EFFECTS OF FIBROUS ROOT INFECTION BY STREPTOMYCES IPOMOEA ON YIELD OF SWEETPOTATO.

Abstract

The effect of fibrous root infection by S. ipomoea on disease on storage roots and production of marketable yield in the susceptible sweetpotato cultivar `Jewel' was evaluated in field experiments in 3 years. Drip irrigation (main plots) reduced disease on fibrous roots in plots not treated with sulfur or fumigated, but did not significantly increase yields in any year. Sulfur (subplots) reduced the severity of the disease on fibrous roots in nonfumigated plots in 2 years, but reduced yields by 21-33% in 2 of 3 years. Fumigation of soil with Telone C-17 (sub-subplots) 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 and reduced the severity of disease on fibrous roots in all years. Only fumigation increased the yield of marketable storage roots by 68% and 19% in two of three years. 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 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.