Responses and relationships among Fusarium species, sweet corn, and western spotted cucumber beetles.

摘要

Sweet corn (Zea mays L.) yields in the Willamette Valley of Oregon have been declining since the early 1990's. Studies were done to determine if there is a relationship between ear weight and several disease parameters including necrotic crowns or stalk nodes, nodal root rot, radicle root rot, and sub-crown internode rot. Regression analysis indicated that plants with more necrotic crown tissues have lower ear weights and crown necrosis was the best predictor of ear weight at harvest. Fungal isolations from crown tissues indicated that presence of Fusarium oxysporum was associated with more necrotic crowns and lower ear weights. Fusarium verticillioides was also associated with darker crowns and nodes. Fluid conductance tests indicated that plants with darker stalk nodes had a reduction in fluid passage through a 30-cm stalk section. Isolates of several species of Fusarium were examined for their ability to cause crown and stalk node rot in two field and two greenhouse trials. Seed kernel inoculation with F. oxysporum and F. verticillioides resulted in plants with darker crowns and lower ear weights, although differences were not always significant. Both of these fungi were recovered from crown tissues of the majority of mature corn plants sampled in pathogenicity trials. It was also observed that seed kernel inoculation with these Fusarium species resulted in plants having greater root damage attributed to the western spotted cucumber beetle (WSCB), Diabrotica undecimpunctata undecimpunctata. Studies were conducted to determine if the beetles show feeding or oviposition preferences when sweet corn plants were grown from kernels inoculated with Fusarium species. Generally, plants grown from Fusarium-treated kernels had significantly more leaf-feeding injury of corn seedling from adult WSCB compared to the disinfested control. Plants grown from Fusarium-treated kernels were also found to have increased larval root feeding in field pathogenicity trials. It appears that sweet corn ear yield reductions in the Willamette Valley is at least partly attributed to crown and stalk node rot incited by Fusarium species, and WSCB injury could be compounded with Fusarium injury, causing further stress of diseased sweet corn plants when these beetles are present.