Weather Enhances Crop Diseases

 

Some crop diseases are intensifying and being enhanced by current weather conditions. Cool night time temperatures (below 70°F), humid conditions, and wet leaves enhance tar spot in corn. Tar Spot is often helped by August and September weather conditions. Red Crown Rot in soybeans is also being found in Ohio. 

Corn tar spot (Phyllachora maydis) is becoming a major corn disease. Tar spot fungus came from Central America and is spreading by wind into corn growing states. Tar spot grows rapidly when temperatures are 60-70°F and humidity is 75% or higher with rainy, foggy, cloudy summer weather spreading this disease. Corn will mature early with reduced ear weight, poor kernel fill, stalk rot, and lodging with yield losses ranging from 0-60 bushel per acre, depending upon disease severity. 

Corn tar spot was spotted earlier this year at the OSU South Charleston research plots. Due to weather conditions at that time, it was at relatively low levels and mild. It was found on the lower leaves and most likely over wintered. Tar spot can go through several cycles and move up the leaf, either from splashing rain water from infected soil or blown in on weather fronts. If the tar spot lesions are only on the top leaves, most likely it came in recently. 

Scout for tar spot from about V10 to R1. Tar spot looks like small raised black spots called stromata on corn leaves. An easy test is to moisten your finger and rub the leaf surface. If it rubs off, it may be dirt or insect frass but if it does not, then it is most likely tar spot. It can affect corn yields from the late reproductive phase R3 to R6. Late planted corn is the most severely affected. 

There are several ways to combat tar spot. No corn hybrid is totally resistant but some varieties (especially early maturing) are more tolerant than others. Fungicides may help but early application is critical for optimal success. The tar spot inoculum can survive the winter, so getting corn leaves to decompose quickly helps reduce the spread. Crop rotation helps but it appears that the inoculum is fairly widespread. Some universities recommend tillage to bury the residue, but that is not a long-term solution. About 90-95% of all cropland being planted to corn is tilled. Long-term no-tillers who use cover crops to decompose corn residue quickly by changing the micro-environment and getting more moisture, microbes, and earthworms to incorporate the leaf residue into the soil which can be as effective as tillage. 

Reducing plant stress with optimal fertility helps reduce tar spot severity. Almost all tar spot corn leaves show signs of zinc deficiency. Adequate zinc is needed as a plant defense mechanism to reduce tar fungal reproduction. Corn zinc deficiency shows up as a white midrib in the center of the corn ear leaf while healthy corn leaf midribs should be dark green. Zinc activates nearly 300 different enzymes in plants. Also associated with tar spot are manganese, iron and copper deficiencies. Manganese deficiency shows up as a yellow midrib in corn. The glyphosate herbicide (Roundup) also ties up or chelates many of these micro-nutrients. It is recommended to apply fungicides early, before the disease becomes severe. 

Red crown rot on soybeans looks like sudden death syndrome and impacts the roots, hurting yields significantly. The big difference is that red crown rot has a very distinct red discoloration at the bottom of the stems. Very small red spherical fungal masses start appearing on the stems as the plant matures. Red crown rot is a soil borne fungal pathogen (Calonectria ilicicola) that can over winter. As a soil borne pathogen, the spread of this disease has been slow, but researchers say they may have misidentified or overlooked it for several years, so it is more widespread than previously thought. 

Key symptoms include yellow blotches on leaves that turn brown, reddish discoloration on the lower stem, and rotted roots. The roots are so rotten that the plants are easy to pull up. Plants are stunted, may have some pods, but the pods are small and often not fully developed. It often appears in small patches in wet areas of the field. Often, the patches may be very small and have healthy plants only a few feet away. 

Ways to control red crown rot include using a fungal seed treatment that controls the red crown rot pathogen. Crop rotation away from soybeans helps reduce the disease. Improving field drainage and late planting may reduce the disease. This is a relatively new disease, so send in samples to the university for confirmation.