Natural Defenses Against Disease
There may be hundreds of species of disease organisms in your garden soil or living in weeds in and around your yard. However, even though disease-causing organisms are nearly always present, that doesn't mean that your garden crops and ornamentals will develop a disease. Untold millions of potential diseases never amount to anything, because conditions aren't favorable for their development.
Plants have intriguing defenses that help protect them from infection. Leaves have a waxy coating called the cuticle that prevents them from staying wet, making it hard for disease-causing organisms to survive. The leaf cuticle may also prevent spore germination and slow the penetration of disease-causing organisms. Leaf hairs trap spores and hold them away from the surface of the leaf. Some leaf hairs actually secrete chemicals that prevent spores from germinating or sticky substances that help catch pathogens and/or the insects that transmit them. Leaves also exude substances that promote the growth of beneficial microorganisms that compete for space with, or are antagonistic to, pathogens.
Plants can also spring to their own defense when pathogens try to invade. For example, some plants can form a corky layer of tissue around the site of attack. Other plants may seal off the diseased part, which then dies along with the disease organisms.
Plants also have natural chemical defenses that repel or damage the pathogens themselves. Some of these defenses are present all the time and some are "turned on" when disease-causing organisms are present.
The discovery of these natural defense mechanisms (and more new discoveries are still being made) has been of great benefit to organic farmers and gardeners, because plant breeders can select for these mechanisms when they're working to develop disease-resistant cultivars. It also appears that products containing harpin and other plant growth promoters help to prevent disease by triggering plants to turn on their natural chemical defenses.
Most of the time, simple preventive controls will stop disease problems from developing or limit their severity. In a few instances, you may have to resort to spraying substances such as neem or sulfur to prevent a disease from ruining a plant or crop. These substances are considered organic because they derive from natural sources—however they are not innocuous. For example, sulfur can burn plant leaves, and spraying sulfur too often can cause soil imbalances that harm beneficial soil organisms.