Who doesn’t look for the easiest way around a problem?
Gardeners can move a plant that gets too much or too little sun. We can fertilize a nutrient deficient plant and correct soil that is too acid or alkaline. We can improve soil drainage and water holding capacity. These are cultural and physical fixes for problems, not cures for disease.
In cases of disease, prevention is by far the best method of control. Generally, plant diseases are bacterial, fungal or viral. It is easier to thwart the pathogens than cure a disease.
Bacterial diseases are associated with rotting plant tissue. External manifestations include cankers (dead or dying areas, usually on branches), leaf spots, scabs, wilts, bacterial ooze, water soaked lesions, leaf spot with a yellow halo and crown gall, for example. Wounds, insects and storms allow the introduction of bacteria into plants. Disease can spread quickly from plant to plant.
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About 85% of plant pathogens are fungal. Fungal disease parasitizes plants. Signs and symptoms include leaf or stem rust, sclerotinia (white mold), powdery mildew (whitish gray), downy mildew (yellowish), black spot, leaf spot, anthracnose, chlorosis, damping off and rot. Wind-blown spores and splashing water transmit fungus, so do insects.
Viral disease is most often transmitted by piercing and sucking insects. Some viruses weaken plants so they fail to thrive; other viruses can kill. Viruses enter plants through points of mechanical injury (including propagation) and puncture wounds from insects (aphids, whitefly, leafhoppers and thrips, for example). There are no known cures for plant viruses. In the case of a virus, you can’t see evidence of damage the way you can with bacteria and fungus, but symptoms like mosaic pattern on leaves, crinkled leaves, yellow leaves and stunting may be visible.
Signs and symptoms from disease-causing pathogens overlap. It is often difficult for a home gardener to diagnose a plant disease much less know what to do about it. Prevention is easier.
Good cultural practices are the best way to control disease. The more deterrents you employ the more success you’ll have.
Plant disease resistant varieties when available.
Plant on raised beds in areas with poorly drained soil.
Use proper plant spacing to allow for air circulation.
Use a balanced fertilizer. Excess nitrogen enhances foliage disease development.
Practice good sanitation. Remove diseased plants or plant parts to keep disease from spreading.
Weeds harbor insects that serve as hosts for a number of viruses. Eliminate summer weeds, and do not allow weed debris to overwinter in your garden.
Don’t overwater. Avoid overhead irrigation if possible. Wet leaves invite fungus.
The best time to water is in late morning after the dew has dried. That leaves time for irrigation water to dry off plants during the day.
If a plant becomes diseased remove and destroy infected plant parts. If disease has become systemic, remove and destroy the entire plant.
Clean and sanitize tools. It is critical to preventing recurrence and spread of disease.
Inspect plants regularly for problems and pests.
Transmission of plant disease is often connected to insect pests. IPM (Integrated Pest Management) begins with monitoring plants and, if necessary, taking early action.
If you must spray three good options are neem oil, horticultural oil, and insecticidal soap.
Neem is oil is an insecticide, fungicide and miticide that controls soft bodied insects at all stages—eggs, larvae and adult. It is also effective against many types of fungus. It is hazardous to bees only if they are directly exposed to the spray. Apply neem oil when bees not visiting target plants, usually early morning or evening.
Insecticidal soap kills soft bodied insects on contact. Use it on vegetables, flowers, fruits and houseplants. It is safe for use up to the day of harvest. Toxicity to honeybees and aquatic invertebrates is low.
Horticultural oil is an insecticide, miticide and fungicide. It smothers insects and interferes with a fungal pathogen’s ability to attach to a host plant. It is toxic to amphibians, fish and mollusks, also bees if exposed to direct treatment.
Always read information and follow all directions on your pesticide label.
Remember, that spraying with pesticide kills the good bugs, too.
Reach Debbie Menchek, a Clemson Master Gardener, at email@example.com.