Brent Van Kooten, Agronomy Sales
Last year, ISU pathologists started mapping the distribution of tar spot throughout Iowa. Signs of the disease were observed in 75 counties across the state, though none were found to be severe enough to cause loss. Experts are concerned, however, about how quickly tar spot spread across the state. It will be a disease to keep an eye out for this year.
Tar spot is a disease in corn caused by the fungus Phyllachora maydis, which produces small, raised, round to irregular-shaped black spots on the upper and lower leaf surfaces. These can occur singularly or in clusters with more severe infections, and cannot be scraped or wiped off the leaf.
Tar spot is most often found in fields that still have green leaves and other diseases – like northern corn leaf blight, southern rust, or grey leaf spot – present in the canopy.
What conditions are favorable for the development of tar spot?
The tar spot fungus prefers cooler temperatures in the 50 to 70 degree range. Extended leaf wetness, high humidity and frequent rainfall promotes disease development. Several areas with fields experiencing severe tar spot symptoms saw high amounts of rainfall in last June, whereas rainfall in July and August varied.
What is the impact of tar spot?
The impact of tar spot depends on how severe and early infection occurs. Last year, initial infections appear to have occurred in June, with secondary infections occurring in July. In some cases, symptoms were extremely severe by August, which may have resulted in premature plant death.
How effective are fungicides?
Fortunately, fungicide trials last year showed that most fungicides have efficacy against tar spot, but timing is crucial. The most effective applications were made when very little disease was visible. Because of the aggressive nature of the disease and hybrid susceptibility, you want to be proactive to prevent infection.
After infection occurs, tar spot lesions can take 14 to 30 days to form. Once the lesions develop, each one can house 1,000 spores, which can travel at least 250 feet to infect new plants when conditions are favorable. Fungicides applied during the vegetative stages or at tassel when the disease was just starting to appear proved to be the most worthwhile investments.
Later-season applications did help limit the severity of the disease, but were less likely to save enough yield to be profitable. Since corn residue appears to house the disease, burying residue with tillage could help lower the level of inoculum within an individual cornfield, but it won’t prevent the disease from infecting neighboring fields.
Why is timing important?
When leaves are severely infected with disease during grain fill, sugars may not be available, and plants may not be able to completely fill ears prior to black layer, resulting in an overall loss in kernel weight and yield. Also, photosynthesis is reduced because of a loss of leaf area, causing stalks to be cannibalized for sugars, which results in poor standability and lodging.
Many fields may see little to no yield loss because the disease came in late or symptoms did not develop to levels that affect yield. Yields in fields with tar spot may be reduced because of many other stress factors, such as other diseases like gray leaf spot, reduced fertility from loss of nutrients, or loss of stalk integrity.
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