First Report of Pythium aphanidermatum Causing Crown and Stem Rot on Opuntia ficus-indica

Opuntia ficus-indica (L.) Mill. (prickly pear cactus) is grown in semiarid regions of Baja California Sur (BCS), Mexico for human consumption and forage. Most of the produce is sold at local markets as a vegetable; however, there is an increasing demand from international markets. O. ficus-indica is propagated using individual cladodes, which are planted with half of the cladode covered with soil. Collapsed plants were observed in a commercial orchard near La Paz, BCS, during March 2000, and in an experimental plot at CIBNOR during August 2000. Disease incidence was 12% in the orchard and 27% at CIBNOR. The initial symptoms were soft, dark brown lesions on the cladode at the soil line. As the disease advanced, the lesions progressed along the soil line and to the upper part of the cladode. In some plants the infection reached upward to the next cladode. Root rot was observed in those cladode areas that were already rotting. Depending of the advance of the rot in the base cladode and the size of the plants, larger plants collapsed more rapidly than smaller plants, but eventually all plants with rot lesions collapsed. The organism consistently isolated from diseased cladodes of several varieties of O. ficus-indica produced inflated sporangia, intercalary antheridia, and oospores described for Pythium aphanidermatum Edson (Fitzp.) (2). To isolate the pathogen, cladodes with lesions were washed with detergent and brush, rinsed with tap water, and then the epidermis was covered with 95% ethanol and the ethanol burned. The epidermis was peeled away from the edge of the lesion and 1 square cm sections were aseptically removed. Tissue sections were plated out on potato-dextrose agar (PDA) plates. Pathogenicity studies were made twice in a greenhouse on a total of 16 potted O. ficus-indica plants with only one cladode. Inoculum was obtained from colonies growing on V8 agar for 7 days producing abundant oospores. Thirty milliliters of an oospore suspension (240 oospores per ml) and V8 agar plugs containing mycelia and oospores were applied next to the crown of the Opuntia plants. Initial symptoms were observed 2 days after inoculation and were similar to those observed on field-grown plants. All inoculated plants were dead 5 to 10 days after inoculation. P. aphanidermatum was re-isolated from diseased cladodes. Water-inoculated plants remained healthy throughout the experiments. P. aphanidermatum has been reported causing root rot in Opuntia sp. (1). This is the first report of P. aphanidermatum on O. ficus-indica. References: (1) S. A. Alfieri Jr. et al. Fl. Dept. Agric. Cons. Serv. Div. Plant Ind. Bull. 11 (rev.), 1984. (2) A. J. Van der Plaats-Niterink. Monograph of the genus Pythium. Studies Mycol. 21:1, 1981.