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Drought slows soybean rust in Southeast

A record planting season drought in the lower Southeast blunted early movement of soybean rust and nagging drought conditions throughout the upper part of the region have not been conducive to disease development.

In North Carolina, North Carolina State University plant pathologist Steve Koennig, says some new weapons to fight soybean rust and other soybean diseases have been granted temporary label status.

Long-time North Carolina State University Soybean Specialist Jim Dunphy says, “at the end of July, I expect about 40 percent of North Carolina's 1.3 million acres of soybeans to be blooming. With current soybean prices being higher than usual, our growers will be more likely to spray a fungicide for diseases other than rust. I anticipate we will still have more soybeans unsprayed than sprayed, if rust doesn't get here before our soybeans are past a treatable stage.”

Dunphy adds, “It will be early October before 80 percent of our state's soybeans are safe from economic damage from rust. If it is dry in September, they may in practice be safe from rust a week or two earlier. I don't know what the chances are that our growers will have to deal with rust; as of July 25, I suspect less than 50-50,” Dunphy adds.

Though hurricane forecasters have backed off their earlier predictions somewhat, based on warmer than normal Atlantic Ocean water temperatures, the chances of a hurricane or tropical storm fronts still pose a threat for moving rust over long distances in a short period of time.

Long-time soybean experts agree that soybean rust is still a formidable potential threat and highly dependent on the weather.

In South Carolina, John Mueller, Extension plant pathologist at Clemson’s Edisto Research and Extension Center says no rust has been found on either kudzu or soybeans. Mueller says sentinel plots monitored in late July showed no signs of rust development at different grower sites and sentinel plots across the state.

Mueller says, “Scouting of individual fields is not needed at this time. Rust has not been observed close enough to South Carolina to warrant scouting by growers or consultants. South Carolina growers should keep track of the results of monitoring plots in the state at the USDA soybean Web site and by subscribing to the Clemson University Soybean Rust News Note,” he adds.

Mueller says, with the high price of soybeans many growers are interested in spraying fields for diseases other than rust. He contends growers should wait until R-3 for the first spray and then spray again approximately 2 weeks later to optimize control of diseases other than rust. If growers wish to spray only once for the "other" diseases they should wait until R4/R5 unless conditions are very wet.

Mueller says when a grower sees an advisory to spray beans between Growth Stage 4 and Growth Stage 6, he or she better know how to identify these growth stages. A simple ride-by isn’t likely to be a good indicator of when, or if, to apply fungicides to soybeans, he adds.

When the first flower appears on the main stem on the majority of plants in a field, the plants, and for evaluation purposes the field, are in Reproductive Growth Stage 1, or R1. When one flower on one of the top two nodes is full bloom, the plant is at R2, or Reproductive Growth Stage 2.

When pods on the top four nodes are less than one quarter-inch in length, the plant is in the R3 stage. When these pods reach one quarter-inch or longer, the plant is at the R4 stage. At R5, seeds are formed, but are less than one eighth-inch in diameter.

By the time the plant reaches mid-R5 stage, and legally by R6, soybeans should not be sprayed for Asian soybean rust. At the R6 stage, seed are larger than one eighth-inch in diameter. At R7, one pod has the color of mature beans and at R8 95 percent of pods are mature.

Mueller says growers should always look at 20-25 plants in a field. If 50 percent of the plants fit into the defined growth stage categories, the grower should rate the field, he says. Looking at one or two plants isn’t likely to give you a good evaluation of the growth stage of an entire field, and in larger fields with large differences in topography and soil quality, more than 20-25 samples may be needed to get an accurate picture of maturity.

The northeastern counties in North Carolina and southeastern Virginia have been plagued by drought conditions throughout the last half of June and all of July. However, most soybeans were planted on time and are progressing toward the flowering stage.

Unless rust makes a dramatic late-season move, these areas should once again avoid significant damage from the disease.

In Alabama, Auburn University Plant Pathologist Ed Sikora says the drought has created a hodgepodge of planting dates for soybeans. In some cases full season beans were planted on or about the same time as double-crop beans because of the severe drought that has plagued most of the state this growing season.

Sikora says there have been no new reports of soybean rust since the disease was detected in a soybean sentinel plot in Baldwin County on June 25.

The majority of soybean sentinel plots in the state are in mid- to late-reproductive stages of growth. Some growers in the southern end of the state, with soybeans in the reproductive stage, and where frequent showers broke the drought cycle, have used triazole or a triazole/strobilurin pre-mix or tank-mix, he adds.

In the Southwest, where record floods have contrasted growing conditions in the Southeast, soybean rust has been a problem. By the end of July rust was reported in soybeans in a second Arkansas county, five new counties in commercial soybean fields in Texas and two counties in Oklahoma.

Rust has been detected in Mississippi at one site, but only on kudzu.

In Texas, soybean rust was found in commercial soybean fields in several counties around the Dallas area, at low levels. This adds to recent finds in commercial soybean fields in Jackson, Fort Bend, and Austin counties. The disease is widespread in this part of the state.

This area is still receiving isolated, heavy thundershowers.

Whether rust will advance on northward into the Midwest soybean belt remains to be seen. For sure the record wet weather to the south has created a good environment for early soybean rust spread.

Conversely, in the Southeast it appears that drought has slowed development enough to allow soybeans to get past the R5 to R6 growth stage, unless tropical weather fronts move the disease spores rapidly.


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