Field Crop Disease Update - July 31, 2012

Published: July 31, 2012

This season has been an extremely challenging one for corn and soybean crops in much of Missouri and it doesn’t look like there is any relief in sight. Both corn and soybean are showing symptoms of drought and heat stress. In some cases diseases may be compounding the damage.

Corn: We have received several samples of corn leaves showing firing from the tip towards the base of the leaf. In most cases the discoloration is a fairly uniform light brown to pinkish-brown in color and may cover more than half of the leaf. Individuals have asked if this is Goss’s wilt or drought and heat stress. Samples have been checked for bacterial streaming (the Goss’s wilt pathogen is a bacterium) and have been tested using the immunostrip assay. So far all samples received this year have been negative for Goss’s wilt. The symptoms are more suggestive of those caused by hot, dry conditions.

Goss’s wilt typically begins as a water-soaking of the tissue. The lesions may turn yellow-green and eventually brown to reddish brown. Lesions may form along leaf margins, at the tips of leaves or in the center of leaves. So there is more variation in lesion color and shape than with heat and drought stress injury. A key symptom of Goss’s wilt is the presence of dark green specks or “freckles” in the lesion or close to the edge of the lesion. Glistening droplets of bacterial ooze on the lesion surface would be another sign that Goss’s wilt is the problem. The University of Nebraska Extension Guide on Goss’s wilt may be found at the following site and has more detailed descriptions and pictures of this disease. http://www.ianrpubs.unl.edu/epublic/pages/publicationD.jsp?publicationId=679

Corn Ear and Kernel Rots: Several fungal ear and kernel rots are showing up on corn, especially on ears damaged by insects or ears showing damage from drought and poor pollination. Dr. Allen Wrather has provided an article focusing on Aspergillus flavus and aflatoxin. In addition to Aspergillus flavus, Penicillium species, Fusarium graminearum and Aspergillus niger have been present on ears collected in central Missouri. In fact some ears have had all four of these ear and kernel rot fungi present. Mold growth tends to be prevalent around sites of insect damage, at the tips of ears and around areas where kernel set was poor. At this point in the season there are no control measures for the ear and kernel rots of corn. Growers should scout fields to determine if ear and kernel rots are prevalent and then take precautions at harvest (see Dr. Wrather’s article).

Soybeans: There have been few reports of foliage diseases in soybean fields this season. Scattered fields are showing symptoms of sudden death syndrome but weather conditions have not been conducive for widespread development of this disease. Most questions are related to wilting, stunting and premature death of soybean plants. In most cases the stunted plants have poor root systems with possible Fusarium root rot and/or Rhizoctonia root rot. So far there have not been many samples with charcoal rot but would expect this disease to show up as the season progresses. But much of the damage to these stunted, wilted or dead plants is due to the unusually hot, dry weather pattern the state has endured for the last three months. The root rot pathogens are taking advantage of the weakened plants.