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No perfect storm for soybean rust yet

By Doug Rich

A late-planted, late-maturing soybean crop had growers and Extension personnel in Tier II states on the alert for Asian soybean rust this fall. Luckily, all of the necessary elements were not in place for rust spores to move north.

"Rust spreads best anywhere in the world, when the weather is cloudy, we have frequent light rains, and there are times when the dew stays on the plants for extended periods of time," Allen Wrather, University of Missouri plant pathologist at the Delta Center in Portageville, said. "This results in a greater number of hours of leaf wetness, and that is required for the rust spores to germinate and enter the plant."

Since Asian soybean rust was first discovered in the U.S., in 2004, it has developed in Missouri and the surrounding states every year except 2005, Wrather said. However, it has always developed in mid-September or later.

"In most years, that is at a time when our soybeans are no longer at risk because all of the plants have reached the R6 growth stage," Wrather said.

The R6 growth stage is when the soybeans in the pod are large enough that they are almost touching. Rust that develops at this point probably would not be severe enough to hurt yields.

"This year, we had some fields that would be at risk even as late as mid-September," Wrather said. "We may have some fields that need to be sprayed with fungicide."

Perfect storm

The perfect storm for soybean rust would be temperatures in the high to low 80s, high humidity, frequent rain, and cloudy weather. Just the opposite of what we have had this fall. Anything that results in drying of the leaves in the early morning, slows the development of soybean rust."

Wind direction also plays a role in the development of soybean rust in Tier II states like Missouri, Kansas, Iowa, Illinois, and Indiana. Asian soybean rust will not survive through the winter in the U.S. except in southern regions like central Florida and south central Texas where rust spores survive and buildup on Kudzu plants in those areas.

This year, wind currents have not brought very many rust spores to the Midwest. Rust spores moved into eastern Arkansas, west Mississippi, and south Tennessee but no further.

"Anything that results in a wind pattern, like a large low-pressure system that results in a counter-clockwise rotation of winds, would bring spores north as the low pressure moves into the central part of the U.S.," Wrather said.

Since 2004 soybean rust has developed as far north as Canada, but it was so late in the season that yield was not affected. Yields have been reduced in some southern states like Louisiana, Mississippi, and Alabama. Wrather said these are Tier I states where rust survives, and will probably always have a little bit of rust every year, and they will have to spray fields to protect their plants.

Early warning

The first line of defense against soybean rust are the scouts who monitor soybean fields in their areas, send in leaf samples, and alert area growers when necessary. Patricia Miller, agronomy specialist and county program director in Vernon County, Mo., is one of the scouts in southwest Missouri.

Miller said, in the past, soybean rust has come to southwest Missouri in a stair-step fashion, working its way from east Texas to Oklahoma, and then to southwest Missouri. This year, dry conditions in both Texas and Oklahoma slowed the development of soybean rust. The year she did find soybean rust, it followed wet conditions in eastern Texas and Oklahoma. Miller said that year was a wakeup call for them. It gave Extension and growers a chance to see what it looked like, but there was no real damage to yield.

"It was a dry run and a dry run with no damage is always good," Miller said.

Reduced funding resulted in a scaled back scouting network this year and Miller is only checking one field in Vernon County. Around the middle of August, Miller started making weekly visits to a field west of Nevada Missouri to collect 100 random leaf samples. These samples are put in a plastic bag and mailed to Allen Wrather in Portageville, Mo., where each sample is examined under a microscope to determine if soybean rust is present.

When they first began monitoring fields for soybean rust, in 2004, scouts would wait for confirmation before alerting area growers. Miller said she can identify rust now and would probably go ahead and let producers know it is present in the county.

"Once you have seen it, it is not hard to identify again," Miller said. "But you can't really tell with the naked eye, you need a hand lens to look at the underneath side of the leaf."

Miller said just walking across a field looking at the upper leaves, it would look a lot like any other leaf spot disease. Look at the underneath side. If it is soybean rust, there should be a volcano like raised point on the leaf where the rust spores are coming out on the underneath side of the leaf.


Funding for the Pest Information Platform for Extension and Education (PIPE) was cut this year, but the United Soybean Board stepped forward with money to keep it going. Negotiations for funding in FY 2010 are ongoing. Bev Paul, Washington Representative for the American Soybean Association, said both the House and Senate Ag appropriations bills fully funded the president's request for the Food and Agriculture Defense Initiative (FADI) at $9.83 million for FY 2010.

"The good news about this is that the inclusion of soybean rust in FADI signals the administrations commitment to PIPE," Paul said. "The bad news is that it doesn't specify a funding level for soybean rust within FADI."

PIPE is critical for control of both soybean rust and soybean aphids. Paul said she expects efforts to continue for funding PIPE at the higher levels seen in the past year on the part of both ASA and USDA.

Rust resistance

An early warning network is critical because there are no soybean rust resistant varieties available at this time. Growers need time to make decisions about whether or not to treat their fields based on maturity of the crop, potential yield, and condition of the crop. Dr. David Walker, USDA research geneticist based at the University of Illinois, oversees research efforts to develop resistant varieties.

"We are doing a lot of field screening in Quincy, Fla., and we have a group of researchers between Louisiana and South Carolina who are helping to screen germplasm from the USDA rust collection," Walker said.

Several researchers have made crosses that seem to be resistant in at least some of the locations in the southeast. The problem is that some of the plant productions that were resistant in South America do not seem to be very resistant here and visa versa. Some of the germplasm that seemed very resistant in Georgia and Florida seem to be somewhat less resistant in Louisiana.

"We are trying to work with all of those, and combine resistance genes by crossing these lines with leading cultivars or advanced breeding lines," Walker said. "It is progressing."

There is always a concern that when Asian genotypes are crossed with North American cultivars or potential cultivars that they will not have the agronomic package that is important to producers. Most of the resistant germplasm comes from Asia. Walker said every time you make a cross of that kind you risk incorporating undesirable genes as well as the resistance genes.

"Even though rust has not been a big problem here yet, people are aware of the massive yield losses that it can cause if the conditions were right for an epidemic," Walker said. "I think there is concern about what could happen if we were to have a mild winter followed by a rainy spring; that would create the conditions for the pathogens to survive the winter in a lot of locations, and then move northward early in the spring."

The perfect storm for Asian soybean rust has not occurred yet, but the potential for serious yield loss is still out there.

Doug Rich can be reached by phone at 785-749-5304 or by e-mail at richhpj@aol.com.

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