Bacterial diseases are controlled by resistant varieties, fungicides and cultural practices. This group of organisms is the most widespread and damaging to fruit and nut crops. Fungi survive on diseased plant material or on alternate crops. Disease problems are most severe during periods of high humidity or when the plant tissue is covered by a thin film of moisture. Temperatures between 70 o and 85 o F are favorable for most fungi.
Viruses are submicroscopic pathogens that increase in number once they are inside the host plant.
Viruses can be spread by insects, nematodes, seeds and infected propagating material and by mechanical methods. Individual virus cells can be observed only with the aid of an electron microscope. Symptoms can often be confused with those of plant mutations, nutrient deficiencies, toxicities or other pathogens. Virus diseases are controlled with the use of resistant varieties, rotations, and weed and insect controls. Plants infected by nematodes develop distinct symptoms based on the type of nematode parasitizing the plant.