Soybean rust moving slowly so far

University of Georgia research has consistently shown yield increases of 10 percent or more on soybeans with an application of Dimilin 2L at 2 ounces per acre along with boron when pod growth begins (pods are 3/16 inch long at the four uppermost nodes of the main stem).

This usually occurs in the first weeks of August but depends on planting date.

Many growers in recent years have combined a fungicide with the Dimilin for insurance against Asian soybean rust, but monitoring for soybean rust may save producers money or allow for a more timely fungicide application.

Sentinel plots throughout Georgia are monitoring for the disease and are an excellent warning system. The sentinel corn plots have been an excellent tool to monitor for the presence of southern corn rust and saved growers a ton of money this year. Growers in southeast Georgia have not had to make expensive fungicide applications due to a lack of Southern rust being found in corn sentinel plots and growers' fields.

Farmers can monitor soybean rust at

Dimilin is not an insecticide, but an insect growth regulator so it is very safe for the environment. Dimilin interferes with chitin deposition (chitin is the protein that serves as the cell wall) and the worms are unable to form an exoskeleton. Worms are unable to successfully molt and go to the next stage after eating Dimilin. It is not systemic so good coverage is critical.

Young velvetbean caterpillars “loop” when they crawl until they are about one half inch long. They can be distinguished from loopers since they have four pair of prolegs (hind legs) and loopers only have two. They also have a yellow head capsule and will wiggle violently when they are disturbed or handled. They are usually green with yellowish-white stripes running down their bodies. Brown or black color phases with yellow stripes may also be found late in the season when populations are high.

Asian soybean rust has still only been detected in kudzu in a patch in Decatur and Grady counties (as of late July). Soybean specialists from Alabama, Mississippi and Louisiana are stunned at the slow spread of the disease given the early detection in all but Mississippi (still not found there yet). The complete lack of any tropical storms has apparently stalled the progress of the disease. However there is still plenty of season left.

There are numerous fungicides labeled for Asian Soybean Rust. The strobilurin fungicides, such as Quadris, Quilt and Headline, have a protective window of approximately three weeks versus two weeks with the triazole fungicides such as tebuconazole (Folicur, Orius and Uppercut), tetraconazole (Domark), flutriafol (Topguard), flusilazole (Punch), metaconazole (Caramba), myclobutanil (Laredo), propiconazole (Tilt, PropiMax, Bumper) and cyproconazole (Alto) but do not have curative activity. Strobilurin fungicides must be applied before rust infects the plant.

Propiconazole (Tilt, PropiMax, Bumper) is a weaker fungicide against rust than our other triazoles such as Folicur, Laredo, Domark, Alto, Punch, Caramba and Punch.

Chlorothalonil products (Bravo WeatherStik, Echo and Equus) were less effective than our other fungicides for rust. Although chlorothalonil is labeled for the control of rust, the University of Georgia Extension advises growers that the optimum timing of chlorothalonil is unclear and we recommend using it cautiously. Chlorothalonil remains an effective tool against diseases such as frogeye leaf spot.

Although the University of Georgia Extension has not had a single trial where we were able to evaluate each fungicide under a severe rust outbreak, we expect excellent control of rust with Folicur, Headline SBR (Headline plus Folicur), Domark 230ME or Topguard.

Headline, Quadris plus crop oil, Quilt plus crop oil and Stratego used preventatively should provide satisfactory results to growers. Quilt from field experience seems less effective than Folicur, Domark or Headline SBR.

Laredo 25EC is a triazole fungicide that performed satisfactorily in trials in 2005 and 2006, though not quite as well as Domark, Folicur or Headline SBR.

The strobilurins, especially Headline and Headline SBR and likely Quadris and Quilt, also have a growth effect and may cause the leaves to stay green and remain on the plant longer than the other fungicides. This may make it more difficult to harvest the soybeans.

Where Folicur and other tebuconazole fungicides were applied in University of Georgia studies, we sometimes observed striking foliar symptoms described as “interveinal chlorosis.” This effect was more severe in 2005 than in 2006-08. The foliage on these soybeans looked like they had been affected by nematodes or sudden death syndrome.

However, we did not find any yield reductions associated with these symptoms. The tebuconazole fungicides provide excellent control of soybean rust.

TAGS: Soybeans
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