EDITOR'S NOTE — The following article was compiled by Philip H. Jost, Bob Kemerait and Layla Sconyers of the University of Georgia Copperative Extension Service.
Now that we have the soybean crop in Georgia harvested and out of the field, we can look back and try to assess the real importance of Asian soybean rust to producers in Georgia in 2005.
To begin, Asian soybean rust was widespread in Georgia. An initial find in late April on volunteer soybean plants in Seminole County did not seem to have any real impact on our crop. The major sustained epidemic was first detected in mid-July in southwest Georgia. The disease continued to spread across the state and by the end of the season had been found in counties bordering Florida, Alabama, South Carolina and Tennessee.
We were never able to find the disease on soybeans in Elbert, Madison, Morgan or Hart counties, but it was likely hidden there somewhere.
We don't know why rust did not become established earlier in the season, despite the movement of hurricanes and tropical storms out of Florida and the Caribbean. Our belief is that the initial amount of spores being carried north to Georgia was quite low. The disease had to “percolate” for a while at low levels on kudzu and soybeans in Georgia before reaching some critical level.
The most dire predictions from Brazil have indicated that with the right environmental conditions, soybean rust can move 300 miles in a single day. This did not happen in Georgia in 2005. As a very rough estimate, based on the amount of time it took to detect rust in Tifton and then in Oconee County just south of Athens, we guesstimate that soybean rust was moving north in the state at approximately 60 miles per week.
Asian soybean rust did not “explode” overnight in a field, though it may have looked that way to some growers. Based on observations, it could be a number of weeks between the time we first observed (using a microscope) the initial few rust pustules in a field and symptoms that would have been apparent from typical scouting with a hand lens.
We estimate that between 60 percent and 70 percent of the soybean acreage in the state was sprayed with a fungicide at least once to combat soybean rust. From field trials in Decatur, Tift, and Appling counties, it appears that the R1 (bloom) growth stage was an appropriate time to begin a fungicide program, if there was indication from sentinel plots that soybean rust could be in the area. Delays in initiating fungicide applications led to reduced yields in several field trials.
Chlorothalonil was much less effective at controlling soybean rust than were other fungicides labeled for use against the disease. Unfortunately, we do not have sufficient data to say which fungicides were the “best.” However we did see benefits from triazoles, strobilurins, and triazole-strobilurin mixes.
We did see significant yield increase in some trials where we used fungicides.
In Attapulgus, two applications of Folicur (four fluid ounces per acre) increased yield by 19 bushels per acre.
In an on-farm trial in Appling County, two applications of Headline SBR increased the grower's yield by 16 bushels per acre over the untreated check. From grower visits around the state, we're confident that some growers who did not use fungicides suffered significant yield losses, whereas other growers in the general area who did treat were not affected.
One of the most dramatic lessons from our fungicide trials was that treated rows of soybeans were not affected by untreated, completely defoliated rows next to them. Growers who treat their crop do not need to worry about the effect of neighboring fields that do not receive fungicides.
We do not know how soybean rust will affect our soybean crop in 2006. However, growers should expect the disease to be at least as bad as it was this past season. Although we do not expect soybean rust to survive the winter in Georgia due to freezing temperatures, we do expect it to continue to build in Florida.
We know it can survive on kudzu in that state during the winter. We also found Asian soybean rust on Florida beggarweed in Attapulgus this season, so the disease may also survive on this weed as well.
There is no question that the disease once again will invade Georgia in 2006. The questions now are, “How early and how much initial inoculum?” Stay tuned.