Interactions between lesion nematodes and corn pathogens
Root-lesion nematodes (Pratylenchus spp.) are migratory endoparasites of a variety of hosts worldwide. They are among the most common parasitic nematodes that feed on maize roots at all plant growth stages. Maize seedlings also are commonly attacked by pathogenic fungi and Oomycetes. The combination of nematode and fungus often results in a synergistic interaction wherein the crop loss is greater than expected from either pathogen alone or an additive effect of the two together. These interactions have been described from several crops, but lesion nematode-seedling pathogen interactions on maize (Zea mays L.) have not been intensively studied. Developments in seed treatment technology now offer new tools for the study and management of nematode-fungus interactions. The objectives of this study were to measure interactions between Pratylenchus penetrans and fungal/oomycete pathogens (Fusarium graminearum, Fusarium verticillioides, Pythium ultimum and Rhizoctonia solani) causing seedling diseases in maize; assess the impact of nematode control with abamectin on these interactions and evaluate added benefit of abamectin combined with fungicide seed treatment for seedling disease management. Pythium ultimum, Fusarium graminearum and Fusarium verticillioides experiments were conducted twice each in a growth chamber and Rhizoctonia solani experiments were conducted twice in the greenhouse at the Iowa State University, Ames, Iowa. Experiments were conducted in 150 ml pots that were filled with an autoclaved sand-soil mixture combined with inoculum of Fusarium graminearum, Fusarium verticillioides, Pythium ultimum or Rhizoctonia solani (colonized corn meal/sand mixture). A suspension of 4000 P. penetrans (adults and juveniles) was added to the pots at the time of planting. A factorial experimental design was used including 8 seed treatments x 4 pathogen combinations x 6 replicates. Four replicates of each treatment were harvested 30 days after planting. Shoot lengths, fresh and dry shoot and root weights, were determined. Digital images of the root systems were recorded with a flatbed scanner and image analysis conducted with WinRhizo software (Regent Instruments Inc.); root length, volume, tips, branching, surface area, discoloration and diameter class distribution were determined. Two replicates were harvested 42 days after planting and nematodes from soil and roots were extracted and counted. The results demonstrated significant effects on root health with interactions between fungal or Oomycete pathogens and nematodes. Seed treatments showed efficacy against fungal and nematode inoculation, improving most measures of seedling health compared to the nontreated control; mainly those seed treatment combinations including abamectin or abamectin with thiabendazole. Root structure analysis from WinRhizo showed that seed treatment significantly improved root system characteristics such as root volume, root length, number of tips, forks, surface area, fine roots and reduced diseased root length and diseased root volume. Fungal inoculation had a stronger effect compared to nematode inoculation, although diseased root length and diseased root volume were significantly affected by nematode inoculation, and seed treatment combinations with abamectin significantly reduced diseased root length and volume when compared to the non-treated check. Abamectin in combination with commercial seed treatment fungicides significantly reduced lesion nematode infection of the maize root system. This study provides the first quantitative evidence of interactions between P. penetrans and maize seedling pathogens in relation to root and seedling health. Overall, seed treatments with abamectin in combination with fungicides, provided the best control of seedling disease symptoms and also nematode feeding.