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Occurrence, Dissemination, and Survival of Plant Pathogens in Surface Irrigation Ponds in Southern Georgia. F. M. Shokes, Former graduate assistant, Department of Plant Pathology and Plant Genetics, University of Georgia, Athens 30602, Present address: Institute of Food and Agricultural Sciences, Agricultural Research and Education Center, P.O. Box 470, Quincy, FL 32351; S. M. McCarter, professor, Department of Plant Pathology and Plant Genetics, University of Georgia, Athens 30602. Phytopathology 69:510-516. Accepted for publication 28 November 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-510.

Eight surface irrigation ponds were surveyed for plant pathogens in 1976 and 1977 to determine whether water used to irrigate vegetable transplants in southern Georgia is a potential source of plant pathogens reinfesting recently fumigated fields. The sampling techniques were: (i) unfiltered samples from three depths at five or six locations taken with a water samples bottle, (ii) filtered debris collected after pumping 48378 L of water through microorganism filters, and (iii) bottom sediment samples taken with an Ekman dredge. Plant pathogenic fungi from samples included Pythium aphanidermatum, P. irregulare, 20 other Pythium spp., Phytophthora spp., Fusarium spp., and Rhizoctonia spp. These fungi were isolated through laboratory assays with general and selective media, baiting techniques, and growth chamber bioassays. One sample taken by filtering water from an irrigation line in the field yielded five Pythium spp., a Phytophthora sp., and 14 lance nematodes. In pathogenicity tests 27 isolates representing 13 Pythium spp. and 14 isolates representing 10 Pythium spp. and two Phytophthora spp. differed widely in virulence on cotton, cabbage, tomato, and pea. P. aphanidermatum was the most virulent pathogen, but P. irregulare was the most frequently isolated pathogen. In a survival study in a pond, P. aphanidermatum in infested ryegrass seed and as oospores was recoverable for 185 days after submersion. Isolates from surviving oospores, after overwintering in the pond, were pathogenic on the four hosts. Zoospores were not recovered after 12 days. Rhizoctonia solani in infested cotton stem sections was recoverable in declining numbers up to 96 days and as sclerotia up to 85 days but failed to overwinter in the pond. Sclerotia of a Sclerotium sp. (probably S. hydrophilum) were 92% viable after 133 days in the pond. Results indicate that the use of contaminated irrigation water may be a major factor limiting the success of general purpose fumigants in transplant production.

Additional keywords: fungal pathogens, reinfestation, Rhizoctonia zeae.