HPIPM:Pythium Wilt Root Rot DB

Author: Howard F. Schwartz, David H. Gent, Gary D. Franc and Robert M. Harveson

Taxonomy

Domain	Eukarya
Kingdom	Chromista
Phylum	Oomycota
Class	Oomycetes
Order	Peronosporales
Family	Pythiaceae

Scientific Name

Pythium

Common Name

Pythium diseases

Identification and Life Cycle

Pythium root rot and wilt is caused by various fungus-like pathogens in the genus Pythium. The diseases are favored by high soil moisture and moderate to high temperature. Varietal susceptibility will also influence disease development. Pathogen survival and inoculum buildup are favored by soils with high organic matter and poor drainage. Susceptibility can be increased if roots are damaged during cultivation or by other soil related problems, such as nematode feeding. The fungus can be transported within and between fields by contaminated irrigation water. Occurrence is also favored by improper crop rotation. The pathogen survives pathogenically on weeds and in the soil as dormant oospores.

Plant Response and Damage

Pythium species may infect planted seeds prior to germination, germinating seedlings, young plants, or older plants during blossoming and pod formation. It is one of the pathogens capable of causing seed decay and seedling death. Initial root rot symptoms appear as elongated water-soaked areas on the hypocotyl and roots, within one to three weeks after planting. The pathogen will extensively prune roots, reduce overall plant growth, and destroy much of the hypocotyl and main root system. The water-soaked region may extend several inches above the soil line, with little, if any, visible evidence of the fungus. The water-soaked area eventually dries out, becomes somewhat sunken, and tan to brown in color. Plants then wilt and die. Disease development later in the season results when pods in contact with the soil become infected. These pods exhibit a watery soft rot, or a mass of white fungal growth, but black sclerotia do not form as with white mold.

Biological Control

Deny (Burkholderia cepacia) is registered for protection of dry bean roots from Fusarium, Rhizoctonia, and Pythium spp. T-22 Planter Box (Trichoderma harzianum strain KRL-AG2) is registered, but cannot by used in alkaline or cold soils. The efficacy of these biological control agents in the High Plains is not known.

Cultural Control

Plant high quality seed in warm (greater than 60°F), well-prepared seedbeds under conditions favorable to rapid seedling emergence; shallow planting can encourage rapid emergence. Avoid excessive irrigation and poor drainage. Planting into raised beds can help avoid water- logging and promote more rapid germination. Crop rotation with non-hosts (i.e., small grains) may provide some reduction in damping-off pathogens, but damping-off and seedling blight pathogens have very broad host ranges and can attack most plants. Conventional tillage tends to reduce damping-off as compared to minimum or no-till operations. Where pod rot is a problem, irrigation intervals should be extended near the end of the season to allow the soil surface to dry, thus inhibiting fungus development.

Chemical Control

Chemical controls are most effective when combined with sound cultural practices.

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