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Asian rust likely overwintering

It's likely that Asian soybean rust will be here a while, unless a winter freeze in south Florida, south Louisiana and the southern Texas Gulf Coast kills the overwintering fungus prior to a growing season.

The good news is that the disease was discovered early enough last year for the soybean industry and USDA to put together a battle plan to control it in 2005, according to a USDA official.

The disease, which is believed to have reached the United States on the winds of Hurricane Ivan around Sept. 16, 2004, “is a threat to agriculture, but we can cope with it,” said Kent Smith, USDA, office of pest management, Washington, D.C. Smith spoke during the National Alliance of Independent Crop Consultants annual meeting in Universal City, Calif.

“We have the tools necessary to manage this disease. We shouldn't have any problem with availability of fungicides, although what we really need are resistant varieties.”

According to Smith, yield losses from Asian soybean rust could range from 10 percent to 50 percent, and could hit 90 percent or greater in certain fields.

However, in a report by USDA's Economic Research Service on the likely impact of the disease on the U.S. economy, yield impacts ranged from less than 1 percent to as much as 10 percent, if conditions are favorable for the disease. The study also assumed adequate availability of fungicides, proper timing and knowledge of how to use fungicides.

Smith noted that this year's winter freeze will eliminate the pathogen throughout most of the soybean-growing region of the United States. But the fungus will likely overwinter in south Florida and along the Gulf Coast. “The farther you are away from these overwintering sites, the less the chance of you taking severe damage from this disease.

“All years will not be the same,” Smith stressed. “Some years, if we have hot, dry weather, there will be very little of this disease. But if we have wet weather and mild temperatures, we could expect this disease to be a serious problem.”

Temperatures suitable for infection range from 50 degrees to 84 degrees under moist conditions. “Hot, dry conditions will virtually stop the disease in its tracks.”

Early symptoms of Asian soybean rust are very difficult to catch, according to Smith. “You see small chlorotic flecks in the leaves. Unless you know what you're looking for, you might not see them. As the flecks develop, you see a bronzing of the leaves and a very unhealthy appearance to the foliage. In the latter stages of the disease, defoliation can occur at a fairly early stage of soybean development.”

The disease begins with an initial infection in which the Asian rust fungus penetrates leaves. Two types of lesions can form. “One is the normal tan lesion, the other is a reddish-brown lesion. The difference is a resistance response, but at this point, we don't have a resistant variety.”

Each lesion, or spore mass, releases thousands of spores. The spore masses, which are most often on the underside of the leaves, can be seen with a hand lens. Individual spores, however, cannot be seen with a hand lens.

The infection cycle can occur six to 10 times during the season, according to Smith. “If you have a massive influx of spores and conditions are right for infection to occur, you can have devastation within a week or two of when you see the first symptoms. The disease can spread before your eyes.

“That's why we want to prevent the initial infection. You want to have your fungicide on before infection occurs or immediately after it occurs. Otherwise, you could have serious problems.”

In situations where conditions are not ideal for Asian soybean rust or just a few spores have entered the field, “you may have focal points in the field, where you have very unhealthy plants in a sea of green.”

Asian soybean rust was first observed in 1902. For the next 90 years it occurred only in southeast Asia and the southwest Pacific. However, USDA's Agricultural Research Service considered it a serious disease as early as the 1960s. It was discovered in Hawaii in 1994, spurring additional research.

“How it got to Hawaii, no one knows,” said Smith. “The fungus is certainly capable of moving hundreds or thousands of miles in spore clouds if the wind currents are right. There is also the possibility that it could have been carried inadvertently on a soybean plant taken into Hawaii by Japanese farmers.”

In 1996, Asian soybean rust was found in Africa. By 2001, it had moved to South America. Its occurrence was reported in the United States in November 2004, “but we think it may have occurred two months earlier than that (with Hurricane Ivan) and had not been detected yet.”

Smith noted that China's experiences with Asian soybean rust could provide some insight into how the disease may behave in the United States. Much of the soybean-growing region in China, which has been fighting the disease for 50 years, is at the same latitudes as soybean-growing regions in the United States.

“They occasionally see the disease in their major soybean-producing area in the north, above 45 north degrees latitude,” Smith said. “Below 37 degrees latitude, which would be equivalent to the northern borders of North Carolina and Tennessee, “you see the disease with some frequency. Asian soybean rust seriously affects soybeans in China below 30 degrees north latitude, which is equivalent to Florida.

“We can't say that this would be repeated in the United States. China has natural barriers, deserts and mountains which might limit the movement of spores from south to north. We don't have such barriers in the eastern part of the United States. In fact, the eastern part of the United States is essentially one big soybean field. So the disease could move very quickly here.”

USDA is focusing on developing Web tools to help U.S. soybean producers track the movement of the disease, according to Smith. “One goal is to develop a Web site that will show the farmer exactly where the disease is on a daily basis. The information is going to be gathered from plots that will be monitored daily.”

A diagnostic tool developed by USDA was very effective in identifying the pathogen in November.

Smith noted that there is some speculation that if the disease does not go any farther than it's been detected this year, it could be killed by a winter freeze, which would eliminate its foothold in the United States. “We don't have any collection data at the moment to indicate that it is overwintering. But most of us suspect that the organism has entered overwintering areas.

“We fully expect it to be there next year, because kudzu (a host plant for Asian soybean rust) grows all over Louisiana,” said Calvin Viator, a crop consultant in south Louisiana, where the disease was first discovered in the United States. “Even if it freezes, we have green material there.”

Viator noted that Louisiana soybean producers generally budget for one application of a fungicide for cercospora and pod, stem and aerial blight. His growers are now prepared to make a second fungicide application.

“For the first application at flowering, we'll probably go with a material that's better for rust. The second material will be more effective on cercospora. That will accomplish two things, rotation of chemistries and protection from rust.”

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