EFFECTS OF IRRIGATION, SULFUR, AND FUMIGATION ON STREPTOMYCES SOIL ROT AND YIELD COMPONENTS IN SWEET-POTATO

The effect of fibrous root infection by Streptomyces ipomoea on disease on storage roots and production of marketable yield in the susceptible sweetpotato cultivar Jewel was evaluated in replicated field studies conducted over 3 yr. Levels of disease were manipulated by soil treatments with drip irrigation (main plots), reduction of soil pH with sulfur (subplots), and soil fumigation with Telone C-17 (sub-subplots). Sweetpotatoes were grown in a field with natural inoculum of S. ipomoea in 1 yr or were artificially infested in other years. Irrigation reduced disease on fibrous roots to the greatest extent in plots not treated with sulfur or fumigated and increased the number of storage roots produced per plant in all years. Irrigation reduced the number of diseased storage roots produced per plant in the low rainfall year of 1990 but did not significantly increase yields in any year. Although addition of sulfur reduced the severity of the disease on fibrous roots in nonfumigated plots in 1988 and 1990, it also reduced yields by 21-33% in 2 of 3 yr because fewer storage roots were produced per plant in sulfur-treated plots. Fumigation reduced the percentage of diseased storage roots produced per plot from 71, 8, and 22% in nonfumigated plots to 52, 3, and 6% in fumigated plots in 1988, 1989, and 1990, respectively. Fumigation also reduced the number of diseased storage roots produced per plant and the severity of disease on fibrous roots in each year. Only fumigation increased the yield of marketable storage roots by 68 and 19% in 2 of 3 yr. Path analysis demonstrated that the severity of disease on fibrous roots had an important direct effect on marketable yield and important indirect effects on marketable yield by affecting the percentage of diseased storage roots produced per plot, the number of diseased storage roots per plant, and the number of storage roots per plant. The severity of disease on fibrous roots was positively correlated with the percentage of diseased storage roots produced per plot (r = 0.84) and the number of diseased storage roots produced per plant (r = 0.64), and the severity of disease on fibrous roots was negatively correlated with the number of storage roots produced per plant (r = -0.66). Yield of marketable storage roots was negatively correlated with both the severity of disease on fibrous roots (r = -0.77) and the percentage of diseased storage roots produced per plot (r = -0.73). These data demonstrate the importance of fibrous root disease in this pathosystem. Management strategies that reduce disease on fibrous roots may ultimately lead to increased yield of storage roots.