Berks County Cooperative Extension

PLANT DISEASES

How disease affects plants.

"Sick as a dog" is a descriptive expression that has come to mean feeling quite ill. How about "sick as a silver maple?" As we all know, diseases attack plants as well as people (and dogs). A disease may affect a plant in the following ways:

Reducing or destroying the food reserves of perennials stored in roots and stems.
Reducing photosynthesis, the process by which a plant manufactures food.
Damaging roots.
Interrupting movement of water, nutrients and food within the plant structure.
Diverting food produced by the plant from plant growth to nourishing the disease that is afflicting the plant.
Inhibiting plant reproduction.

Symptoms of plant disease.

What are the plant world's equivalents of coughing, a runny nose, sore throat and a fever? Symptoms to look for in a plant suffering a disease include:

Wilted stems and leaves.
Cankers - isolated areas of dead bark and wood - on trees and shrubs.
Leaf distress, which may appear as spots, or dead areas along the edge of or between the veins of a leaf; in some cases, complete loss of leaves may occur.
Rotting of fruit.
Sooty mold, a black fungal growth that develops on trees and shrubs during the summer months. When insects infest a plant, their waste products (referred to as "honeydew") provide a fertile growing medium for the mold. If the insect infestation is brought under control, the sooty mold will begin to dissipate.
Blight - the rapid death of leaves and branches.
Rust, a fungal disease symptom that materializes as dry, powdery, reddish-orange spores.
Galls - abnormal growths on the plant.

Controlling plant diseases.

The war against plant diseases has been waged since the first hole was poked into the earth to accept a seed. It is possible to battle plant diseases by using three tactics. First tactic: monitor your plants. Keeping a close eye on plants allows one to detect pathogens (the agent that causes the disease) and to take prompt action to eradicate or control them. Second tactic: Prevent disease by purchasing disease-free plants, treating the soil to eliminate pathogens before planting and using appropriate chemicals at the opportune time to control disease. Third tactic: manage a disease by taking prompt action to control it or to lessen its severity or spread. Specific means of controlling diseases include:

Crop rotation. A plant that is susceptible to attack by a specific pathogen is referred to as a "host plant." Rotating crops removes the host plant so that while the pathogen may remain in the soil, it has no plant to attack. Since similar pathogens attack tomato, eggplant and potato, rotation of these plants is not recommended; tomato followed by corn, beans or lettuce is a desirable rotation.
Sanitation. Raking fallen leaves eliminates a home used by many pathogens to survive the winter. Other desirable sanitation practices include pruning infected twigs and branches and removing diseased plants from the garden or landscape. Gardeners may unwittingly spread a pathogen to healthy plants by pruning a diseased plant part and then using the pruning shears on a healthy plant. To avoid spreading disease in this manner, disinfect tools using a mild bleach solution after using them and before proceeding to the next plant.
Drip or trickle irrigation. Irrigation by the drip or trickle method makes moisture readily available to roots, while keeping foliage dry. Dry foliage discourages bacteria and fungi from developing on plants. Sprinkle irrigation, on the other hand, may encourage pathogen invasion by creating a wet environment on foliage and by splashing pathogens from the soil to plants or from plant to plant.
Resistant varieties of plants. A resistant plant is one which is does not readily become diseased. For example, it is possible to purchase varieties of tomato that bear the label, "VFN resistant." This indicates that the tomato variety is resistant to fusarium and verticillium wilt (two diseases that can wipe out an entire crop) and nematodes (tiny worms that attack plant roots).
Chemicals.The key to using chemicals to manage plant disease is to first determine what is causing the disease. To assist home gardeners and others to properly identify plant diseases, Penn State University professors have compiled an exhaustive series of Ornamental and Flower Disease Fact Sheets. Without knowing the precise disease that is attacking a plant, application of chemicals is worse than shooting in the dark, since the wrong chemical or improper application may damage or kill a plant. After the disease is ascertained, it is imperative to correctly apply the appropriate chemical at the proper time.

Source: The Penn State Master Gardener Manual
Document created: 15 December 1999; Revised: 3 March 2001