The torrential rains that northern Iowa experienced last week may have favored several diseases of corn including crazy top, Physoderma brown spot and node rot, Goss’s wilt and bacterial stalk rot. While there are no in-season management practices for these diseases, correct diagnosis can help with disease management options in future years including choosing hybrids with resistance.
Crazy top
The most characteristic symptoms of crazy top are distorted tassels with prolific leaf-like growths, or stunted plants with numerous tillers. Crazy top is caused by a soilborne oomycete that produces swimming spores (zoospores) in flooded conditions. The zoospores infect the growing points of young plants.
Physoderma brown spot and node rot
This disease is most common when heavy rains occur when the corn is at growth stage V5 to V9. This articledescribes the characteristic symptoms of the two phases of the disease. Note that hybrids that are susceptible to Physoderma node rot rarely have Physoderma brown spot symptoms on the leaves; while hybrids that get Physoderma brown spot, rarely have Physoderma node rot.
Goss’s wilt
It’s been a few years since Goss’s wilt was prevalent in Iowa, however I still hear reports of it each year. While most hybrids have good resistance to the bacterium, there are some hybrids that are susceptible. Remember to look for dark “freckles” in the lesions, which become translucent when they are backlit.
Bacterial stalk rot
We rarely see bacterial stalk rot in Iowa but it’s possible to find this disease in areas where water sat, and where plants may have been wounded. This stalk rot often occurs mid-season. Infected plants may be scattered across the field and die prematurely. The stalk is soft, slimy and brown; and perhaps the most characteristic symptom – infected plants stink.
As mentioned earlier, there is no in-season treatment for these diseases, therefore fungicide applications are not efficacious.
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- Professor of Plant Pathology and Microbiology
- Extension Field Crops Pathologist