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Diplodia Ear Rot: An Old Disease Reborn

Don White Don White
Professor
Department of Crop Sciences
(217) 333-1093
d-white@illinois.edu
  Dean Malvick Dean Malvick
Assistant Professor
Department of Crop Sciences
(217) 265-5166
dmalvick@illinois.edu

In the summer of 2000, Diplodia ear rot caused by Stenocarpella maydis (Diplodia maydis) was a serious problem in much of central Illinois, Indiana, and Ohio. The disease was considered to be the most important ear rot disease from the early 1900s until the late 1950s; however, it almost disappeared with the widespread use of fall plowing in the 1960s and 1970s. Corn is the only host for S. maydis, and it's a poor competetor with other fungi and bacteria found in soil. The fungus overwinters on previously diseased stalk and ear tissue that’s produced on stalks and ears are spread by windblown rain to the area between the stalk and ear shoot, where the fungus penetrates the ear shank or the base of the ear. We don't know how far the spores of S. maydis can be spread by windblown rain, but several hundred yards is very likely.

Diplodia ear rot with the white fungal mycelium between kernels

Figure 1. Diplodia ear rot with the white fungal mycelium between kernels.

The incidence and severity of Diplodia ear rot and stalk rot began to increase in Illinois in the late 1970s with the increased use of conservation tillage. It has taken a number of years of conservation tillage for S. maydis to become reestablished as an important stalk and ear pathogen in Illinois. For several years, Diplodia stalk rot has been common in most Illinois cornfields. Diplodia ear rot has been occurring in scattered locations, usually in conjunction with wet weather at flowering.

Diplodia ear rot was not uniform from field to field in the summer of 2000. This is likely due to the differences in Diplodia stalk rot on previous corn crops, planting dates, weather, and hybrid susceptibility. Greater damage occurred in fields where corn followed corn and stalk debris from a previous crop was on the soil surface. The disease also was common where corn followed soybeans and cornstalks from previous crops were present.

A wide range in susceptibility of corn hybrids to Diplodia ear rot was observed in the summer of 2000, and this has been documented in experiments at the University of Illinois. It's interesting to note that there were similar levels of susceptibility in hybrids grown 20 years ago, although the fungus was not prevalent at that time. We suggest that potential new hybrids be evaluated for susceptibility, and those with extreme a poor susceptibility should not be grown.

Fortunately, there have been no reports of problems with feeding grain damaged by Diplodia ear rot to allowed to remain on the soil surface. Spores swine or cattle. S. maydis has been reported to produce a mycotoxin that results in a nervous condition in cattle and sheep grazing on moldy ears. This disease syndrome is usually not fatal and has been reported know how far the spores of only in southern Africa. We don't know if the isolates of the fungus that occur in North America are capable of producing a toxin; however, it's likely that the fungus in Africa originated from North America.

Another potential problem with Diplodia ear rot is that the fungus may cause ear rot of inbred seed parents. Previous work in the 1930s and 1940s indicates that seed that appears to be normal but is infected with S. maydis may germinate, although the resulting plants can be stunted and low yielding.

It's very risky to predict which disease will be a problem in the future. If rain occurs at flowering time, we suspect that Diplodia ear rot will be present in 2001. If wet weather doesn’t occur at flowering time, the disease may not be common. This report was written in April, and by the time of Agronomy Day, the effect of S. maydis on seed quality and plant health during the summer of 2001 will be known.

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Department of Crop Sciences
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