Symptoms

The expression of bacterial wilt symptoms varies with different crop species. Cucumber and melon are severely affected by the disease. Individual runners or whole plants wilt and die rapidly (Fig. 1). Affected runners appear dark green at first and then become necrotic as the wilt becomes irreversible. Symptoms appear at all stages of plant development, but wilting is most severe early in the season, when the plants are growing rapidly.

Summer squash and pumpkin also develop bacterial wilt. Plants may wilt dramatically during the heat of the day but partially recover by morning. Foliage of affected plants often appears chlorotic. A distinct necrosis and chlorosis develops around leaf margins before the entire plant collapses and dies. Young pumpkin plants normally lose vigor and die within 2 weeks of the initial signs of wilt. Summer squash may remain vigorous and continue to produce for weeks after wilt symptoms appear.

A common diagnostic test for bacterial wilt involves cutting a wilted runner close to the crown of the plant, rejoining the cut surfaces for a moment, and then slowly drawing apart the cut ends (Fig. 2). The presence of bacterial slime (masses of bacteria streaming from xylem tissues) extending from one cut surface to the other is a positive indication of bacterial wilt. A variation of this test involves placing the cut runner or crown in water. Bacteria will ooze from the exposed vascular elements of infected plants in 5–10 min, forming milky strands flowing from the cut surface into the water.

Causal Organism

The causal agent of bacterial wilt is Erwinia tracheiphila (Smith) Bergey et al., a gram-negative, rod-shaped, motile bacterium with peritrichous flagella. It is transmitted by insect vectors, primarily the striped cucumber beetle (Acalymma vittatum (Fabricius)) and the spotted cucumber beetle (Diabrotica undecimpunctata howardi Barber). The western spotted cucumber beetle (D. undecimpunctata undecimpunctata Mannerheim), the banded cucumber beetle (D. balteata LeConte), and other insects that cause wounds, such as grasshoppers, may also transmit the bacteria.

Disease Cycle

Cucumber beetle vectors are responsible for bacterial dispersal within and between fields. The pathogen is transmitted mechanically by contact with contaminated beetle mouthparts; it does not reproduce within the insects. The adult beetles feed on stems and leaves. The bacteria multiply at the wound site, enter the xylem vessels, and then move down the petiole and stem. Vascular plugging by masses of bacteria and the subsequent formation of gums and resins are the primary mechanisms of wilting. The bacteria spread to unaffected runners through the dissolved walls of adjacent xylem vessels. Infected plants serve as the source of inoculum for subsequent infections.

The overseasoning of E. tracheiphila remains uncertain. Early theories, based on circumstantial evidence, held that it overwinters in the intestinal tract of adult cucumber beetles in fence rows and wooded areas. Subsequent histological investigations of the association of the pathogen and vector did not support this theory. More recent studies suggest that asymptomatic weed hosts may play a major role in the survival of the wilt bacterium. E. tracheiphila remains viable in dried plant debris for very short periods of time. In plant stems, the pathogen dies as the stems deteriorate. It is not seed-transmitted, and there is no evidence that it survives in soil.

Epidemiology

The incubation period in the field from infection to symptom expression ranges from several days to several weeks and is influenced primarily by the age of the plant and the point of inoculation. The rate of bacterial wilt in the field is most rapid while plants are young and succulent. Several factors contribute to a dramatic decrease in the rate of spread as plants mature: beetle populations naturally decline in midsummer; the older, more brittle leaves and stems are not preferred food sources; and bacteria introduced into new growth at vine tips must travel greater distances in the plant to induce wilting. Weather has little effect on wilt incidence, but it may influence the rate of symptom expression. Ideal growing conditions for crops, including warm weather, adequate soil moisture, plentiful sunlight, and balanced nutrient concentrations, also appear to favor the development of the disease.

Control

Control of bacterial wilt depends on control of the cucumber beetle vectors. In areas where the disease is a problem, the use of a systemic, soil-applied insecticide and well-timed foliar application of contact insecticides are prerequisite to profitable melon and cucumber production.

The future holds some promise for the development of wilt-resistant cucumber cultivars that limit the multiplication and movement of the bacteria within the host. No such resistance has been reported in melon.

Roguing wilted plants and the use of trap crops have been suggested to slow the progress of the disease, they are of no practical value unless appropriate insecticide programs are implemented. Bait formulations, which consist of volatile attractants, a feeding stimulant, and a small amount of insecticide, are being developed and show promise.

Selected References

Bassi, A., Jr. 1981. Epidemiological study of Erwinia tracheiphila with immunofluorescence technique. (Abstr.) Phytopathology 71:859.

Leach, J. G. 1964. Observations on cucumber beetles as vectors of cucurbit wilt. Phytopathology 54:606-607.

Main, C. E., and Walker, J. C. 1971. Physiological responses of susceptible and resistant cucumber to Erwinia tracheiphila. Phytopathology 61:518-522.

Rand, F. V., and Cash, L. C. 1920. Some insect relations of Bacillus tracheiphilus Erw. Sm. Phytopathology 10:133-140.

Rand, F. V., and Enlows, E. M. 1916. Transmission and control of bacterial wilt of cucurbits. J. Agric. Res. 6:417-434.

Wei, C. R., Walker, J. C., and Scheffer, R. P. 1952. Plant nutrition in relation to disease development. VII. Cucurbit wilts. Am. J. Bot. 39:245-249.