Effect of Stem Polyphenol Content on the Susceptibility to Foot Rot Disease in Sweetpotato [Ipomoea batatas (L.) Lam.]

Foot rot ( Diaporthe destruens ) of sweetpotato generally infects the stem of the plant and then spreads throughout the plant. In Japan, foot rot is prevalent in the main area of sweetpotato production, and there is a pressing need for the development of resistant cultivars. It is generally believed that polyphenols are involved in defense against stress. However, little is known about the relationship between polyphenols and the susceptibility to foot rot in sweetpotato. We thus analyzed the polyphenol of the disease-free healthy stems and investigated their relationship between susceptibility to the disease to establish basic techniques for selecting cultivars resistant to foot rot. Polyphenol was analyzed by the Folin–Ciocalteu method and high-performance liquid chromatography. Resistance tests were conducted using the direct inoculation method. We observed a correlation between the total polyphenol content and the degree of foot rot disease severity. Surprisingly, cultivars with a low stem polyphenol content (‘Tamaakane’) had lower susceptibility to foot rot, whereas the cultivars with high stem polyphenol content (‘Kokei No. 14’) had higher susceptibility to foot rot. In particular, high positive correlations were shown with several polyphenols, including 3,4-dicaffeoylquinic acid and chlorogenic acid. Furthermore, the results of in vitro experiments also suggested that foot rot may metabolize trace amounts of chlorogenic acid. This discovery has potential applications in the selection of resistant cultivars, contributing to the streamlining of disease resistance tests in breeding programs. It could ultimately shorten the breeding period and help overcome foot rot disease in sweetpotato.