Fungi pathogenic to the corm of arracacha (Arracada xanthorrhiza) in Puerto Rico

Abstract

Phytophthora palmivora, Rhizoctonia sp. and two isolates of Fusarium spp. were identified and associated with the arracacha (Arracada xanthorrhiza) corm rot in pathogenicity tests conducted in vitro and in vivo. In vitro tests revealed that these fungi were pathogenic to the corm. Fungi caused lesions to the corm under greenhouse conditions. Three major symptoms were observed: necrotized roots; necrotized roots with a dry, brown, hard injury to the internal corm tissue; and wet lesions with brown shades. The highest incidence of affected corms occurred with P. palmivora in combination with Rhizoctonia sp., and the lowest was observed in corms inoculated with Rhizoctonia sp. alone. The most severe symptoms were caused by P. palmivora alone and in combination with Rhizoctonia sp. In vitro tests were conducted to evaluate the efficacy of metalaxyl, etridiazofe thiophanate-methyl, and fosetyl-al against the isolated fungi at doses of 5 to 200 mg ai/kg. Among the fungicides tested, etridiazole thiophanate-methyl was effective against all fungi associated with corm rot. Micelial growth of P. palmivora was completely inhibited with metataxyl at all doses and by etridiazole thiophanate-methyl with doses of 20 or more mg ai/kg.