|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
Alternaria alternata has a wide host range, causing leaf spots and blights on many plant parts. This disease summary will discuss the affects Alternaria alternata has on papaya fruit.
Please see the Alternaria Primer summary if you want more general information.
Alternaria fruit spot was a major papaya fruit disease in orchards located in dry areas of Maui island in Hawaii. Since papayas are no longer grown in these areas the disease is seldom seen. This disease can be serious and has the potential for becoming a serious problem if papayas are grown in dry areas. This same fungus has been reported on papayas in India. Other Alternaria spp. have been reported on papayas in Florida, Australia and Brazil.
Alternaria fruit spots are characterized by depressed, circular to oval, lesions that eventually become black as a result of mass sporulation by the pathogen. Lesons are restricted to the surface of the fruit and do not cause extensive rotting of the flesh. However, lesions from multiple infection sites can coalesce as they expand and eventually cover the entire fruit surface.
The fungus is commonly found colonizing many kinds of dead or dying plant material. Conidiophores are brown, up to 50 x 3-6 um. Conidia are brown, smooth or verruclulose, muriform, short beaked, formed in long chains, and 20-60 x 9-18 um.
Alternaria fruit spot rarely develops on fruits that are kept or ripened at room temperatures after harvest. Fruits that are kept in cold storage, e.g. 10 C for 14 days, often results in more than 80% of the fruits infected with the fungus.
The pathogen readily infects senescing petioles and forms large numbers of conidia. Fruit surfaces typically have large numbers (13,700 to 36,900 conidia per fruit) at harvest. This disease appears to be restricted to drier environments where the other fruit deseases, that occur more commonly in wetter areas, are much less common.
A single postharvest water dip of 20 min. at 48 C will reduce the incidence of Alternaria fruit spot.
Chlorothalonil or mancozeb sprayed once every two weeks can reduce Alternaria fruit spot by about 50%. However, an orchard spray program alone does not provide adequate economic control necessary for surface shipment to export markets. The above water dip will help.
Alvarez, A.M., J.W. Hylin, and J.N. Ogata. 1977. Postharvest diseases of papaya reduced by biweekly orchard sprays. Plant Dis. Rep. 61:731-735.
Alvarez, A.M., and W.T. Nishijima. 1987. Postharvest diseases of papaya. Plant Dis. 71:681-686.
Saxena, R.M., and D.D. Sharma. 1981. Deterioration of papaya fruits by fungi. Agric. Sci. Digest 1:140-142.