|Crop Knowledge Master||Fungi|
|fruit spot of papaya (Plant Disease Pathogen)|
Wayne Nishijima, Extension Plant Pathologist
Department of Plant Pathology
University of Hawaii at Hilo
This summary focuses on the affect of this pathogen on papaya. This fungus is also known to infect a variety of hosts including the Genera: Antirrhinum, Callistephus, Capsicum, Chrysanthemum, Dianthus, Juncus, Lycopersicon, Salvia, Solanum, and Stokesia.
Stemphylium fruit spot of papaya was first observed in 1978 in Hawaii and is now widespread in the tropics.
Early symptoms of Stemphylium fruit spot are small, round, dark brown lesions. The lesions become sunken and develop reddish-brown to purple margins as they enlarge. A velvety, dark green spore mass forms in the lesion center. White to gray mycelia grow over the lesion in the advanced stages. Internally, the infected tissue is discolored from a reddish brown to a dark brown color, appears dry, and small air pockets may develop.
The fungus is also capable of causing a stem-end decay. Stem-end decay characteristics are similar to the fruit spot except the infection begins on the broken end surface or near the base of the peduncle and later spreads to the surrounding tissue. Internally the vascular tissue is more intensively pigmented than the surrounding parenchyma tissue.
S. lycopersici (syn.: S. floridanum) grows well on V-8 juice agar (VJA) in culture with optimal temperature for mycelial growth at 26ûC. Sporulation occurs on cultures under continuous fluorescent light from 10 to 14ûC. However, at 18 to 26ûC cultures produce conidiophores but require an additional 8 to 10 hr of darkness for spores to form. No sporulation occurs above 30ûC.
Conidia germinate in 4 hr, when incubated on VJA and water agar, but in 7 hr, at reduced percentages, in papaya latex. Conidia take longer than 24 hr to germinate in distilled water at room temperature. Conidia (72.5 - 45 x 27.5 - 15 µm) are light brown, minutely warted with two or three transverse septa. The medial transverse septum is most prominent.
Papaya fruits are apparently quite resistant to infection by S. lycopersici. Injury or stress such as mechanical wounds, heat treatment, and cold storage are known to increase susceptibility. Ripe fruits are more prone to infection than less ripe fruits. The fungus does not invade tissue beyond the wounded area in fruits inoculated and incubated at room temperature.
This disease is unusual on fruits harvested and kept at room temperature. Various hot and cold treatments that papayas go through to meet quarantine requirements and the long term storage and shipping are responsible for making the otherwise resistant papaya fruit susceptible.
The standard single hot water dip is effective in controlling this disease. At 48ûC a 20 min hot water exposure will kill more than 98% of the conidia. Avoid wounds, prolonged cold storage and immediately cool fruits to ambient after required heat treatment.
Regular field sprays with protective fungicides help to keep the inoculum level in the field low.
Chau, K.F., and Alvarez, A. M. 1983. Postharvest fruit rot of papaya caused by Stemphylium lycopersici. Plant Dis. 67:1279-1281.
Glazener, J.A., and Couey, M. H. 1984. Effect of postharvest treatments on Stemphylium rot of papaya. Plant Dis. 68:986-988.