Introduction
Something is wrong with your plant. What’s the cause? You can begin to determine the cause of the problem by taking on the role of Sherlock Holmes – be a keen observer and ask many questions. Diagnosing plant problems is often a difficult task. There can be many different causes for a given symptom, not all of them related to insects or diseases. The health of a plant may be affected by soil nutrition and texture, weather conditions, quantity of light, other environmental and cultural conditions, and animals, including humans. Complicating this scenario is the fact that any two of the above factors can interact to give rise to a problem. For example, a prolonged period of drought may weaken plants so that they are more susceptible to pests; this is typically observed with boxwoods.
The most effective approach is to know what questions to ask to narrow down the possibilities. For example, you need to consider recent rainfall and the fertilization schedule if you notice brown, dry edges on the leaves of your plants since both dry weather and excess fertilizer can cause such damage. In another example, either excessively dry soil or waterlogged soil can cause the same plant reaction: wilt. Even insect damage can sometimes be confused with plant diseases caused by microorganisms. For an excellent guide on questions to ask see Virginia Tech Publication 426-714.
Best Management Practices
- Learn the diseases that are common to the plants growing in your landscape / garden
- Look for ways to improve the cultural conditions as a means to reduce plant disease (too little watering, too much watering, over fertilizing, etc.) Note: see the below listing under “Additional References” for an extensive listing of cultural practices that can reduce disease and pests
- Monitor plants in the landscape frequently so problems can be observed early
- Plant crops and varieties that are well-suited to the soil and climate and recommended by Virginia Cooperative Extension.
- When seeding directly, use disease-free, certified seed, if available.
- Select for maximum insect- and disease-resistance in vegetable varieties.
- Select healthy, sturdy transplants with well-developed root systems. Diseases and insects in young seedlings may start in greenhouses or plant beds and cause heavy losses in the garden.
- Buy plants from a reputable grower who can assure you that they are disease- and insect-free, or grow your own from seed.
- Often hand picking or pruning diseased portions of the plant is the most effective solution
- Provide a sample of the diseased plant to the Help Desk for diagnosis Mondays and Tuesdays are the best days to bring in samples. Hours are 8 am – 5pm at 8033 Ashton Ave. Suite 105 Manassas, 20109. To save time it is recommended that one call or email before bringing in a plant or insect. (Call 703-792-7747 or email mastergardener@pwcgov.org)
- If a disease problem requires chemical controls, contact the Help Desk for more information. Use the least toxic materials according to the label. A certified nurseryman or Extension agent can help you identify the proper and legal pesticide and the method to use it.
References
Diagnosing Plant Problems, Virginia Tech Publication 426-714.
Plant Disease Diagnostic Form, Virginia Tech Publication 450-097.
Additional Resources
Plant Diseases, Virginia Tech Publications and Educational Resources provides numerous links to detailed plant diseases and problems.
Improving Cultural Conditions to Minimize Disease (from above VT reference)
- Water in the morning so plants have time to dry before the cool evening. Drip irrigation systems prevent foliage from getting wet when watering.
- Use interplantings in the vegetable garden as opposed to solid plantings of a crop. This can slow the spread of diseases and insects, giving you more time to deal with them if they occur.
- Space plants properly and thin young vegetables to a proper stand. Overcrowding causes weak growth and reduces air movement, resulting in increased insect and disease problems.
- Keep down weeds and grass. They often harbor pests and compete for nutrients and water. Leaf and other organic mulches are extremely effective for weed control, as are inorganic weed mats, plastic, and other fabrics.
- Use a mulch to reduce soil splash, which brings soil and soil-borne diseases into contact with lower leaves.
- Rotate your garden plot, if you can. Do not grow the same kind of produce in the same place each year. Use related crops in one site only once every three or four years. Avoid mixing soils in areas by forming permanent raised beds with distinct borders.
- Avoid injury to plants. Broken limbs, cuts, bruises, cracks, and insect damage are often the site for infection by disease-causing organisms.
- Stay out of the garden when the plants are wet with rain or dew to prevent spreading diseases.
- Do not use tobacco products, such as cigarettes or cigars, when working in the vegetable garden. Tomato, pepper, and eggplant are susceptible to a mosaic virus disease common in tobacco and may be spread by your hands.
Remove and dispose of infected leaves from diseased plants as soon as you observe them. Remove severely diseased plants before they contaminate others. - Clean up crop refuse as soon as you are finished for the day.
- Sanitize stakes and wire cages prior to use with a light bleach solution.
- Keep old sacks, baskets, wooden stakes, decaying vegetables, and other rubbish, which may harbor insects and diseases, out of the garden.
- Staking tall flower and vegetable plants or planting them in wire cages prevents the blossoms or fruit from coming in contact with the soil.
- Time plantings in such a way that the majority of your crop will avoid the peak of insect infestations. For example, plant squash as early as possible to avoid borers, which lay eggs in July. Keep a record of the dates insect problems occur.
- Plant warm-weather crops after the soil has warmed to avoid problems with seed and root rots and to promote vigorous growth.
- Inspect plants for egg clusters, beetles, caterpillars, and other insects as often as possible. Hand-pick as many pests as you can. Avoid sprays until the population of insects has reached a critical threshold level.
- Where slugs are a problem, use approved baits and traps and try to create drier conditions. Heavy mulches may encourage slugs. Diatomaceous earth, crushed eggshells, and hydrated lime near plants may help deter slug activity.
- Enlist the aid of birds in your garden. Overall, they do more good than harm. Consider planting shrubs and trees with fruits that attract them. Keep in mind, however, if you attract wild birds, you will have to protect ripening fruit (and even some vegetables) by using bird netting or scare devices (aluminum pans banging in the breeze are fairly effective) if damage is noted
Some Specific Diseases and Solutions
There are numerous diseases that have evolved over the years to thrive on ornamental plants, vegetables, fruit trees, woody shrubs, lawn turf and virtually every botanical species. So do not be afraid to ask for assistance. While not complete, the following are diseases that are commonly diagnosed by our Help Desk volunteers:
Anthracnose: Lesions are usually quite large and follow the veins of the leaf. Trees may drop some affected foliage. These diseases normally occur on large trees and do no permanent damage – practice good sanitation by cleaning up fallen leaves. You may choose to apply a preventive fungicide treatments next spring. A certified pesticide applicator and certified arborist can make these treatments for you. Plants susceptible: ash, maple, sycamore and oak trees.
Azalea Leaf and Flower Gall: Commonly seen on azaleas especially those grown in shade or partial shade. It is caused by a fungus. Thick fleshy growths may be white or pinkish in color. Hand picking is the best option.
Fireblight: Caused by a bacteria. The most typical symptom is branch die back from the tip down typically resembling a shepherd’s crook. Affected branches usually start dying in mid-May or June. Prune out dead branches about a foot below the dead tissue. Sterilize pruning shears between cuts. Plants susceptible: cotoneaster, pyracantha, apples, pears and Bradford pears.
Juniper Tip Blight: Characterized by dead brancon h tips in juniper plantings. Two different fungi cause this blight. Small outbreaks can be pruned out. Fungicides are also an option. The timing of the treatments depends on the causal fungus. Blight caused by Phomopsis is treated in the spring. Blight caused by Kabatina is treated in the fall.
Oak Leaf Blister: The light green to yellow spots will turn dark brown or black later in the season. This disease is primarily cosmetic and requires no action.
Rose Mosaic: Frequently seen as yellow patterns on the foliage of rose bushes. Infected plants usually grow slower and have fewer flowers. There is no cure and the disease does not spread from plant to plant.
Slime Molds: (ex. ‘dog vomit fungus’): This is not an actual disease and appears in mulch beds. They are harmless, but do look ugly. Physically break up with a rake or hard stream of water.
Wet wood (Slime flux): Caused by a bacterial infection in the heartwood. Affected trees ooze a sticky, sour smelling liquid through the bark, and sometimes from old pruning cuts. There is no treatment for wet wood. However, trees can live for years with no visible affects. Plants susceptible: numerous hardwoods.