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Delayed planting could lead to increased risk from soybean rust

Corn plantings are up in the Southeast and the record Easter freeze made replanting necessary on large acreages in South Carolina and Georgia. Add to the planting delays due to a prolonged drought in many areas of the Southeast, and the sum result is large acreages of soybeans planted later than usual across the Southeast.

The delayed planting could put many soybeans at risk for Asian soybean rust, if the disease develops and spreads northward. Active rust is still being monitored at six sites in Florida, but little movement has been detected.

As of mid-May, the drought in Georgia and Alabama had stopped rust development cold, though rust was found on kudzu in Louisiana earlier in the month. The disease was also detected on soybeans in one county in Texas, but that field has since been cultivated and planted with corn.

Conditions remain critically dry in Florida, with fires hampering many outside activities, including planting. University of Florida researchers say, “kudzu is growing rapidly and in those sites that are infected the rust is progressing slowly. We have not had any sites become negative since the winter, but thus far we have found fewer positive sites than in previous years.”

In Alabama, Auburn University Plant Pathologist Ed Sikora says, “Soybean rust has not been detected in our soybean sentinel plots or on kudzu in south and central Alabama as of mid-May. The state is still relatively dry, though some areas have received significant amounts of rainfall from widely scattered thunderstorms.”

Georgia has been ravaged first by the record Easter freeze and since by extreme drought conditions that have prevented planting of many crops, including soybeans. In the northern half of the state, kudzu monitoring sites have remained dormant. Though some of the kudzu sites in south Georgia have emerged from dormancy, no rust has been found on any kudzu in the state.

University of Georgia Researcher Bob Kemerait says, “Intensive scouting occurred on April 24 at sites where Asian soybean rust was found to over-winter in 2006-2007. We are no longer able to detect Asian soybean rust at any location in Georgia on kudzu at this time. The only Asian soybean rust that was found were pustules on long-dead leaves. These leaves have been dead for likely 8 weeks, so it is unlikely that the remaining spores are viable,” according to Kemerait.

Soybean planting is behind its normal schedule in South Carolina. In the Savannah Valley extreme drought has brought planting of all crops almost to a halt. Where moisture or irrigation are present, cotton and peanuts have a higher planting priority at this point, according to Clemson University Plant Pathologist John Mueller.

In the northern half of the state moisture in some areas has been excessive. Even where moisture is good soybean planting has taken a backseat to finishing cotton and peanut planting. All planting has been delayed due to the replanting of more than 25 percent of the corn crop due to the Easter freeze, Mueller adds.

Regardless of when soybeans are planted, a critical factor in managing soybean rust is to know when these plants are most vulnerable to the disease. The sentinel plot system is so accurate in monitoring movement of rust that farmers can wait until they get an advisory to spray. Still, they must know whether their beans are vulnerable to rust damage.

Pawel Wiatrak, an agronomist at Clemson University's Edisto Agricultural Research and Education Center in Blackville, S.C., says when a grower sees an advisory to spray beans between Growth Stage 4 and 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 fourth-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 of 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.”

Though rust appears to be delayed in the Southeast, active rust in Louisiana could spell trouble for growers in the Mid-South and Midwest. The find in New Iberia Parrish is 53 days earlier than 2006, which brings up questions about the initial inoculum source and the overall implications for soybeans in Louisiana and the mid-section of the country, according to LSU Plant Pathologist Clayton Hollier.

Hollier says, “such an early find in the Deep South has implications for our industry and the industry north of us. It was indicated last year (2006) that Louisiana was probably the source of inoculum for the Mid-South and the Mississippi and Ohio River states north of us. That being said, the same could happen this year, especially if the weather is wetter than last year's.”

While rust remains a threat to soybeans nationwide, it appears development of the disease has been severely hampered in the Southeast by drought. In 2006, a prolonged drought in Georgia and Alabama blunted northward movement of the disease, delaying it until there was little threat to soybeans in the Carolinas and Virginia.

Thought no accurate predictive models of soybean rust movement have been developed, researchers in the Southeast agree that the Easter freeze and prolonged drought in the lower Southeast have delayed development of rust.

Whether these conditions delay movement of rust into soybean producing areas of the upper Southeast remain to be seen.

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