Horticulture Principles and Practices

TABLE 7–2
Selected Bacterial Diseases of Horticultural Plants
Common Name Pathogen Plant (Host) Attacked
Bacterial wilt Erwinia tracheiphila Cucumber and other cucurbits
Bacterial soft rot Erwinia carotovora Vegetables, fruits, and tubers
Crown gall Agrobacterium tumifasciens Tree fruits and woody ornamentals
Bacterial canker Pseudomonas syringae Pitted fruits: cherry, peach, and plum
Common blight Xathomonas phaseoli Field bean, lima bean, and snap bean
TABLE 7–3
Selected Viral Diseases of Horticultural Plants; Viruses Are Given Descriptive
Names Based on the Diseases They Cause
Tobacco mosaic virus (TMV)
Tomato ring spot virus (TomRSV)
Tomato spotted wilt virus (TSWV)
Potato leaf roll virus (PLRV)
Potato virus Y (PVY)
Cucumber mosaic virus (CMV)
Bean common mosaic virus (BCMV)
Curly top virus of sugar beet (CTV)
Prunus necrotic ring spot virus (PNRV) (affects most stone fruits)
to thirty days (heat therapy) has been used to inactivate certain viruses. Actively growing
tips of plants are virus free and provide another effective means of producing diseasefree
plants from infected plants (see propagation of plants by tissue culture). Viruses are
often described according to the species they infect (host specificity) (e.g., lettuce mosaic
virus, maize dwarf mosaic virus, and sugar beet curly top virus). They are also described
according to serological properties, particle morphology, mode or modes of transmission,
morphology, and nucleic acid type (DNA or RNA).
Common symptoms of viral infection are yellowing of leaves, loss of vigor, poor
growth, and stunting. Viruses are primarily systemic in the host plant’s vascular fluids
and hence transmitted readily by sucking insects. They commonly remain indefinitely in
biennial or perennial hosts for as long as the host remains alive (except tobacco mosaic
virus [TMV], which can thrive on dead plant tissue). To control viruses, one may use
resistant cultivars, control the vectors, or limit their transmission in vegetative plants or
their parts. Some viruses are transmitted by seed. Sometimes infected plants may show
no symptoms (symptomless).
7.5.4 MYCOPLASMA-LIKE ORGANISMS
Mycoplasmas, which occur in the phloem of plants, are parasites that are intermediate
in size between viruses and bacteria. Like viruses, mycoplasmas are technically not
animals. Some of the symptoms associated with mycoplasma-like infections include
yellowing, stunting, wilting, and distortions, as in viral infections. Aster yellows have
been identified in, for example, carrot, strawberry, lettuce, phlox, and tomato.
232 Chapter 7 Biological Enemies of Horticultural Plants
7.6 OTHER PESTS OF HORTICULTURAL PLANTS
Apart from insects, fungi, bacteria, viruses, and mycoplasma-like organisms, horticultural
plants are plagued by other animal species. They may not cause diseases but can
nonetheless cause economic loss to growers. The major ones include small animals,
birds, nematodes, and snails and slugs.