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Corn+Soybean Digest

PRR 42-Million-Bushel Sneak Thief

A 45-bu/acre soybean yield may seem like a decent crop. But you could see an additional 5- to 10-bu/acre yield boost by identifying a major disease threat you don't even realize you have. Phytophthora root rot (PRR) robbed U.S. soybean growers of an estimated 42.2 million bushels in 1998. That's about $215 million lost to the fungal thief.

"Our bean yields aren't bad, but they have not increased the way corn yields have," says Ron McNeall, Keytesville, MO. "We worry more about phytophthora in our heavy soils and grow resistant varieties there. But we are less careful about soybeans planted in upland soils, and we may be sleeping through things that are hurting us."

That's a fairly safe bet, says George Smith, University of Missouri plant pathologist and soybean grower.

"We're dealing with what I call the 40- to 45-bu syndrome," says Smith. "And many growers don't realize it, because a 45-bu yield isn't too bad, and plants don't always show any obvious symptoms of disease. But it's there, and soybeans are bumping into a yield ceiling because of it."

If you grow soybeans, odds are better than even that your fields are infected. While PRR is found across soybean country, it typically is a more chronic problem in the eastern Corn Belt.

"We have great soils for PRR - old lake beds - heavy soils that do not drain well," says Anne Dorrance, Ohio State University plant pathologist. "It doesn't take much rain to saturate these soils, and the weather here usually warms up early in planting season."

Unlike some fungal diseases, PRR favors wet, warm conditions.

"That's when it hits hardest," agrees Smith. "Beans that have been in the ground less than 72 hours before a rain usually are hurt - especially if soil temperature is above 60 degrees."

The PRR fungus has an uncanny ability to outwit bred-in resistance by forming new races of the organism.

"Single-gene resistance works as long as you're fighting the same race of PRR," says Dorrance. "But the Ohio population of PRR is very diverse. We have all the combinations, plus another whole set."

It's a numbers game, she points out. The eight basic strains or races of PRR can join in a number of combinations.

Whipping PRR begins when you buy seed.

"Order at least 70% of your seed with a broad-spectrum fungicide plus Apron/Allegiance," advises Smith. "And choose a top-yielding variety with good field tolerance or resistance. Apron-type products give protection against PRR until resistance or field tolerance kicks in."

Smith orders treatment when he books his soybean seed. Minimum rate needed for PRR control per 100 lbs of seed is 0.32 oz of Apron XL-LS, or 0.75 oz of Allegiance.

"It costs me about a dollar extra per bag of seed, but it's good insurance," he adds. "I make sure I have enough treated seed for beans I'll plant early and for those planted no-till. PRR is worse in no-till because the fungus survives for several years in crop residues."

If you can find it, choose an adapted variety with genetic resistance that is effective against the newer races of PRR.

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