Pythium spp., Fusarium spp., Rhizoctonia solani
Pre-emergence damping-off is generally caused by the invasion of the host by the fungus prior to plant emergence from soil. This is due to conditions that inhibit or slow seed germination, while allowing the pathogen to grow. Post-emergence damping-off occurs on young seedlings at or near the soil line, although, Pythium spp. may infect at the roots or root hairs. The host tissue appears water-soaked and constricted, eventually leading to seedling collapse. Damping-off becomes less of a problem as the host plants mature. A hypocotyl or stem infection of older plants by Rhizoctonia solani may produce a canker. Infected stems may be somewhat smaller in diameter than normal, but tough and wiry; hence, the name “wirestem.” This disease is most problematic on slow-growing and deep-seeded plants.
Conditions for Disease Development
These fungi may be present in the soil for a long time, but will not generally affect plants until the right environmental conditions, such as wet soils and cool temperatures, are met. Disease damage is generally greater in soil with infected, non-decomposed plant debris.
Fumigate, manage irrigation water and rotate to non-hosts to help reduce inoculum levels. Sow fungicide-treated seed to help control these pathogens.