|Crop Knowledge Master||Fungi|
|(Plant Disease Pathogen)|
Wayne Nishijima, Extension Plant Pathologist
Department of Plant Pathology
University of Hawaii at Hilo
The genus, Mycosphaerella, has a large host range. This summary only discusses its affects on papaya.
Dry rot of papaya is a common disease wherever papayas are grown commercially.
In India and Brazil, fruit surface lesions are most common while in Hawaii it is the most common cause of postharvest stem-end rot. It also is reported to cause spots on leaves, flowers and young fruit.
Fruit surface lesions are slightly sunken, circular, black and up to 4 cm in diameter. Margins of lesions are light brown and translucent. The lesion surface dries and wrinkles with age and turns black as it becomes covered with hyphae and pycnidia of the imperfect stage. Under humid conditions it is common to see light-colored tendrils of pycnidiospores oozing from pycnidia. The infected tissue is dry, firm and is initially tan in color and eventually turns black.
Stem-end rot caused by this fungus takes on the characteristics similar to fruit surface lesions.
Infection of flowers and young fruit are initially brown then become dark and sunken. Lesions often extend through the peduncle to the succulent stem tissue where it too takes on the characteristic dark, sunken appearance.
Perithecia are dark brown to black, flask-shaped to oval and 100-180 x 70-200 µm. Asci are 29-53 x 7-13 µm, ascospores are straight to slightly curved, hyaline, septate, constricted in the middle and 8-15 x 3-5 µm. Conidia are oval with one or two cells 9-15 x 3-5 µm and are produced in pycnidia.
In Hawaii perithecia were not found on living leaf tissue but were abundant on dead leaves and petioles. Ascospores are discharged within 1 hr after the relative humidity reached 100% or after the start of rain.
Ascospores are capable of germinating within an hour on water agar and frequently reach 100% germination rate in 5 hr. Papaya latex is a suitable medium for ascospore germination, mycelial growth and pycnidial and perithecial production by the pathogen.
The fungus colonizes senescing leaves and petioles and produce abundant fruiting structures on dead leaves and petioles and serve as the primary inoculum in the field. Conidia and ascospores are deposited on the fruit surface during rainy episodes and remain until wounds occur nearby. Wounds created during harvest and postharvest handling are quickly colonized during storage if conditions for spore germination and hyphal growth exist. Since papayas are susceptible to chilling injury at temperatures below 7ûC, normal storage and shipping temperatures are suitable for spore germination and disease development. In Hawaii, typical incidence of postharvest stem-end rot caused by Mycosphaerella sp. in unsprayed fields is about 30 to 40% if fruits are not postharvest heat treated.
Phoma carica-papayae (Tarr.) Punith (previously referred to as Ascochyta carica Pat. and A. caricae-papayae Tarr.), is the imperfect stage of this pathogen.
Hot water treatment of 48ûC for 20 minutes is an effective method for preventing stem-end rots from occurring. Fruits treated within 40 hours after inoculation showed no signs of infection. The single hot-water treatment typically reduced stem-end rot caused by Mycosphaerella sp. to less than 4%. The double hot-water dip and the vapor heat quarantine treatments are also effective in controlling this disease.
Orchard sprays have been shown to decrease inoculum levels in the field. It is effective in reducing stem-end rots, caused by Mycosphaerella sp., by 24 to 43% when sprayed biweekly over a 50 week period.
Chau, K.F., and Alvarez, A.M. 1979. Role of Mycosphaerella ascospores in stem-end rot of papaya fruit. Phytopathology 69:500-503.
Chowdhury, S. 1950. A fruit rot of papaya (Carica papaya L.) caused by Ascochyta caricae Pat. Trans. Br. Mycol. Soc. 33:317-322.
Hunter, J.E., and Buddenhagen, I.W. 1972. Incidence, epidemiology and control of fruit diseases of papaya in Hawaii. Trop. Agric. (Trinidad) 49:61-71.
Punithalingam, E. 1980. A combination in Phoma for Ascochyta caricae-papayae. Trans. Br. Mycol. Soc. 75:340.
Ullasa, B.A., Sohi, H.S., and Ganapathy K.M. 1974. Ascochyta leaf spot of papaya and its perfect state. Indian J. Mycol. Plant Pathol. 4:218-219.