Diagnose bean seedling diseases

So far, planting and growing conditions in 2011 here in southern Iowa have been far more favorable than those we encountered during the 2010 planting season. It was nice to see dust rolling behind planters rather than mud. Soybeans emerged in less than a week with soil temperatures in the mid-60s.

However, as I write this article, soil temperatures dropped from 67 to 52 degrees F in just five days, plus we’ve had almost 2 inches of rain in the same period. Cool temperatures and wet soils are conducive for seedling diseases to develop in bean fields. Be sure to examine your fields for damping off or other seedling diseases.

Two of the more common pathogens that reduce soybean stands are pythium and phytophthora. Both are fungal-like organisms classified as “water molds.” Both can cause seed rot, damping-off belowground and postemergence damping-off.

Symptoms include brown-colored, soft-rotted seeds, or collapsed and soft-rotted hypocotyls belowground. Often you may see green cotyledons aboveground, but the emerged hypocotyls or stems will be collapsed and rotted. Both pythium and phytophthora cause similar symptoms, and lab tests are needed to determine which water mold caused the disease.

We tend to think pythium is most active in cooler, wetter soils, thus more of a problem in early-planted fields. There are several species of pythium. Once the bean seed is planted, there isn’t much you can do. There are no varieties resistant to pythium. Seed-applied fungicides are the most common control strategy, and seed treatments containing metalaxyl (e.g., Apron or Allegiance) or mefenoxam can protect the seed and seedling for about two weeks. However, research has also shown some isolates of pythium aren’t controlled by seed-applied fungicides.

Disease prefers warmer soils

Phytophthora tends to be more active in warmer soils than pythium. While phytophthora can cause seed and seedling disease like pythium, the pathogen can continue to kill soybean plants throughout the growing season. Once plants get past the V2 growth stage, phytophthora can cause root rot. Diseased plants wilt, leaves remain on the dead plant, and the lower stem turns a brownish-purple color.

Seed-applied fungicides can reduce “damping-off” but don’t last long enough to reduce root rot. The most effective control is to plant quality seed that has major resistance genes such as Rps1a, Rps1c and Rps1K. Other such genes are being produced. Occasionally in fields, the major genes have been defeated by an aggressive race or isolate of phytophthora. Multiple races can exist in the same field.

Watch for seedling blight

Rhizoctonia is another fungus that can damage seed or cause seedling blight. Look for red-brown lesions along the hypocotyl or stem at the soil line. Unlike pythium or phytophthora, a rhizoctonia lesion is more of a firm, dry rot than a soft rot. If the lesion is on a small part of the stem, the plant may recover. If a lesion girdles the stem, the plant will die. Rhizoctonia seedling blight is more common in warmer, drier weather and in later-planted soybean fields.

Fusarium is another fungus that can cause disease in beans and is often overlooked or misdiagnosed. Fuzzy, white rotted seed is a symptom of fusarium seed rot. Symptoms of fusarium root rot are brown or necrotic, sunken lesions on soybean roots. If a taproot is severely infected, the plant may die. In other cases, the fungus may only infect secondary roots, impeding root function and reducing yield with no obvious aboveground symptoms.

Different species of fusarium prefer different conditions. Some cause disease during warm, dry soil conditions. Others are more aggressive if it’s cool and wet. Fusarium graminearum, a species causing root rot, stalk rot and ear rot of corn, as well as scab or head blight of wheat, has been isolated from soybean roots. Several fusarium species have a wide host range. Seed-applied fungicides that contain fludioxonil or a strobilurin product may reduce fusarium infections.

It’s important to correctly diagnose seedling diseases. Most disease-causing organisms produce structures for long-term survival. For example, pythium that causes damping off in 2011 will be in the field in 2013 or the next time the field is in beans. Knowing which pathogens are present will assist in future planning — choosing resistant soybean varieties, selecting appropriate seed-applied fungicides or maybe waiting until soils have warmed to plant.

Do you have damping-off?

Iowa State University has received a grant from USDA to study pythium and phytophthora. The goal is to determine which species of pythium and phytophthora are affecting soybean stand establishment. The information will be used to develop DNA-based diagnostic tools to use in a field. The tools will enable you to identify pathogens responsible for the problem in an affected field, and ensure more successful management of damping-off by seed treatment fungicides or soybean varieties. If you find damping-off in your fields, contact Alison Robertson at alisonr@iastate.edu or your local ISU Extension field agronomist.

Carlton is the ISU Extension field agronomist at Albia in southern Iowa.

This article published in the June, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.