Wet conditions in southern Iowa have resulted in an outbreak of wheat scab. This disease is caused by a number of species of Fusarium fungi, especially Fusarium graminearum (Gibberella zeae), which also causes Gibberella ear and stalk rot of corn.
Scab causes premature ripening of wheat.
The first noticeable symptom of scab is premature bleaching of heads. Part or all of the head will die prematurely. After the wheat has turned, scab-infected heads may have a pink or orange fungal growth at the base of or on the dead glumes. Plants may be in patches or scattered throughout a field. In some fields, most of the plants are infected this year.
Black head mold can appear on a prematurely-killed head.
Premature death of wheat plants may result from other problems, such as root rot, so do not assume that it is always scab. If heads die early and the leaves and roots are still healthy, this is a good indication that scab is the cause. Heads with scab will have no grain or very shriveled grain that has a chalky-white appearance. Premature ripening often is followed by a darkening of the head as secondary fungi, such as Alternaria and Cladosporium, colonize the dead tissue. This condition often is called black or sooty head mold and is a sign that heads died prematurely but does not indicate what caused the original problem. Black head mold is not harmful in itself.
The scab fungus survives in crop residue and windborne or splashed spores infect the heads during flowering. Moist weather and moderate temperatures are favorable for infection. Scab can be devastating to yields if a large proportion of plants are infected. Some fields may not be worth harvesting for grain. F. graminearum also can produce harmful mycotoxins, especially deoxynivalenol (vomitoxin). If a badly affected field is harvested and much of the grain looks shriveled, it can be cleaned and most of the shriveled seed will be removed. This will significantly reduce the risk of mycotoxins.
Samples can be tested for toxins by private laboratories, by a rapid immunoassay kit, or by Iowa State University. Samples sent to ISU must be submitted either through a veterinarian or the ISU Plant Disease Clinic. Care must be taken to get a representative sample; at least a few pounds should be collected from various places in a truck or bin. Scabby grain is a poor candidate for storage.
Scab is difficult to control if weather is favorable for the disease. Fungicidal control has not been effective. The fungus infects wheat and corn, so these crops should be rotated with a legume crop. Tillage will reduce the survival of the fungus. Some varieties are less susceptible, but none are highly resistant. Scabby seed should not be planted because emergence is poor. Seed treatments can improve emergence of scabby seed, but they do not prevent infection of the plants later in the season.
This article originally appeared on page 130 of the IC-476(18) -- July 15, 1996 issue.