Several bacteria cause serious problems on fruit plants. Fire blight of pear and apple, bacterial leaf spot and bacterial canker of peach and plum are three of the more frequently observed diseases in the home garden. Bacterial diseases are found in all areas, but they are generally more severe in areas of high to moderate rainfall. 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.