Organic farming, strictly defined, excludes the use of toxic and synthetic solutions for combating plant diseases caused by bacteria, fungi, nematodes, mycoplasmas, protozoans and viruses. Most diseases that afflict fruit trees are best controlled organically by preventing infection rather than treating the trees after they are damaged. Active infections are difficult to control organically.
There are varieties of fruit trees and rootstocks that resist certain diseases. Research diseases that are prevalent in your area or commonly afflict the kind of tree you want to grow, and select cultivars that are most likely remain free of disease. Some fruit tree varieties resist one type of disease but are susceptible to others.
Root rots and other fungal diseases are ordinarily caused by wet soil that is poorly drained. Pear and apple rootstocks can tolerate poorly drained soils. Plums are intolerant. Research the fruits you want to grow, and match the trees with your soil and typical rainfall. If your soil is not well drained, take steps to ensure that it is.
The Ashburner system developed in Australia suppresses disease in soils by using lime and cover crops combined with applications of chicken manure, straw and other organic materials. The Australians use this system to control root rot in avocado trees. An alternative approach is to use organic compost teeming with bacteria that can form a barrier around tree roots, helping combat infection by nematodes.
Preventing Fruit Rots
Fruit rots are common fungal and bacterial afflictions of soft fruit with high sugar content. These fruits need to be kept dry; therefore, make sure the trees get adequate sunlight and air flow. Prune trees to prevent crowded centers; this encourages the circulation of air and provides additional sunshine to the fruit.
Care and Maintenance
Healthy trees are better able to resist disease. Make sure your trees receive the correct nutrients and water. Some plants host diseases that can spread to your trees. Find out what they are and get rid of them. Clean up the debris in your orchard, especially diseased branches that have been removed from trees.
Oils, Dusts, Sprays and Soaps
Oil sprays help repel water that fungi need to grow. Vegetable and neem oils are easily biodegradable. Additionally, oils made from basil, clove, cumin, eucalyptus and mint have been used to combat fungi.
Sprays of garlic extracts have been shown to have antifungal properties. Sprays containing extracts of the plant horsetail, containing 15 to 40 percent natural silica, have been used to combat fungal disease.
A giant knotweed plant, Reynoutria sashalinensis, has been found to have antifungal properties. Dried knotweed extract is sold as a fungicide under the brandname Milsana.
Baking soda is cheap, readily available and can be effective against powdery mildew. Commercial soap solutions marketed for organic growers can help control black spot, canker, leaf spot, powdery mildew and rust.
Additionally, nurseries often sell cheap, nontoxic biodegradable antitranspirants that protect against fungal disease. The Israelis have recently developed Timorex, an organic distillate of tea tree oil, to combat powdery mildew.
Copper and Sulfur
A solution to fungal and bacterial disease that is accepted by some organic growers but scorned by others is the use of copper, sulfur or what is called the Bordeaux mixture. The Bordeaux mixture, which can be applied as a liquid or a dust, contains copper sulfate and lime used to control apple scab and leaf spot. The ingredients are naturally obtained but are toxic; the corrosive Bordeaux mixture can burn eyes, skin or the gastrointestinal tract if ingested.