|Crop Knowledge Master||Fungi|
|wet fruit rot of papaya (Plant Disease Pathogen)|
Wayne Nishijima, Extension Plant Pathologist
Department of Plant Pathology
University of Hawaii at Hilo
This summary deals specifically with the host, papaya. However, there are a large number of Phomopsis spp. that infect a wide variety of other hosts.
This fungus occurs in Hawaii on papaya, but other Phomopsis spp. can be found worldwide.
Wet fruit rot in its early stages resembles Rhizopus watery soft rot. Its incidence is sporadic but damage can be extensive when it does occur. Wet fruit rot occurs most frequently as a stem-end rot but any part of the fruit can be affected. It is not known to infect other parts of the papaya plant except senescing petioles.
The infected area is slightly depressed, soft and translucent. The fungus is rapid growing causing lesions to expand very quickly with the infected area extending to the seed cavity. The cuticle over the infected area remains intact and develops a delicate, wrinkled pattern that is parallel to the leading edge of the lesion. Black, scattered pycnidia usually form in the infected area after about seven days. The infected tissue is soft, mushy and wet but does not usually leak liquids that is characteristic of Rhizopus watery soft rot. Under conditions of high humidity the infected area can become covered by white to gray mycelium.
The fast growing mycelium is initially white and gradually turns to a light gray color on potato-dextrose agar (PDA). Pycnidia are formed in PDA under continuous fluorescent light in about three weeks and sporulates about one to two weeks later. Conidia are of two types: §-conidia are hyaline, fusiform, unicellular, 6.4-8.0 x 2.7-3.1 µm; and §-conidia are 13.7-20.0 x 1-1.8 µm. These conidial dimensions differ from P. caricae-papayae, which has been reported from India causing a fruit and stem rot of papaya. Phomopsis caricae-papayae also differs from wet fruit rot in the symptoms it causes on papaya fruit (lesions dark brown to black, depressed, and cracked at advanced stages) and the lack of formation of the §-conidia by pycnidia produced on host tissue. The Phomopsis sp. that causes wet fruit rot readily produces §-conidia on infected fruit and petioles. No perfect stage is known to be associated with either fungi.
Phomopsis sp. is typically found sporulating abundantly on dried petioles that remain attached to the tree. During rainy periods, conidia are discharged and deposited on fruit surfaces. The fungus lacks cutinases and, therefore, the ability to penetrate through intact cuticle. It requires wounds such as the peduncle broken during harvesting, fruit fly oviposition puncture wounds, and abrasions on the fruit to infect fruit. The disease is rarely seen in the field on green fruits and is more usually seen on fruits that have fully ripened. Phomopsis sp., like Rhizopus stolonifer, is capable of utilizing anthracnose and Cercospora black spot lesions as courts of entry into ripe papaya fruits.
Although removing dead and senescing leaves and petioles from the field is not practical, they should be removed from the tree because they become heavily infected with Phomopsis sp. and other fungi as they senesce and die. This is best accomplished by periodically cutting petioles drooping below the horizontal about 30 cm from the stem and removing them after about a week later when the abscission zone forms but before the petiole stub has dried. Also, if left on the tree they can be an obstruction to the target when spraying and may cause mechanical injury to fruit as they are being harvested.
The prevention of mechanical wounds during and after harvest also is important. Postharvest hot water treatment for 20 minutes at 48ûC is also an effective control measure when used in conjunction with regular field preventive sprays.
Regular field sprays with protective fungicides reduce inoculum levels and prevent infection through wounds that might occur in the field.
Alvarez, A.M., and Nishijima, W.T. 1987. Postharvest diseases of papaya. Plant Dis. 71:681-686.
Dhinga, O.D., and Khare, M.N. 1971. A new fruit rot of papaya. Current Science, 40:612-613.
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. 1985. Phomopsis caricae-papayae. Descriptions of Pathogenic Fungi and Bacteria, No. 827. Commonwealth Mycol. Inst., Kew, Surrey, England, 2 pp.