Agronomic Crop Diseases

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Wheat
The wheat crop is made and now drying down. There was a late season disease that popped up that was not seen until now. Sharp eyespot is a disease that is caused by Rhizoctonia cerealis and causes stunting and “whiteheads” when it is severe. It can cause areas in the field that vary in size to be stunted and mature prematurely. As with take-all, the best control for sharp eyespot is rotation of at least one year out of small grains. The disease begins on the outer leaf sheath near the base of the plant. Lesions on the stems are light-brown to straw-colored with a sharply defined dark brown border. This disease could be confused with take-all but there is generally little root rot associated with sharp eyespot. Generally the disease is not severe enough to warrant control measures other than rotation with legumes or other nonhost crops.

Sharp eyespot on stems

Note sharp eyespot symptoms on lower leaf sheath.

Scab or head blight is present at varying levels in some fields and others have none. Fortunately for most growers, scab was not as bad as it could have been this year. There were some fields, mostly in Kent County so far, where scab is severe and growers should check for scab and adjust their combines accordingly. See the last issue for more info on scab.

Corn
Pythium root rot
has been the most common problem we have seen in the lab so far this season. With corn under water earlier in many places these conditions are extremely favorable for Pythium root rot. Seed treatments with metalaxyl or mefanoxam (Apron, Apron MAXX, Apron XL and others) should provide good control. Corn treated with Dynasty alone would not be effective under severe conditions for Pythium and should be combined with an Apron product for optimum disease control.

Seedling anthracnose was also diagnosed this week. Seedling anthracnose often occurs when corn follows corn especially in no-tillage systems. Initially, small watersoaked spots are seen and become tan with red or red brown borders as they age. Eventually the small hairs or setae of the fungus can be seen with a hand lens in the center of the lesions.  Anthracnose rarely causes any loss at this stage of growth and the plants grow out of the initial infections, which can recur later in the season if the weather is favorable for anthracnose leaf blight. Burying crop residues may be helpful in reducing these early season infections but do little or nothing in reducing the late season leaf blight and stalk rot phase of anthracnose.

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