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Disease Cycle and Epidemiology

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Epidemiology

Infection by A. euteiches can occur at any time during the growing season although it frequently occurs during the early phases of seedling emergence. The symptoms of ARR are sometimes difficult to distinguish from those caused by other root-infecting organisms such as Pythium, Rhizoctonia, and Fusarium. However, unlike Pythium, A. euteiches seldom causes seed rot or pre-emergence damping-off. The lesions caused by Rhizoctonia are sunken and cankerous whereas infection by Fusarium is distinguishable by the black or reddish discoloration of the vascular tissues, which can be readily observed in cross-section.

ARR is considered a monocyclic disease, which means only one infection cycle per season. Oospores within decaying host residue or in the soil germinate when chemical signals from host roots are detected. Germination can occur directly as a germ tube that ramifies by the formation of hyphae, or indirectly through the production of sporangia, which release primary spores and zoospores. Whether hyphae are derived from an oospore or from an encysted zoospore, infection can occur at any stage during the host’s development and within a large range of temperatures. However, infection and disease development are exacerbated when seedlings are infected and temperatures are optimal for host growth (22-28° C). Zoospores require water for movement to host roots. Standing water also predisposes host tissue to infection. Although water-saturated soils favor infection, symptoms and effects on yield are greatest if warm and dry soil conditions occur post-infection. Under optimal conditions, symptoms of disease are observed within 10 days following infection. Oospores form within necrotic tissues between 7 and 14 days. Oospores will either remain in host tissue or be released into the soil as roots decompose. Once formed, oospores remain dormant and can persist in the soil for more than 10 years, serving as primary inoculum when production of a host crop is resumed.

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