Pythium aphanidermatum Causing Collar Rot on Papaya in Baja California Sur, Mexico

Demand from international markets for organically grown papaya (Carica papaya L.) from Baja California Sur is increasing. Occasional rains during the summer of 2000 provided extra moisture to the soil in most papaya farms in the state. Collapsed plants of the Hawaiian type cv. Sunset were observed in 20 commercial orchards in Pescadero, Baja California Sur from July through September 2000. The average disease incidence per orchard was 2%, although there was one orchard with 25% of diseased plants. The initial symptoms were soft, watery lesions at the soil line. As collar rot progressed, foliage wilted. In plants with severe collar rotting, lesions girdled the stem, causing the foliage to be completely wilted and the plants to collapse. Root rot was not observed in plants with collar rot. To isolate the pathogen, 15-cm-long portions of the stem with rot lesions were excised, washed with soap and brush, and rinsed with tap water. Transverse sections of the stem were lightly sprayed with 95% ethanol, and the ethanol was ignited. The superficially burned tissue was removed aseptically, and 1-cm-square sections were cut from the remaining tissue. These sections were plated on potato dextrose agar. The fungus consistently isolated from disease stems grew optimally at 37°C, producing lobate sporangia, antheridia mostly intercalary, and aplerotic oospores characteristic of Pythium aphanidermatum (Edson) Fitzp. (3). Pathogenicity studies were conducted twice in a screened house on a total of 36 4 to 6 month-old Sunset papaya plants 85 to 100 cm tall. Two longitudinal wounds (0.5 cm long and 0.2 cm deep) were made on opposite sides at the base of the stem using sterile razor blades. Pathogen inoculum was obtained from 7-day-old V8 agar cultures. Thirty milliliters of an oospore suspension (200 oospores per ml) and V8 agar plugs containing mycelia and oospores were applied next to the crown of wounded and nonwounded plants. Initial symptoms were observed 3 days after inoculation and were similar to those observed on diseased plants in the field. Wounded, pathogen-inoculated plants were dead 6 days after inoculation. P. aphanidermatum was reisolated from diseased plants. Nonwounded pathogen-inoculated plants, wounded water-inoculated plants, and nonwounded water-inoculated plants remained healthy throughout the experiments. Pathogenicity experiments suggest that field grown papaya plants might be predisposed to infection by P. aphanidermatum due to mechanical damage to the base of the stem caused by abiotic factors such as wind driven sand. P. aphanidermatum has been reported to cause root rot in C. papaya in Tabasco, Mexico (2), and the United States (1). This is the first report of P. aphanidermatum causing collar rot on C. papaya in Baja California Sur. References: (1) Anonymous. 1960. Index of Plant Diseases in the United States. USDA. Handbook. No. 165. Washington, DC. (2) M. I. Saldaña et al. Rev. Mex. Fitopatol. 3:14, 1985. (3) A. J. Van der Plaats-Niterink. Studies Mycol. 21:1, 1981.