The Effect of Fallow Periods on Common Root Rot of Wheat in Rio Grande do Sul, Brazil

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Abstract
In 1979, the incidence and intensity of common root rot of wheat in 17 fields in Rio Grande de Sul, Brazil, increased during the season from 31 and 9%, respectively, at growth stage 8-9, to 82 and 47% at growth stage 11.1-11.3. Much less disease developed in fields that were fallow for 3 or 4 yr or not previously cropped to wheat than in those cropped annually or after 1 or 2 yr fallow. Near plant maturity, the incidence and intensity averaged 68 and 25% in the fomer and 98 and 72% in the latter fields. The estimated yield loss in these groups of fields averaged 9.1 and 23.1%, respectively. The dominant pathogen isolated from underground parts of infected plants was Cochliobolus sativus. The relative abundance of its conidia in the upper 3 cm of soil sampled when wheat was maturing averaged 264 spores/g in fields cropped annually or after 1 or 2 yr fallow, compared to 114 spores/g in fields cropped less frequently.

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