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

Phytophthora palmivora, Rhizoctonia sp. and two isolates of Fusarium spp. w ere identified and associated with the arracacha (Arracacia xanthorrhiza) c orm rot in pathogenicity tests conducted in vitro and in vivo. In vitro tes ts revealed that these fungi were pathogenic to the corm. Fungi caused lesi ons to the corm under greenhouse conditions. Three major symptoms were obse rved: necrotized roots; necrotized roots with a dry, brown, hard injury to the internal corm tissue; and wet lesions with brown shades. The highest in cidence of affected corms occurred with P. palmivora in combination with Rh izoctonia sp., and the lowest was observed in corms inoculated with Rhizoct onia sp. alone. The most severe symptoms were caused by P. palmivora alone and in combination with Rhizoctonia so. in vitro tests were conducted to ev aluate the efficacy of metalaxyl, etridiazole thiophanate-methyl, and foset yl-al against the isolated fungi at doses of 5 to 200 mg ai/kg. Among the f ungicides tested, etridiazole thiophanate-methyl was effective against all fungi associated with corm rot. Micelial growth of P. palmivora was complet ely inhibited with metalaxyl at all doses and by etridiazole thiophanate-me thyl with doses of 20 or more mg ai/kg.