemly appearance, and are often difficult to remove. Sooty molds have high allergenic potential, particularly the Cladosporium and Aureobasidium components found in sooty molds of the Eastern United States.
- Sooty mold can be managed by reducing populations of insects that excrete honeydew. Pressure washing can help dislodge insects from trees.
- An important biological consideration is ant management. Ants are attracted to, and use honeydew as a source of food. As such, they protect insects that produce honeydew from predators and parasites. If ants are eliminated, predators and parasites will become more prevalent. As their populations increase, they begin feeding on scale insects, aphids, psyllids, whiteflies, and mealybugs. Ant stakes and other baits can be placed under trees and shrubs to prevent ants from foraging. Sticky compounds are another effective deterrent; they may be placed around the base of trees.
- If insect populations fail to decline, horticultural oils, insecticide, fungicide, miticide, or insecticide soap can be applied to suppress insects; one or more applications may be required.
- Neem oil is an organic broad spectrum pesticide that can used to quash insect populations on house plants, flowers, vegetables, trees, shrubs, and fruit. It is a biodegradable substance, and has not been shown to be toxic to mammals, birds, bees, earthworms, or beneficial insects.
- Judicious pruning cuts should be applied to remove infested plant parts.
- Branches close to buildings or other access points should be trimmed back to prevent insects from invading the tree.
- Trees should be fertilized in late spring or early summer to maintain tree vigor.
- Ensure trees are sufficiently watered, especially during periods of extreme heat.
- Apply a layer of organic mulch around the base of trees to improve soil quality, moderate soil temperature, and maintain soil moisture.
- Outdoor furniture can be cleansed with water during periods of honeydew excretion, particularly during drought.
- Sooty mold fungi growth can be inhibited by preservatives used in treated wood.
- The following cleaning solution can be used to remove sooty mold from plastic or painted surfaces:
– 1/3 cup of powdered house detergent
– 2/3 cup of trisodium phosphate
– 1 quart of household liquid bleach
– 3 quarts of water
- Always wear rubber gloves when cleaning with this solution.
- A mixture of lukewarm water and mild soap can be used to remove sooty mold from fruits and vegetables. Fruits and vegetables covered with sooty mold remain edible.
Photo courtesy of Jeff Kubina CC-by-2.0