First Report of Phytophthora nicotianae Causing Leaf Blight, Fruit Rot, and Root Rot of Papaya in American Samoa

Abstract

Papaya trees (Carica papaya) were observed in 1997 with symptoms of acute chlorosis and wilting of foliage, circular whitish lesions with necrotic centers on fruits, and root and trunk rots that killed the diseased trees. Identical symptoms were observed on many trees in fields interplanted with other crops and in home gardens on two of the five islands of American Samoa. A Phytophthora sp. was consistently isolated on water agar from symptomatic stems, fruits, and roots. The fungus was grown in pure culture on V8 juice medium at 25°C under continuous fluorescent illumination, and was identified, on the basis of spherical to broadly ovoid (34 to 67 × 40 to 50 µm), intercalary to terminal sporangia, chlamydospores (20 × 40 µm), and uniform to uneven hyphae (5 to 7 µm wide), as Phytophthora nicotianae Breda de Haan (= P. parasitica Dastur) (1,2). Ten 4-week old papaya seedlings grown on pasteurized soil in 15-cm pots were inoculated with a 2-ml suspension of 100 sporangia per ml in sterile water applied to the soil surface. Six uninoculated plants were used as controls. Two mature fruit that had reached the climacteric point were inoculated with 200 µl of sterile water containing approximately 20 sporangia. Inoculum was placed inside a 1-cm-diameter rubber circle that had been attached to the surface of the fruit to prevent run-off. Two uninoculated fruits served as controls. Inoculations of seedlings and fruit were conducted three times. Wilting symptoms developed on inoculated plants in less than 5 days and plant death accompanied by crown rot occurred in 14 days. Inoculated fruit developed lesions covered with a whitish fungal growth within 10 days after inoculation. Plants and fruits exposed to sterile water were healthy. The pathogen was reisolated from the diseased plants and fruits and it was morphologically identical to the original isolate, confirming its role as the causative agent of the disease. The wide distribution of diseased plants over an estimated one-quarter of the area of the island of Tutuila (approximately 24 sq. km) and on the adjacent island of Ofu indicates that the disease was not of recent introduction. References: (1) H. H. Hob et al. The genus Phytophthora in Taiwan. Inst. Bot., Ac. Sinica, Monogr. Ser. 15., 1995. (2) G. M. Waterhouse and J. M. Waterhouse. C.M.I. Descriptions of Pathogenic Fungi and Bacteria No. 34, 1964.