Effect of Sodium Tetrathiocarbonate on Growth and Viability of Sclerotinia minor and S. sclerotiorum and Development of Lettuce Drop. M. E. Matheron, Extension Plant Pathologist, University of Arizona, Yuma Agricultural Center, Yuma 85364. J. C. Matejka, Research Specialist, University of Arizona, Yuma Agricultural Center, Yuma 85364. Plant Dis. 77:995-998. Accepted for publication 21 June 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0995.

Studies were initiated to determine the effect of sodium tetrathiocarbonate (STTC) on mycelial growth and viability of sclerotia of Sclerotinia minor and S. sclerotiorum and to evaluate the efficacy of this material for control of Sclerotinia leaf drop of lettuce. No growth of mycelia from agar disks containing either S. minor or S. sclerotiorum was apparent 72 hr after exposure to STTC at 122 µg/ml, but mycelia resumed growth when placed on potato-dextrose agar (PDA). Mycelia of S. minor and S. sclerotiorum were killed following exposure at rates of 245 and 1,225 μg/ml, respectively. There was a significant linear relationship between the concentration of STTC applied to soil in which sclerotia of S. minor or S. sclerotiorum were buried and the subsequent percentage of sclerotia that germinated. Only 3% of sclerotia of S. minor in soil treated with STTC at 3,675 μg/ml subsequently germinated when placed on PDA. In contrast, the germination rate for sclerotia of S. sclerotiorum treated with STTC at the same rate was 95%. For sclerotia of S. minor, a significant linear relationship was observed between the duration of exposure to STTC at 2,450 μg/ml and subsequent germination. Germination ranged from 75 to 5% after exposure to STTC for 8 and 96 hr, respectively. When compared with nontreated soil containing sclerotia of S. minor, a preplant drench with STTC at 3,675 μg/ml resulted in a significant increase in plant survival and fresh weight of leaf tissue of lettuce plants subsequently seeded and grown. These investigations demonstrate the potential utility of STTC for control of leaf drop of lettuce caused by S. minor.