The effects of herbicides on axenic growth of Cylindrocladium crotalariae and on the development of CBR in the greenhouse and in the field was studied. In laboratory tests, dinitramine at 1, 5, and 10 ug/ml increased axenic growth of two isolates of C. Crotalariae in herbicide-amended potato-dextrose broth (PDB). Dinoseb and dinoseb + naptalam at the same rates also increased growth, whereas alachlor, benefin, diphenamid, vernolate and 2,4-DB were found to have no consistent effect. Treatment of infested soil with dinitramine at 1, 5, 10, 50, and 100 ug/g soil had no significant effects on populations of C. crotalariae microsclerotia (ms) in a Ruston or a Woodstown loamy fine sand. Dinoseb reduced ms populations significantly (P=0.5) in Woodstown soil at 5, 10, 50, and 100 ug/g soil and in Ruston soil at rates of 50 and 100 ug/g soil. In greenhouse tests, soil treatment with dinitramine at 0.56 kg/ha but not at 0.84 kg/ha increased significantly (P=0.05) the severity of CBR in Ruston and in Woodstown soil. In Ruston soil, dinitramine increased the severity of CBR only at an inoculum density of 5 ms/g soil, but in CBR only at an inoculum density of 5 ms/g soil, but in Woodstown soil, the herbicide resulted in disease increase only at 50 ms/g soil. Additional tests showed that disease severity was increased when peanut seedlings were pretreated with dinitramine and transplanted to herbicide-free infested soil. Dinitramine treatment was postulated to alter growth and development processes in ‘Florigiant’ peanut in a way to result in increased susceptibility to CBR. Dinitramine treatment of soil did not increase disease severity in ‘NC3033’ peanut, a CBR-resistant cultivar. Dinoseb at 1.68 kg/ha increased significantly (P=0.05) the severity of CBR in Woodstown soil, but did not in Ruston soil. Higher rates of dinoseb (3.36 and 6.72 kg/ha) were found to have no effect. Field tests were conducted in microplots (77-cm diameter) to determine the effects of herbicides on CBR. The severity of pod rot caused C. crotalariae was increased significantly (P = 0.05) by pre-plant soil treatment with dinitramine at 0.56 kg/ha, but not at 0.84 kg/ha in 1979. Although not significant (P=0.05), substantial increases in root rot and top symptom severity occurred in 1979 in response to this treatment. Pre-plant soil treatments with dinoseb at 1.68, 3.36, and 6.72 kg/ha did not affect disease development in microplots in 1979. In 1980, dinitramine (0.56 kg/ha) increased the severity of both root and pod rot significantly (P=0.05) in two separate microplot tests. Dinoseb at 1.68, but not at 3.36 or 6.72 kg/ha also increased significantly the severity of CBR in these tests.. Soil assay results indicated that herbicide treatments had no consistent effect on populations of C. crotalariae ms, .Macroposthonia ornatum or Meloidogyne hapla in microplots. These results provide evidence that certain herbicides can affect the development and severity of CBR in peanut. Furthermore, the present study emphasizes that knowledge of the non-target effects of pesticides on the biosphere can be significant and must be considered in tbs development of effective disease control programs. / Ph. D.

Seed transmission of <i>Cylindrocladium parasiticum</i> was investigated as a possible explanation for the higher-than-expected incidence of Cylindrocladium black rot (CBR) of peanut in fields fumigated with metam sodium. Sixty-three commercial seed lots from Virginia production fields were examined for the presence of seed with speckled testae, a symptom of seed infection by <i>C. parasiticum</i>. Speckled seed was present in conditioned seed lots in 1998, 1999, and 2000 at an average rate of 1%. Soon after pods were harvested, the fungus was recovered from speckled seed at high frequencies. During winter seed storage, pathogen survival remained high in seed stored at -13 and 4 C, but declined in some seed stored at 15 C and ambient temperatures. Speckled and normal seed with and without fungicide treatment was planted in steam-treated soil in the greenhouse and metam-treated field plots in 1999 and 2000. Speckled seed treated with captan + pentachloronitrobenzene (pcnb) + carboxin transmitted the pathogen to emerging plants in the greenhouse and field. Transmission levels depended on the amount of viable inoculum present in seed at the time of planting. In the field, yield losses were significant when 20% or more of the seed planted was speckled. The low incidence of speckled seed in commercial seed lots would not result in a loss of yield for growers, but may contribute to disease spread. Testing of additional seed treatment fungicides indicated that fludioxonil, tebuconazole, and thiram may offer the best protection against disease spread from seed transmission of <i>C. parasiticum</i>. / Master of Science