Pest Alert: Soybean Rust Update

— Written By NC State Extension

Date: September 28, 2012

From: Jim Dunphy, Extension Soybean Specialty, and Steve Koenning, Extension Plant Pathologist

Asiatic soybean rust has been confirmed during the past two weeks in quite a few counties in Mississippi and Alabama, but not many on the east coast. Kentucky now has its first confirmation of rust in 2012. South Carolina still has found rust in only 5 counties, all in the southern half of the state. More than two weeks after we found rust in Robeson and Johnston counties in North Carolina, we still haven’t found any more rust, including in those two fields, and in several nearby fields. All of our sentinel plots have been checked in the past two weeks as well. This reinforces the principle that if weather conditions do not favor the development of this disease, it is likely to develop more slowly than we know it can develop with ideal weather conditions. Currently, rust has been confirmed approximately 105 miles from Charlotte, 140 miles from Elizabeth City, 15 miles from Fayetteville, 15 miles from Murphy, 35 miles from Raleigh, 80 miles from Washington, 75 miles from Wilmington, and 80 miles from Winston-Salem, North Carolina.

We do not recommend spraying soybeans with a fungicide to control Asiatic soybean rust if they are blooming (or past blooming) but rust has not been confirmed within 100 miles, or if full sized seeds are present in the top of the plant (stage R6). The higher labeled rates tend to provide more days of prevention, and may thus require fewer applications. The triazole fungicides, alone or in combination with a strobilurin fungicide, will probably provide better prevention of rust than a strobilurin alone. Be sure to check the fungicide label to see how many times it may be used in a season.

Soybeans that have just reached full bloom (stage R2) typically have 65 days until they’re safe from rust or frost (stage R7) if they are full-season soybeans, or closer to 55 days if they are double-crop soybeans. If they have small pods in the top of the plants (stage R3), they have 55 and 47 days, respectively, to R7. With full sized pods in the top of the plants (stage R4), they have 45 and 38 days, respectively, until R7. From stage R5 (small seeds in the top of the plant) they typically have 35 and 30 days, respectively. From stage R6 (full sized seeds in the top of the plants), they typically have 20 and 17 days, respectively.

Rust will typically take 10-20 days from initial infection to develop to detectable levels. It will take another 7-14 days to spread to other leaves on the same plant, and another 10 days to cause significant defoliation. This assumes optimal conditions for rust – “65-85 degrees, and either overcast or rainfall” – through much of this period. This is not common in North Carolina in September and October, but can and has occurred.

The rust prediction models say there was a fair to good chance rust spores were deposited in North Carolina three weeks ago. If so, we expect to detect rust in about three weeks in sentinel plots, which would be about October 1. It will likely take at least another two weeks with optimal conditions for rust to increase to damaging levels.

The current status of soybean rust in the U.S. can always be found at http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi.

Updated on Mar 18, 2014
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