When plants are well nourished and well taken care of, they have a natural ability to resist disease. And since keeping plants healthy is at the heart of organic gardening, disease problems tend to be minor in organic gardens. Organic gardeners feed the soil regularly with compost and organic matter, which keeps disease organisms in check both by producing stronger plants and by encouraging beneficial soil organisms that actually fight against pathogens.
Factors other than diseases can produce disease-like symptoms, and these problems are called plant disorders. Extreme weather conditions or soil imbalances may be the cause, and, in fact, people are often indirectly the cause of common plant disorders such as salt damage or ozone damage. Choosing plants that are well suited to the site is the best way to prevent many frustrating, costly, and sometimes fatal plant disorders.
Figuring Out What's Wrong
If a plant in your garden has, say, leaves that are turning yellow or black spots on its fruit, how can you tell what's causing the problem—an insect, a disease, a nutrient imbalance, or something else?
Start by ruling out insect damage and cultural problems. Inspect the plant carefully, using a magnifying lens. If it's an insect problem, you can usually find some evidence of the insects on the plants. Review the symptoms of nutrient deficiencies and consider recent unusual weather patterns and nearby sources of pollution.
If you can rule out insects and cultural problems, then you'll need to consider diseases or other kinds of disorders. Become familiar with the general types of plant diseases and the common disease problems in your area. Or you can consult books on plant diseases and garden problem solving, which often are organized by plant and list the diseases that each can get.
A Who's Who of Plant Pathogens
Microorganisms that can cause diseases include fungi, bacteria, viruses, and pathogenic nematodes.
Fungi: Fungi are primitive plants. They don't have chlorophyll, the pigment that allows green plants to convert sunlight and air into food. Instead, fungi obtain nutrients by inserting special rootlike structures (called haustoria) into host plants or dead organic matter. Many fungi live on and decompose dead organic materials. These beneficial fungi are an important ally in the garden. Parasitic fungi, on the other hand, are a leading cause of plant disease. Some attack only one species of plants, while others attack a wide array of plants.
Fungi produce tiny spores that are spread by wind, water, insects, and gardeners. Spores germinate to form mycelia—the body of the fungus. Mycelia rarely survive winter, but spores easily survive from season to season.
It's often easy to spot signs of fungi—mushrooms are the most common example. Often the structures that produce fungal spores look like dots or discolored areas on leaves, stems, or fruit. These structures are one of the best ways to distinguish fungal diseases from other plant problems.
Bacteria: Most bacteria are beneficial—they help to break down dead organic matter. Some, however, cause plant diseases. Bacteria usually reproduce asexually—the cells simply split in half. Bacteria spread via wind, water, insects, garden tools, and gardeners' hands. Bacterial diseases are usually more difficult to control than fungal diseases, and they spread more quickly than other types of diseases. You need a microscope to see actual bacterial cells, but some of the disease symptoms they cause, such as dark streaks on leaves or stems and bacterial slime are easy to see with the naked eye. (The slime often smells bad, too.)
Nematodes: Nematodes swim freely in the film of moisture surrounding soil particles and plant roots. A few types are barely visible to the naked eye. Many nematodes do not cause plant diseases and are important members of the soil community. In fact, beneficial nematodes often prey on the nematodes that attack plants.
Parasitic nematodes can be very destructive. They lay eggs that hatch into tiny larvae. Larvae molt several times before maturing to adults. Nematodes puncture plant cell walls, inject saliva, and suck out the cell's contents. Some species move from plant to plant to feed; others attach themselves permanently to one root. They can travel short distances on their own but are spread through the garden by water and on tools or gardeners' hands.
Viruses: Viruses are so small, they're difficult to see even with a microscope. Viruses are not complete cells and must be inside living cells of a host in order to reproduce. The symptoms they cause vary widely from distorted growth to mottling of leaves to stunting. Viruses are transmitted by vegetative propagation, in seeds and pollen, and on tools and gardeners' hands. Viruses are also transmitted by aphids and other insects, mites, nematodes, and parasitic plants.