Like human beings, plants are most vulnerable to diseases when they are malnourished or in an unhealthy environment. Naturally, plants are most capable of resisting plant diseases when they are healthy and well-fed.
In order to maintain healthy plants and to prevent plant diseases, gardeners should strive to constantly improve their soil’s organic matter content and to maintain a suitable environment for beneficial organisms.
Plant diseases are caused by living organisms. Many times, plants’ leaves may turn yellow or fruits may develop spots that appear to be diseases but are in reality nutrient deficiencies. These are not considered plant diseases because they are not caused by living organisms. However, it is important to consider such symptoms as potential causes of plant problems.
When trying to identify plant problems, gardeners should not immediately assume that they are caused by disease. Nutrient deficiencies, weather patterns, and pollution should be considered as more likely culprits. If these are ruled out, books on plant disease can be consulted to learn about individual plants and the specific plant diseases that afflict them.
The four main kinds of microorganisms that cause plant diseases are fungi, bacteria, viruses, and nematodes.
Parasitic fungi get their nutrients by feeding on host plants. Fungi are usually visible as dots on discolored parts of a plant.
Microbial fungicides, mixtures of beneficial fungi that destroy parasitic fungi, can be purchased for treating fungal infections. A garlic fungicide can be made by blending 5 garlic cloves with one pint of water and then straining. Sulfur is an option for soils severely infested with fungal diseases.
It must be emphasized, however, that all fungicides should be considered a last resort. The best way to prevent fungi and all plant diseases is to nurture soil by adding organic matter and by purchasing disease-resistant plant varieties when available.
Disease-causing bacteria cannot be seen with the naked eye, but their effects on a plant are usually clearly visible. These bacteria are very hard to control, but the best way to prevent them from spreading is by properly disposing of plant debris and always washing garden tools so as not to spread bacteria.
Viruses are extremely small plant diseases that can rarely be seen, even with a microscope. Viruses reproduce while inside plant cells. They can be spread through garden tools and on hands, as well as on aphids, insects, and nematodes.
Parasitic nematodes attach to plant roots and lay eggs on them. These parasites puncture plant cells and suck out their contents. Nematodes spread slowly on their own but can be easily spread by water and on garden tools. Nematode infestations can be prevented by adding nitrogen to soil.
The absolute best way to control all plant diseases is by maintaining well-nourished soil. Compost and other organic matter should be added to soil regularly. Organic fertilizers like seaweed extract and fish emulsion are also beneficial.
For soil that is severely diseased, there are several options. Gardeners can take soil samples to a diagnostic library, where they will receive a complete analysis of their soil and recommendations as to what should be done.
Another option for eliminating plant diseases is soil solarization. This involves covering the entire soil area with a sheet of black plastic for 4-6 weeks in mid summer. The heat from the sun will destroy soil borne insects and plant diseases. Afterwards, gardeners should amend the soil with organic matter to prevent plant diseases from arising in the future.