Knowing growth stages key to managing soybean rust

Soybean rust made its way late into the upper Southeast, leaving a small percentage of the more than two million acres of beans subjected to damage by the fungus. Knowing the growth stage of beans is critical to making cost effecting decision as to whether to spray fungicides to control or prevent rust.

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 in full bloom, the plant is at R2, or Reproductive Growth Stage 2.

When pods on the top four nodes are less than one quarter-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.

Clemson University Plant Pathologist John Mueller says, “Asian soybean rust is a wimpy little fungus,” and if the right fungicide is applied, the disease is easy to control.

In South Carolina, he says many growers didn’t need to spray at all this year, and if a grower sprayed more than once for soybean rust, he probably sprayed too much, he contends.

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