Disease Cycle and Epidemiology Oospores typically serve as long-term survival structures for Pythium spp., although sporangia and hyphal swellings may also allow survival between seasons in some instances. Survival structures remain dormant until stimulated to germinate by an exogenous source of nutrients, such as seed or root exudates. Germination can either be direct (filamentous hyphae) or indirect (zoospores) depending on the species, fungal structures present, and perhaps environmental conditions. Zoospores swim limited distances through water films in soil and encyst along roots near sites where leakage of nutrients is high, such as root tips or subepidermal “short cells”. Encysted zoospores germinate and produce hyphae, which infect and colonize the roots and cause roots to decay. Zoospores released by sporangia in infected tissues can act as secondary inoculum. Oospores also are produced in infected tissues (Fig. 12). Pythium spp. that infect onion are ubiquitous in agricultural soils. The occurrence of Pythium-induced onion diseases in the field almost always is a result of environmental conditions that favor disease development rather than of the spread of pathogenic Pythium spp. into noninfested fields. In greenhouses, however, where efforts are made to follow sanitary production practices, secondary spread of Pythium spp. from an initial infestation may be important. Pythium-induced diseases of onion are most severe under conditions of high levels of soil moisture and soil temperatures below 18°C. In studies conducted to date, Pythium spp. that readily produce zoospores were most commonly isolated from onion roots during periods of high rainfall, while species producing propagules that germinate directly were more common during periods of moderate rainfall.
Spores can be dispersed with anything that moves soil, such as equipment and boots. Active zoospores can also be dispersed in water, such as irrigation water, runoff from rain, or splashing water.
Management
Agricultural practices that minimize periods of excessive soil moisture are recommended for managing Pythium diseases of onion. Practices such as breaking up compacted soil horizons, land leveling, installing tile drains, or planting on raised beds are typically used to reduce the likelihood of seedling diseases and to eliminate significant root rot problems. Seed and soil treatments with fungicides also are recommended practices. Treating onion seeds provides protection against serious stand loss from Pythium-induced seedling diseases for very little cost and poses insignificant environmental and food-safety hazards. Use of fungicides to manage Pythium root rot of established onion plants is neither recommended nor necessary. Selected References Agrios, G. N. 2005. Plant Pathology, 5th ed. Academic Press, San Diego, CA. Bruckart, W. L., and Lorbeer, J. W. 1982. Pythium species pathogenic to onion seedlings grown in organic soils in New York. Phyto pathology 72:469- 475. McKeen, C. D. 1950. Preliminary studies on a Pythium root rot of spanish onion seedlings. Sci. Agric. 30:125-131. Robertson, G. I. 1976. Pythium species in market gardens and their pathogenicity to fourteen vegetable crops. N.Z. J. Agric. Res. 19:97-102. Shishkoff, N. 1989. Zoospore encystment pattern and germination on onion roots, and the colonization of hypodermal cells by Pythium coloratum. Can. J. Bot. 67:258-262.
Fig. 12. Disease cycle of damping-off and seed decay caused by a Pythium sp. (Reprinted, by permission from Elsevier, from Agrios, 2005)
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