Following a Wet Planting Season - Will Crop Diseases Flourish?

This year's wet beginning has some similarities to 1993.

The extremely wet planting season in Iowa in 2008 raises questions about what will happen with corn and soybean diseases this year.

"So far, this season has some similarities to what happened in 1993, my first year at Iowa State University, and those experiences have stayed with me," says X.B. Yang, an Iowa State University plant pathologist.

"That year planting was delayed so much that planting-related questions were a major topic for ISU agronomy Extension, even in early summer field days. Looking at what happened in 1993 may provide some clues to disease management this season if the weather pattern continues to follow the similarity."

With corn, watch for two diseases

In corn, two diseases were prevalent in 1993: crazy top and corn rusts which include both common corn rust and southern corn rust.

Corn plants which stood in water for a long time had relatively high incidence of crazy top, a disease caused by downy mildew fungus. Southern corn rust showed up as early as late June and was widely spread.

However, southern corn rust may not be a problem this year because in 1993 the disease spread to Iowa from the south. This spring has been dry in Florida and Texas, both of which are the source regions of the fungus. This disease is unlikely to arrive so early. If the disease were prevalent, you might see a financial return from the use of a foliar fungicide for control.

Soybean diseases were the cause of production problems in 1993, says Yang. Brown stem rot, a disease occurring in cool and wet weather, was prevalent that season, causing lodging in severely infested soybean fields. Iowa's average soybean yield that year was 20% less compared to the statewide average yield in 1992 and 1994.

Sudden death syndrome may be problem

SDS - sudden death syndrome - was prevalent in many north-central states and was found in Iowa for the first time in 1993. "In 2008 this soybean disease may be a problem, even though we had a late planting. It could be a problem because prolonged cool weather during the planting season is favorable to the fungus," he says. "Because the management measures for these two diseases (Brown stem rot and SDS) occur before or during planting soybeans, there is nothing we can do about these two diseases now, except for scouting to gather management information for future crop years."

In the fall of 1993, premature defoliation (loss of leaves from the plants) from foliar diseases was prevalent, and many soybean fields turned yellow before September. Two reasons caused the premature defoliation that year, says Yang.

First, fields were flooded for so long the soybeans did not have good root systems. Second, the excessive rainfall led to outbreaks of brown spot, a rain-borne fungal disease, and other diseases which defoliated soybean leaves.

Keep an eye on foliar soybean diseases

"So we advise growers to keep an eye on foliar diseases, especially brown spot this season," says Yang. "Preventative measures can control this disease. The first is to use mid-season cultivation which can improve root growth. Secondly, we can control foliar diseases with the fungicides if the disease risk is high this summer. Numerous studies consistently show that fungicide sprays pay off when a significant number of diseases are present."

Soybean white mold has been a concern to many farmers in eastern Iowa, and the disease was prevalent in every even year in the last 10 years. This season, the disease poses less threat because of delayed planting, making it unlikely the canopy will close early, a necessity for a white mold outbreak to occur. White mold was not a problem in 1993.

It is difficult to predict what the weather will be for the rest of growing season. "Farmers should watch national weather forecasts and scout for diseases for help in making good management decisions," says Yang.

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