|Crop Knowledge Master|
Papaya Bunchy Top, MLO
|(Plant Disease Pathogen)|
(In:Wayne Nishijimas papaya compendium)
Papaya is the only known host of this pathogen.
Papaya bunchy top has been observed throughout much of the Caribbean region, from Cuba southward to Trinidad, since it was first reported in Puerto Rico in 1931. Although the occurrence of the disease appeared to be restricted to the Caribbean islands, recent observations suggest that it is present in Central America and the northern part of South America. The disease has not been reported in Hawaii. Bunchy top is a devastating disease and can severely limit commercial papaya production.
The first symptom of bunchy top is a faint mottling of the upper leaves. The laminae of infected leaves progressively become more chlorotic, especially in interveinal areas, and may eventually exhibit marginal necrosis. Leaves and petioles show reduced growth, and become rigid. Internodes become progressively shortened, and petioles tend to assume a more horizontal position. Oily appearing spots are often present in the upper parts of stems and in petioles. Apical growth ultimately ceases, which, with the shortening of internodes, imparts a "bunchy top" appearance to affected plants. With the cessation of apical growth and decline of plant vigor, the oldest leaves progressively abscise, leaving fewer, more stunted leaves at the top. Eventually, plants may be entirely killed; however, new shoots may sprout from the lower, non-affected portion of stems. If fruits are set on infected plants, their flavor may be bitter due to the disease.
Although bunchy top was originally thought to be caused by a virus, the disease is most likely caused by a mycoplasma-like organism (MLO). MLOs have been observed in the phloem of infected plants using transmission electron microscopy, and infected plants treated with tetracycline-based antibiotics exhibited a remission of symptoms.
Although the absence of latex flow from fresh puncture wounds in leaves, petioles, stems, and fruits was considered diagnostic for bunchy top, a more recent study indicated that this technique is unreliable. The presence of MLOs in plants with foliar symptoms of bunchy top and normal latex flow was confirmed by electron microscopy.
The DNA-binding fluorochrome, 4,6-diamidino-2-phenylindole (DAPI), can be used to stain MLOs in the phloem of numerous plant species, thus, making their presence detectable by epifluorescent microscopy. This technique has been used to detect MLO's in petiole and leaf vein tissue of papaya trees with bunchy top symptoms. The reliability of this method to diagnose bunchy top has not been established.
Two leafhoppers, Empoasca papayae Oman and E. stevensi Young, transmit the bunchy top agent, and the occurrence of the disease coincides largely with that of these vectors. Empoasca papayae has most frequently been associated with the disease, but in Trinidad, where E. papayae is not established, E. stevensi was found to be a vector. Empoasca papayae has rarely been found on other plant species and is the only leafhopper known to breed on papaya. Empoasca stevensi was originally described from specimens collected from papaya in Florida in 1940, but little is known about the ecology and distribution of this species. Interestingly, both bunchy top and E. papayae are not known to occur in Florida.
Single insects of both vector species can transmit the bunchy top MLO. The MLO also can be transmitted by grafting. Symptoms usually begin to appear 30 to 45 days after inoculation. Some papaya cultivars are more tolerant to the disease than others, but immunity is not known. The degree of tolerance affects the rate and severity of symptom expression. However, it is not known whether vector preference and the ability to transmit the bunchy top MLO are affected by cultivar differences.
Currently, the use of tolerant cultivars is the only practical means of control of bunchy top, but it has limited application. Where bunchy top occurs, tolerant cultivars can only be grown commercially in locations with low disease pressure. Disease pressure varies with geographic location, presumably due to insect vector ecology. Application of insecticide to reduce the incidence of leafhopper vectors may be beneficial.
In the past, the disease has been managed through a program involving leafhopper control throughout the productive life of the crop and the removal of sources of inoculum. The latter was attained by rouging infected trees or the topping of infected plants below the point where latex exudes after wounding. Axillary shoots that develop after topping are often free of the disease. However, the success of this program depended largely on the availability of a pesticide with long residual activity, namely DDT, which is no longer approved for use in many areas. A suitable replacement for DDT has not been identified.
Antibiotic therapy is an effective control measure from an experimental standpoint but has not been applied commercially. Drenching the soil around infected plants with chlortetracycline or tetracycline hydrochloride and drenching combined with root-dip treatments were both successful. The efficacy of foliar application or trunk injection have not been established.
Bird, J., and Adsuar, J. 1952. Viral nature of papaya bunchy top. J. Agr. Univ. Puerto Rico 36:5-11.
Haque, S. Q., and Parasram, S. 1973. Empoasca stevensi, a new vector of bunchy top disease of papaya. Plant Dis. Rep. 57:412-413.
Martorelli, L. F., and Adsuar, J. 1952. Insects associated with papaya virus diseases in the Antilles and Florida. J. Agr. Univ. Puerto Rico 36: 319-329.
Story, G. E., and Halliwell, R. S. 1969. Association of a mycoplasma-like organism with bunchy top disease of papaya. Phytopathology 59:1336-1337.
Webb, R. R., and Davis, M. J. 1987. Unreliability of latex-flow test for diagnosis of bunchy-top of papaya caused by a mycoplasmalike organism. Plant Dis. 71:192.