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Corn ears normally remain erect until after physiological maturity (black layer) when the shanks eventually collapse and the ears droop down (Figure 1). Recently, ears have begun to droop in drought-stressed fields that have not reached physiological maturity. Drooping ears suggest a loss of turgidity in the shank due to stress combined with cannibalization.
In response to stress, corn plants will mobilize sugars to fill the kernels thus resulting in reduced sugar content of stalks. This process is referred to as stalk cannibalization and causes disintegration of the pith cells. The weakened stalks are more susceptible to colonization by fungi and to physiological stalk lodging. While this is often called stalk rot, the fungi are primarily colonizing tissues that are predisposed due to any condition that reduces photosynthesis and the production of carbohydrates needed to fill grain and maintain stalk integrity. Physiological stalk lodging is favored by good growing conditions early in the season, followed by stress after pollination. Stresses can include a lack of moisture, nitrogen deficiency, foliar disease, hail damage, and prolonged cool, cloudy weather conditions. Extended periods of dry or wet weather prior to pollination, followed by abrupt changes for several weeks after silking, also can cause poor stalk integrity and occurrence of physiological stalk lodging
Corn stalk rots are caused by fungi that are capable of invading otherwise healthy tissues. Stalk rots tend to be a complex of several disease-causing fungi. It is common for multiple causal organisms to be isolated from a single disease sample. Fields where stalk rot is developing should be identified and targeted for early harvest to minimize grain losses. The following are common stalk rots found in corn.
Symptoms usually occur just before the plant matures. Late in the season a shiny black discoloration develops as blotches or streaks on the stalk surface, especially on lower internodes, (Figure 2). Internal stalk tissue may become dark and soft, extending several internodes. Sometimes a portion of the plant above the ear dies prematurely (top dieback). More often, the entire plant is killed and several internodes are rotted. Lodging typically occurs higher on the stalk than with other stalk rots.
When a corn plant is infected by this disease lower internodes become straw-brown, spongy, and dry. The pith tissues disintegrate, leaving vascular strands intact. White fungal mycelium may appear on the stalk surface. Tiny, dark fungal, structures called pycnidia form just under the stalk surface (Figure 2). The pycnidia are embedded inouter stalk tissue and cannot be easily scraped off.
Infection commonly leads to rotting of roots, crown, and lower internodes. Stalks prematurely dry down and are susceptible to breakage. When stalks are split, a whitish-pink to salmon discoloration may be visible. Disintegration of internal stalk tissues begins at the nodes (Figure 3). Crowns are frequently rotted.
Affected plants may wilt with leaves turning a dull gray-green. The lower stalk softens and becomes straw colored as plants die (Figure 3). Pith tissue disintegrates, leaving only vascular strands intact. The inside of a rotted stalk has a pink to red discoloration. Small, dark fungal bodies called perithecia form on the surface of the lower stalk. These perithecia are superficial and can be easily scraped off.
Fields should be scouted after pollination for visual symptoms of stalk rot such as discoloration on stalks or early drydown. Different corn products and fields with different management practices should be evaluated separately.
When scouting, walk a zigzag pattern through the field and test stalk firmness by squeezing or pinching each stalk a couple of nodes above ground level. Healthy stalks are firm and cannot be compressed. If a stalk feels soft, it may be cannibalized and/or rotted and likely prone to lodging. Check at least 100 plants per field. If more than10 to15% of the stalks in a field are rotted, significant lodging is possible.2
A second method for determining potential stalk lodging is to randomly select 10 plants in a row and push each stalk 45 degrees (or about 5 to 8 inches) from upright. Repeat at 10 different locations within a field. If more than 10 to 15% of the stalks lodge or feel spongy, then the field should be slated for early harvest.3,4
Fields with considerable lodging or elevated lodging potential should be harvested early to help minimize the risk of further lodging and ear rots. Although drying cost is a concern when harvesting wet grain, this expense will likely be a better option compared to potential yield loss due to increased lodging and damage from the ear rots. The following are some harvesting tips to protect yield potential:
Once the grain is harvested, there is still a risk for loss. The following are some tips to help minimize grain storage losses:
Take the time to scout fields, regardless of when they were planted. Identify which fields may develop lodging issues and target these for an early harvest to help prevent potential harvest losses. When selecting corn products for next season it is important to note that ratings for stalk strength, lodging and different diseases vary by product and can be influenced by genetic background, fertility levels, and management practices.
Sources: 1 Corn stalk rots. Dec, 1995. University of Illinois Extension. RPD No. 200 2Munkvold, G. 2002. Time to start scouting for corn stalk rot. Integrated Crop Management. Iowa State University Extension. http://www.ipm.iastate.edu. (verified 9/2/14); 3Vincelli, P. and Herrshman, D.E. Corn stalk rots. PPA-26. University of Kentucky. http://www.ca.uky.edu. (verified 9/2/14). 4Corn stalk rots. Univ. of Illinois Extension. RPD No. 200. December 1995. (verified 9/2/14). 5McNeill S. and M. Montross, M. Corn harvesting, handling, drying, and storage. www.ca.uky.edu. (verified 9/214). Additional sources: Compendium of corn diseases. APS Press. 130911070137