EFFECT OF RESISTANCE TO STREPTOMYCES-IPOMOEA ON DISEASE, YIELD, AND DRY-MATTER PARTITIONING IN SWEET-POTATO

The effect of resistance to Streptomyces ipomoeae on disease, yield, and dry matter partitioning in two sweetpotato (Ipomoea batatas) cultivars commonly grown in North Carolina was evaluated in field studies conducted over 3 yr. Cultivar Jewel, which is susceptible to S. ipomoeae, and cultivar Beauregard, which is resistant, were planted in field plots that were not infested or infested with S. ipomoeae. The percentage of diseased storage roots produced per plot and the severity of disease on fibrous roots were greater on Jewel than on Beauregard in each of the 3 yr. Jewel plants produced significantly more storage roots and more diseased storage roots per plant than Beauregard plants. Marketable yield was not affected by disease because disease incidence on storage roots was less than 20%. Dry matter production at harvest was greater in both shoots and fibrous roots of Jewel than of Beauregard. Beauregard partitioned less dry matter to both shoots and fibrous roots over the season than Jewel. Disease did not affect dry matter partitioning to shoots or fibrous roots over time because incidence was low. However, disease significantly reduced dry weights of storage roots of Beauregard at harvest compared to noninoculated controls in 2 of 3 yr. Roots of Beauregard plants may have contacted less inoculum in soil because their fibrous root systems were smaller than those of Jewel plants. In addition, S. ipomoeae colonized fibrous roots of Beauregard less extensively than it colonized those of Jewel. These results may be useful in understanding resistance to S. ipomoeae in Beauregard. Further studies with more sweetpotato genotypes are needed to critically evaluate the role of fibrous root development in resistance to Streptomyces soil rot. Although disease incidence was low, the techniques described may be useful to plant breeders interested in selecting for resistance to Streptomyces soil rot when field nurseries with uniform natural inoculum are not available.