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Asian rust: Does it pay to spray, anyway?

Agriculture.com Staff 05/17/2006 @ 11:53am

Asian rust: Does it pay to spray, anyway? The question of applying fungicides, come rust or not, is getting extra attention this year from soybean growers. If you get Asian rust, you won't have much choice but to spray a fungicide to protect or enhance yields. But if you spray and there is no outbreak of rust, did you waste the money?

Fungicide manufacturers say that spraying at the R3 or R4 stage (pod set and pod filling) will often pay for itself - with or without Asian rust- because there are usually other foliar diseases present that the fungicide will control. That keeps the plant green longer, giving a more complete seed fill.

In Illinois last year, university researchers conducted a study on 70 farms in which a fungicide was sprayed at R3. Wayne Pedersen, retired professor of plant diseases, told about the study this week in an online seminar sponsored by Syngenta.

Pedersen said a whole host of foliar fungal diseases were present on these farms: a nthracnose stem blight, Septoria brown spot, pod and stem blight, and others. But, none of the farms had Asian rust. The plots on these farms that were treated with fungicide averaged 71.3 bushels an acre. The untreated check plots averaged 65.8 bushels. Of the 70 plots, 49 showed a yield advantage for spraying of 3.4 bushels an acre or more, which is considered the economic threshold (yield gain justifies the cost of spraying).

"Routine spraying with a fungicide isn't a practice that will get you an extra 20 bushels an acre," said Pedersen. "But there are other diseases [besides rust] that you will control."

Pedersen said he sees farmers falling into 4 groups when it comes to spraying soybeans for Asian rust and other foliar diseases: Group 1 will almost never spray. Group 2 will only spray when the disease is actually detected in their fields. They will lose yield with this approach. Group 3 will spray when they think they must to protect yields because the disease is close. Group 4 will spray every year. It won't always pay for them, but this group is intent on getting top yields.

Syngenta also announced at this online seminar that they have recently received expanded clearance to use their new Alto fungicide to prevent or treat Asian rust. It can be used in most major soybean states under a Section 18 emergency clearance. It is of the triazole chemistry class, has a long residual activity, and can be used as both a preventive and a curative. In Brazil last winter, a mix of Syngenta's Quadris and Alto fungicides gave the best performance against rust, according to fungicide brand manager Marty Wiglesworth. He also said that a little more rust over-wintered this year along the Gulf Coast and in Mexico than last year. Precipitation and wind patterns in the next few weeks will impact how rapidly it spreads.

Asian rust: Does it pay to spray, anyway? The question of applying fungicides, come rust or not, is getting extra attention this year from soybean growers. If you get Asian rust, you won't have much choice but to spray a fungicide to protect or enhance yields. But if you spray and there is no outbreak of rust, did you waste the money?

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