With the exception of tobacco mosaic virus, relatively few viruses or viroids are spread extensively in the field by contact between diseased and healthy leaves.

All viruses that spread within their host tissues (systemically) can be transmitted by grafting branches or buds from diseased plants on healthy plants. Natural grafting and transmission are possible by root grafts and with parasitic dodder (Cuscuta species). Vegetative propagation often spreads plant viruses. Fifty to 60 viruses are transmitted in seed, and a few seed-borne viruses, such as sour-cherry yellows, are carried in pollen and transmitted by insects.

Most disease-causing viruses are carried and transmitted naturally by insects and mites, which are called vectors of the virus. The principal virus-carrying insects are about 200 species of aphids, which transmit mostly mosaic viruses, and more than 100 species of leafhoppers, which carry yellows-type viruses. Whiteflies, thrips, mealybugs, plant hoppers, grasshoppers, scales, and a few beetles also serve as vectors for certain viruses. Some viruses may persist for weeks or months and even duplicate themselves in their insect vectors; others are carried for less than an hour. Slugs, snails, birds, rabbits, and dogs also transmit a few viruses, but this is not common.

A small number of plant viruses are soilborne. Viruses causing grape fanleaf, tobacco rattle, and tobacco and tomato ring spots, as well as several strawberry viruses, are spread by nematodes feeding externally (i.e., ectoparasitic) on plant roots. A few soilborne viruses may be spread by the swimming spores of primitive, soil-inhabiting pathogenic fungi, such as those causing big vein of lettuce, soilborne wheat mosaic, and tobacco necrosis.

Viruses often overwinter in biennial and perennial crops and weeds (plants that overwinter by means of roots and produce seed in their second year or during several years, respectively), in plant debris, and in insect vectors. Plants, once infected, normally remain so for life.

Control

After a plant is infected with a virus/viroid, little can be done to restore its health. Control is accomplished by several methods, such as growing resistant species and varieties of plants or obtaining virus-free seed, cuttings, or plants as a result of indexing and certification programs. Indexing is a procedure to determine the presence or absence of viruses not readily transmitted mechanically. Material from a “test” plant is grafted to an “indicator” plant that develops characteristic symptoms if affected by the viral disease in question. In addition, more drastic measures are sometimes followed, including destroying (roguing) infected crop and weed host plants and enforcing state and national quarantines or embargoes. Further control measures include controlling insect vectors by spraying plants with contact insecticides or fumigating soil to kill insects, nematodes, and other possible vectors. Growing valuable plants under fine cheesecloth or wire screening that excludes insect vectors also is done. Separation of new from virus-infected plantings of the same or closely related species is sometimes effective, and the simple practice of not propagating from plants suspected or known to harbour a virus also reduces loss.

Infected peach, apple, and rose budwood stock and carnations have been grown for weeks or months at temperatures about 37 to 38 °C (99 to 100 °F) to free new growth from viruses. Soaking some woody plant parts or virus-infected sugarcane shoots in hot water at about 50 °C (120 °F) for short periods also is effective. Both dry and wet heat treatments are based on the sensitivity of certain viruses to high temperatures. Rapidly growing dahlia and chrysanthemum sprouts outgrow viruses so that stem tips can be used to propagate healthy plants. With certain carnations, chrysanthemums, and potatoes, a few cells from the growing tip have been grown under sterile conditions in tissue culture; from these, whole plants have been developed free from viruses.

Examples of virus and viroid diseases are characterized in the table.

General characteristics

Bacteria are prokaryotic microorganisms—i.e., single-celled microorganisms in which the nuclear substance is not enclosed in a membrane. There are two major types of bacteria, the eubacteria and the archaebacteria, and they are distinguished by differences in the composition of their cell wall and cytoplasmic membrane and by certain metabolic features. Plant pathogens belong to the eubacteria. The eubacteria can be divided into three groups: gram-negative bacteria, gram-positive bacteria, and the mycoplasmas and spiroplasmas, referred to as mycoplasma-like organisms (MLOs). Gram-negative and gram-positive bacteria are distinguished on the basis of their cell wall structure, which affects the ability of the bacterium to react to the Gram stain—one of the most useful stains in bacteriologic laboratories. The distinguishing characteristic of MLOs is their lack of a cell wall; their outer boundary is instead a cytoplasmic membrane, which imparts some unusual properties not found in most eubacteria. MLOs belong to the taxonomic class Mollicutes. Plant diseases caused by MLOs are grouped as agents of “decline” (characterized by loss of vigour, decrease in yield of fruit, and eventual death) and agents of virescence (the greening of flowers) and developmental abnormalities.

The principal genera of plant pathogenic bacteria are Agrobacterium, Clavibacter, Erwinia, Pseudomonas, Xanthomonas, Streptomyces, and Xylella. With the exception of Streptomyces species, all are small, single, rod-shaped cells approximately 0.5 to 1.0 micrometre (0.00002 to 0.00004 inch) in width and 1.0 to 3.5 micrometres in length. Streptomycetes develop branched mycelia (narrow, threadlike growth) with curled chains of conidia (spores) on the tips of the mycelia. Streptomyces are gram-positive; most species of the other genera are gram-negative.