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September 2005 Archives

Corn stalk rots in Minnesota this year

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Dean Malvick Extension Plant Pathologist
Dave Nicolai Regional Extension Educator-Crops
The corn crop generally looks good across Minnesota this year, however, a problem may be lurking in many fields that may reduce yields and set the stage for lodging and harvest challenges. This problem is corn stalk rots, a common problem that is increased by stress conditions in the mid to late growing season. Corn stalk rot has been reported this season in many areas of the state but particularly in areas which suffered from drought stress and/or corn rootworm damage earlier in the season. General information about corn stalk rots and a perspective on this problem for Minnesota are addressed in this article.

Corn leaves changing from to dull green or gray is often an early symptom of severe stalk rot . Wilting, straw or black-colored lower stalks, discoloration and decay of internal pith tissue, and drooping of the ears may occur as the stalk rot develops. Stalk rot shouldn't be confused with innocuous purple leaf sheath, which is seen as irregular-shaped purple to brown discolored patches on and under leaf sheaths caused by fungi and other microorganisms growing on pollen and other nutrients trapped between the stalk and leaf sheath.

Conditions have occurred in Minnesota this summer that may result in increased corn fungal stalk rots. But why may stalk rots be a problem this year in many fields? Plants under stress are more susceptible to stalk rot. The pathogens that cause most stalk rots tend to be widespread fungi that take advantage of plants weakened by various stresses. These may include water stress, susceptible hybrids, root damage, leaf disease, conservation tillage, high plant populations, insect damage, early maturation, low N in mid to late summer, high fertility- especially very high N, and low P and K.

In addition to these potential stress factors, high yield environments can increase stalk rots. For example, large ears require much energy and nutrients to fill the kernels, however, stress conditions that reduce photosynthesis and carbohydrate production by the plants may cause a shift of nutrients from the stalk to the ears, resulting in stalks more susceptible to stalk rots. No-till environments and continuous or short rotations out of corn favor survival and infection by the stalk rot pathogens because the pathogens survive in corn residue on or near the soil surface.

Stalk rot is decay of internal stalk tissues caused primarily by different types of fungi including Colletotrichum (Anthracnose), Giberella, and Fusarium. Charcoal rot and Diplodia stalk rots are important stalk rots that are common in states south of Minnesota, but these as well as Pythium and bacterial stalk rots could also occur occasionally in Minnesota.

Anthracnose stalk rot often appears prior to normal senescence, which is earlier than most other common types of stalk rots. It can cause a top die-back of the stalk which can be confused with early dry-down. In this phase the plant is killed from the top down as the fungus progressively colonizes the stalk. Also, unlike most stalk rots, anthracnose causes development of a black color on the outer surface of the stalk and may cause rot of several internodes. The black, blotchy lesion areas on the stalk surface cannot be scraped away with a thumbnail.

Fusarium stalk rot often occurs late in the season. Yield loss may be more related to corn lodging and broken stalks than poor ear development. Fusarium stalk rot can be distinguished by a whitish-pink to salmon discoloration of the internal stalk tissues. Symptoms of Fusarium stalk rot can easily be confused with Gibberella stalk rot, which produces a pink to reddish discoloration of the internal stalk tissues. Rotting commonly affects the roots, crown and lower internodes. A diagnostic sign of Gibberella is the presence of small, black specks (perithecia) on the surface of the stalk rind and frequently clustered near the nodes. The perithecia may be easily scraped away from the rind tissues with the thumbnail. The fungus that causes Gibberella stalk and ear rot also can cause head scab of wheat and barley.

Consider scouting fields for stalk rots. Test 20 plants in five different parts of a field with the "pinch" or "push" test. For the "pinch test", the lower internode will easily compress when pinched firmly. For the "push test", stalks will break or remain bent over when pushed 10" to the side at ear height. If 10-15% of plants in a field have stalk rot, then the potential for significant lodging is high and early harvest should be considered.

Stalk rot damage is difficult to manage but can be minimized by reducing as many stresses on the corn crop as possible throughout the growing season and by harvesting early to minimize losses. Corn growers should select hybrids that have stalk rot and leaf disease resistance, good standability, and high yield potential. Balanced soil fertility, control of corn borers and corn rootworms, and appropriated plant populations, as may be suggested with particular hybrids, are also important in reducing stalk rots.

Wet conditions lead to harvest delays

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Lizabeth Stahl
Regional Extension Educator-Crops

Soggy fields and submerged crops can be found in areas throughout the state due to recent rainfall events. What effect will these late-season wet conditions and even flooding have on corn and soybeans as we wait for field conditions to improve and harvest to resume?

Wet soil conditions this spring, pressure from corn rootworm and European corn borer, and plants attempting to a fill large number of kernels per ear have set some fields up for potential stalk rot problems. Although some of the fungi causing stalk rots are a player in the natural process of recycling nutrients and organic matter back into the soil, decay of stalk pith prior to harvest can increase lodging potential. Harvest delays increase the chance of lodging due to stalk rots and lodged plants decrease harvestability, leading to ears being left in the field.

To check for stalk rot, stalks can be split (check for stalk tissue disintegration) or squeezed between your fingers (check if stalks are easily crushed) or pushed from vertical (severely stalk rotted plants will kink or lodge). Fields with stalk quality issues should be targeted for harvest as soon as is feasible because of increased lodging potential. Fields with heavy infestations of European corn borer should also be targeted for harvest as soon as is feasible.

Corn grain quality can also be a concern when wet conditions delay harvest. Be on the lookout for ear rots and if grain becomes molded, be sure to test for mycotoxins prior to feeding to livestock.

In soybean fields where pods were submerged by standing water for a significant time period, seed quality is a concern. Wet conditions combined with warm temperatures increase the risk of damage from fungal pathogens. If areas were submerged for several days, stems may weaken or rot, increasing the chance of lodging and harvest losses. Prior hail damage or previous stem diseases may predispose plants to further injury. Pod shatter prior to harvest is also of concern, especially if soybeans go through several cycles of drying and re-wetting.

If soybeans were submerged in areas of the field, consider segregating these potentially lower quality soybeans. Segregation may be particularly beneficial if soybeans were intended to be sold to a specialty market where premiums are based on soybean quality.

Delayed harvest will also influence the ability to conduct fall tillage operations and fertilizer applications. The main focus is, of course, to get crops out of the field. As field conditions improve and harvest resumes, keep an eye on crop status when deciding which fields to harvest next to help maximize crop quality and yield.

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