Thus far, Georgia is the state most affected by Asian soybean rust. The week of Aug. 15 was especially worrisome: several new rust sites were located along with the disease spreading in spots it’s already known to reside.
“On Aug. 16, we found rust in a Putnam County sentinel plot,” said Phil Jost, University of Georgia Extension agronomist, on Friday morning. “That’s the furthest north it’s been found. Yesterday (Aug. 18), it was found in Plains (in Sumter County) in a sentinel plot at the Southwest Georgia Research and Education Center.”
Nine Georgia counties are now known to harbor the rust. However, the disease is yet to be found in a commercial field. Jost suspects it soon will be.
“A continuing spread of the rust is a very reasonable expectation, especially if we stay in the current humid, rainy conditions.”
There have been showers somewhere on the Coastal Plain almost daily for weeks.
“Soybeans like a good shower but so does rust. And there are several tropical storms threatening. Our beans’ growth stage in conjunction with the weather points to rust building. Those factors are helping this problem get worse.”
Following discovery in sentinel plots, the rust typically spreads.
“Generally, the initial finds are one or two pustules from a 100-leaf sample pulled in a sentinel plot. That’s a very minor infestation. Over the next couple of weeks, though, we’ll return to the plot and find the infestation has spread — more pustules on more leaves. It’s still not at an explosive level, but it’s spreading. That’s the pattern we’re seeing.
“By far, the most severe cases found are in Attapulgus. The disease there seems to be spreading more rapidly. That’s also the case, to a slightly lesser degree, around Tifton.”
Having found rust only in sentinel plots points to the plots’ value, said Jost. “In my opinion, that’s a great testament to the necessity of sentinel plots. An early warning is what’s needed and it’s being provided.”
Could it also point to the fact Georgia farmers are spraying fungicides?
“Yeah, that’s true. There are a lot of farmers spraying. Seeing as we’ve got soybean rust sporadically identified across the entire state, we’ve been recommending producers make their first fungicide application.
“The sentinel plots we’re finding rust in are planted in more mature beans with a heavy pod load. We know with this disease, the more pods and more reproductive the plants are, the more susceptible they are.
“We also know the key time to control the disease is early, before establishment. That’s why it’s crucial to get the first spray out.”
The majority of Georgia’s crop is just beginning to bloom heavily. Beans are weeks away from finishing.
“Obviously the inoculum is building and spreading. Our beans, in the next few weeks, are going to be in susceptible stages. Producers need to get a fungicide — a combination strobilurin and triazole — out. That combination will provide some curative action along with a couple weeks worth of residual protection.”
Dealing with soybean rust over the last few weeks has provided many lessons, said Jost.
“Our scouting intensity has been very good. We’ve found this at very low infestation levels. We’ve also found scouting is much more intense and involved than we originally thought it would be. It’s very difficult to find this disease. You’re talking about finding a single pustule on 100 leaves — that’s tough.”
With rust threatening, Georgia has a “very good-looking” soybean crop that has caught timely rains. Recently, stink bugs have found their way into the crop. Frogeye and downy mildew are also showing up.
“If a grower needs further reasons to make a fungicide application, there are two more,” said Jost.