For the past several years vegetable growers in Oregon's Willamette Valley
have experienced reduced yields in their sweet corn plantings. We conducted
studies to 1) describe the symptomology and etiology of the disease, 2) test a root
rot rating system we developed to evaluate factors in the disease syndrome, and 3)
evaluate the effect of soil applied herbicides on severity of root rot. Plant sampling
from commercial fields indicated root rot, not stalk rot, is the disease affecting
sweet corn plantings. Fumigation with methyl bromide and chioropicrin reduced
root rot severity 68-89% at harvest and increased yields by up to 50%, indicating
the primary cause of root rot is soilborne and biotic. Pathogenicity trials showed
several organisms cause root rot, and a rating system we developed for root rot
indicates symptoms differ among pathogens. Pythium arrhenomanes reduced dry
weight and caused limited necrosis of the radicle and nodal roots, but did not cause
rot of the mesocotyl. Phoma terrestris and Drechslera sp. caused rot of all three
components of the root system and reduced plant dry weight. Fusarium
graminearum primarily caused rot of the mesocotyl and reduced dry weight. F.
oxysporum did not cause root rot or reduce plant bio mass. The rating system also was used to evaluate the effect of four herbicides, two of which contain safeners,
and a safener alone on root rot of sweet corn grown in soil artificially infested with
Pythium arrhenomanes, Phoma terrestris, Drechslera sp., or Fusarium
graminearum. In soil infested with P. arrhenomanes, dry weight of sweet corn
seedlings was reduced when treated with the herbicide/safener combination EPTC
+ R-29 148. EPIC 4- R-29 148 also increased severity of rot of the nodal roots
caused by Phoma terrestris. In contrast, in soil infested with Drechslera sp.,
metolachlor increased severity of rot of the radicle and nodal roots and decreased
plant dry weight. Metolachior also reduced biomass of plants grown in soil
infested with F. graminearwn. Atrazine, metolachior + benoxacor and benoxacor
alone generally had no effect on plant dry weight or root rot of sweet corn seedlings
grown in soil infested with any of our pathogens. Our results indicate the effect of
soil applied herbicides on root rot of sweet corn is pathogen species dependent