Factors Affecting Symptom Appearance and Development of Phymatotrichum Root Rot of Cotton

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Abstract
Soil temperature and moisture, sclerotial placement and flowering were evaluated as factors affecting the initial appearance and subsequent development of Phymatotrichum root rot (PRR) [Phymatotrichum omnivorum] of cotton. In a greenhouse study, nonflowering cultivar of cotton, T-25, and a flowering cultivar, ''GP-3774'', which was defruited, both died from PRR. This indicated that flowering as such is not a prerequisite for development of PRR. Additional greenhouse studies showed that sclerotial placement and soil temperatures can affect the initial appearance of PRR symptoms. When soil temperature was maintained at 27.degree. C and sclerotial inoculum was placed at a depth of 5 cm, 50% of the test plants had died 40 days after emergence. When the inoculum was placed at 60 cm, a depth at which sclerotia are commonly found in the field, it took 21 additional days to reach the same disease level. Reducing soil temperature delayed plant death even more. No foliar disease symptoms developed until the soil temperature at the depth at which the inoculum was placed was above 22.degree. C. Soil moisture was the main factor affecting PRR development, and soil moisture levels between -12 to -16 bars reduced the rate of disease development in 1982 and 1983.