Crops: Crop Scene Investigation - 16 - Why are Rick's beans defoliating?

Tuesday, February 3, 2009

by MIKE FOLKARD

When a crop suddenly takes a turn for the worse, don't wait until it's too late. Get some advice to preserve your yield potential. That's what Rick did when he called me about one of his soybean fields in the early part of August.

"My beans are dying," said the Norfolk County grower. "They have been great all summer, but now the leaves are starting to drop off." Rick's soybeans were at the R1 to R2 stage of growth, and should have been blooming.

Before I could ask any questions to help diagnose the problem, Rick proposed a hypothesis. "It's worse on the edge of the field by the ditch banks and it moves into the field from there. It's like a drift pattern," he said.

"The County used an herbicide to control the weeds in the ditch and I'm wondering if it drifted and killed the beans."

I'd have to see the damage before I could confirm Rick's suspicions. Once in the field, it didn't take me long to rule out herbicide injury. The affected soybeans were defoliating – leaves were brown, folded up and lying on the ground. I explained to Rick that herbicidedamage doesn't typically take the leaves off the plant, it just curls them up at the edges.

I dug into the ground to check out the roots of the affected plants. Everything looked healthy and intact. Above ground, I looked for signs of foliage damage from diseases like brown stem rot or white mould. Not surprisingly, I didn't find any symptoms. These diseases are more prevalent in wet seasons, and I could see by the soil that the opposite conditions were at play. "It's been rather dry," confirmed Rick. "We haven't had any rain for about a month." 

Dry conditions are ideal for insect populations to build and there appeared to be something feeding on Rick's soybeans. "But I used a seed-applied insecticide, and I haven't seen any pests on the affected plants," he said.

Some pests you can't see with the naked eye, so I pulled out a trusty diagnostic tool – a hand lens – to take a closer look. I could see webbing on the underside of the leaves. I was tempted to put the case to rest right then, but I tried another simple yet effective test to be sure. I tapped an affected leaf onto a piece of white paper and watched to see if anything dropped off. Dust-like specks moved around the white surface. I had caught the tiny yield robbers red handed.

Can you positively identify the problem with Rick's soybeans? Send your solution to Better Farming at: rirwin@betterfarming.com or by fax to: 613-678-5993.
Correct answers will be pooled and one winner will be drawn for a chance to win a Wireless Weather Station.

The correct answer, along with the reasoning followed to reach it, will appear in the next issue of Better Farming. BF

Mike Folkard, CCA, is a Field Agronomist for NK® Brand, Syngenta Seeds. He is based near Belmont.