Corn production has faced challenges in fields across Minnesota this growing season. The season started wet with planting delays in numerous areas and is ending with warm weather after frost hit many fields last week. Diseases were also highly variable as usual across Minnesota. This article highlights frequently reported disease problems across the state, namely rust, northern leaf blight, Goss's wilt, and most recently stalk rots.
This is the second year in a row that higher than normal levels of common rust developed in central and southern Minnesota. Only common rust was confirmed in the limited sampling that was done, and the potentially more damaging southern rust has seems to have remained south of Minnesota, although that could change in any year. Common rust was favored again by the cooler than normal July temperatures and slowed down when temperatures rose above 80°F. For those that saw rust at uncommonly high levels, the obvious question is how did the disease affect yields? Studies indicate that yield is reduced about 3% to 7.0% for every 10% increase in rust severity in pre-dent crop stages. Thus, it takes severe levels of rust to result in clearly measurable yield losses on a field level. Further, the level of loss declines when rust takes hold in later growth stages.
Northern leaf blight
This fungal disease was reported in many fields, especially in southwestern and central Minnesota. The canoe-shaped lesions, usually about 2-3' long, are generally easy to recognize when they are isolated on the leaves, but can cause confusion when so numerous that they grow together to kill large sections of leaves. The reasons for the significant disease levels in many fields is not well understood. We know that cool to moderate temperatures (66-80F) and extended periods of leaf wetness favored this disease. But whether the disease was favored by weak resistance in hybrids or a change in races of the pathogen seems to be uncertain.
Goss's leaf blight and wilt
Goss's wilt developed this season in fields scattered across Minnesota in northwestern, central, southwestern, and southeastern parts of the state. The disease was noted on small patches in many fields and caused more extensive damage in some fields. It seemed to be more common where hail occurred, but there was no consistent pattern to why it appeared in some fields and not others in areas where there was no hail or severe storms. The use of resistant hybrids is still the main recommended management strategy for Goss's wilt in or near fields where this disease has been seen. Photos and more information are available at http://www.extension.umn.edu/agriculture/crop-diseases/corn/gossbacterialwilt.html.
In the past two weeks we have received many reports of high levels of stalk rot in fields in central and southern Minnesota. As is often the case with stalk rots, the main cause of the problem is difficult to determine. It appears that Fusarium and/or Gibberella stalk rots may be widespread. The potential red root rot with beet-red roots and associated stalk rot that we reported last year (but weren't able to confirm) is also a possible problem. Anthracnose stalk rot is generally common across Minnesota but has not been reported in many fields yet this season. This stalk rot, which causes a nearly black coloration on the outer stalk, is one of the easiest to recognize when it fully develops. We are interested in knowing the cause of the stalk rot problems and whether red root rot is a part of the puzzle. If you have possible red root rot, please consider sending clear whole plant and close-up photos of stalks (inside and outside) and roots with symptoms to me, and then we can then decide which samples may be of greatest interest to look at closely .
Hail and wind damage
Finally, to cap off the challenging crop production year in the Waseca area, 'sudden defoliation syndrome' hit fields in a major way on Sept 20. Hail, wind, and rain caused severe physical damage to corn and soybean in some fields, resulting in nearly complete defoliation and some lodging.