Field Crop Disease Observations- May 30, 2006
By Laura Sweets

Corn

Anthracnose leaf blight on upper leaf.

With the recent swing to above normal temperatures, young corn plants have taken off and are looking much better in both color and size. Much of the yellowing and stunting of corn plants that had been reported over the last week was related to below normal temperatures and correspondingly cool soil temperatures.

Anthracnose may be evident on the very lowest leaves of plants as brown to reddishbrown elliptical lesions. These areas may expand along the margin of the leaf and can be mistaken for herbicide injury.

Older lesions of Stewart's bacterial wilt.

Stewart's bacterial wilt is also showing up in very low levels. Right now Stewart's wilt would be evident as long, pale green to yellow green streaks that tend to follow the veins of the leaf. As the lesions mature they will turn a lighter brown or papery brown in color.

Crown decay has not been as widespread nor severe as might have been expected from the vacillating weather conditions in much of the state this spring. Still when trouble shooting in corn fields with uneven stands or stunted plants, it would be prudent to split plants open and look for signs of discoloration or decay of the lower crown tissues. Healthy plants should be white to light cream in color and have a solid texture in the lower crown. With crown decay, the lower crown tissues may be spongy or pithy and may range in color from light brown to dark brown or almost a blackish-brown.

Soybeans

Thus far there have been few questions or problems on soybeans. A few scattered plants are showing single lesions of Septoria brown spot or frogeye leaf spot. If Pythium and/or Phytophthora root rot are going to be problems this season, I would expect to see symptoms developing over the next few weeks.

Wheat

Wheat across the state ranges from good to not so good. Barley yellow dwarf has been quite prevalent in almost all areas of Missouri this season. The purple to pinkish-red discoloration of the flag leaves is still quite evident in some fields. However, in other fields the shift to higher temperatures has caused barley yellow dwarf infected plants to fire and leaves are drying out and turning brown prematurely.

Barley yellow dwarf symptoms on young plants.

Septoria leaf blotch, leaf rust and stripe rust are being reported in scattered fields throughout the state. In most cases these diseases have come in late enough in the season that they should not have a major impact on yield.

Areas which had rain or extended periods of overcast weather with frequent rains as the wheat crop was flowering may be seeing symptoms of scab or Fusarium head blight on heads. Symptoms of Fusarium head blight are most evident during the grain fill stages before heads normally begin to turn. Scab would be evident as bleached areas anywhere on the head. Areas of pink mold growth may be obvious at the base of glumes.

Laura Sweets
573-884-7307