EVALUATIONS OF STEROL-INHIBITING FUNGICIDES FOR CONTROL OF PHYMATOTRICHUM ROOT ROT OF COTTON.

Eight "sterol-inhibiting" fungicides were evaluated for in vitro activity against Phymatotrichum omnivorum, and all proved to be very active (EC-50 values ranged from 0.001-0.038 ug/ml). Propiconazol received the major emphasis of this research, which included evaluations of phytotoxicity, soil persistance, resistance development, systemic translocation in cotton, and field evaluations for control of Phymatotrichum Root Rot of cotton. Although propiconazol significantly inhibited mycelial growth of P. omnivorum (in vitro) at concentrations as low as 0.0001 ug/ml within 96 hr, hyphal growth from sclerotia was not affected at 0.1 ug/ml within the same time frame. However, after 168 hr, or if sclerotia were pre-germinated for 120 hr before exposure, growth inhibition was comparable to that of the mycelial cultures. It was speculated that the greater level of sterols in sclerotia masked the inhibition of sterol synthesis by propiconazol until these sterol reserves were depleted. After 5 exposures to propiconazol over a 5 mo period, mycelial growth inhibition was the same as that of mycelium with no previous exposure. This indicated that resistance development should not be an immediate concern. Greenhouse evaluations of phytotoxicity and soil persistance of propiconazol determined that this fungicide can severely stunt cotton if planted into treated soil. Using stunting of seedlings as a bioassy of the persistance of propiconazol in non-sterilized soil, it was found that propiconazol was still active 3-5 mo after the initial soil treatment. Through the application of C-14 labeled propiconazol to the leaves of 5 and 8-wk old cotton plants, it was determined that no more than 0.23% of the applied radioactivity was translocated to the roots. However, based upon these percentages, root weights of the plants, and the amount of propiconazol applied to each plant during a field application, the theoretical concentration in the roots would be 30-700X the in vitro concentration necessary to inhibit mycelial growth of P. omnivorum (EC-50 = 0.003-0.006 ug/ml). Field evaluations of propiconazol were conducted in 1982 and 1983, while etaconazol, triadimefon, triadimenol, imazalil, and XE-779 were evaluated in 1983. These evaluations determined that propiconazol, etaconazol, and triadimenol showed promise for control of this disease, while imazalil, triadimefon, and XE-779 did not. . . . (Author's abstract exceeds stipulated maxium length. Discontinued here with permission of author.) UM