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Why Were There Stalk Rot Increases in 2000 and 2001?
by Jim Dodd
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Stalk rot of corn is hardly a new phenomenon. It probably occurred in the earliest of varieties and especially as humans attempted to increase the grain productivity of the species. I suspect that, from the beginning, it occurred erratically, affecting some plants and not others. In fact, this sometimes confusing, pattern of distribution of stalk rotted plants also offers clarity in understanding the biology of stalk rot of corn explanations of specific epidemics of stalk rot.

Research in the 50's and 60's centered on trying to understand its host-pathogen interactions. It became understood that most fungi associated with stalk rot were not aggressive pathogens. It was shown that the stalk pith tissue senesced sooner in plants that developed stalk rot. Research in Canada proved that stalk rot was always preceded with root rot. Many had noted that stresses such caused by leaf diseases, low light, hail damage and drought was associated with stalk rot. A summary of literature describing these factors was presented in 1976 to the NCR-25 committee of corn pathologists. That summary and bibliography is offered in this manuscript for those wishing to study further this foundation of understanding corn stalk rot.

Evidence of the carbohydrate sink size of the grain also influencing the occurrence of stalk rot supported the photosynthetic stress-translocation balance concept of stalk rot (Dodd, 1980). Stalk rot occurs in a plant in which the amount of carbohydrate available to roots is insufficient to maintain viability of root cells. Resistance to the soil microorganisms surrounding roots is dependent upon energy for production of the phenols and other anti-microbial agents needed in all healthy root tissue. Translocation of photosynthetic products are directed in plants by auxins and cytokinins, usually most concentrated in meristematic tissue. Such tissue is found at the tips of roots and stem growing points. Each embryo likewise has a meristem. After flowering, a major shift in carbohydrate flow towards the ear reduces the availability to the root tissue. The kernel number and the rate per kernel determine the strength of the flow in an individual corn plant. Genetics and minerals such as nitrogen determine rate per kernel. If the carbohydrate supply is insufficient to meet the demand of the ear and needs of the root tissue, root cell senescence increases. Eventually the plant's roots are destroyed to the point that insufficient water is moved to the leaves to meet the demands of transpiration, causing the plant to wilt. This causes the familiar "sudden" gray appearance, sometimes referred to as premature death, that always precedes stalk rot. Wilting results in collapse of the pith tissue, pulling it away from the rind. This alone changes the stalk structure from a rod to a tube, weakening its strength by one third. Senescing and dead rind tissue goes through a color change from green to yellow to brown as cells die and are invaded by fungi of the Genera Diplodia, Gibberella, Fusarium, Colletotrichum, Penicillium, Aspergillus, Macrophomina, Cladosporium, Nigrospora, Trichoderma, etc. Cellulase and pectinase enzymes from these fungi further weaken the stalk tissue. Higher temperatures probably reduce the time between wilt and lodging because heat increases the metabolic rate of the fungi. I believe there is no exception to this phenomenon. The process is always the same, although there is one other aspect that probably influenced the 2000 summer epidemic. The individual plant that develops stalk rot did not produce enough carbohydrate to meet the demands of both the grain filling and metabolism of the root. The cause, therefore, is either or both insufficient photosynthesis or over-commitment to the ear.

Photosynthesis in corn is limited by light intensity, effective leaf area, minerals, water and metabolic efficiency. Although each is significant, and obviously interacts, probably the light factor plays the most consistent role in stalk rot predisposition. Photosynthetic rate in corn leaves directly increases with light intensity. A leaf in the shadow of another leaf receives 1/20th the intensity of the fully exposed leaf. High plant density reduces the light per plant, even if the light intercepted per acre is increased. Photosynthesis is also reduced by moisture deficit but this seems to have more of a threshold affect rather than gradual. Plant densities also affect water availability for each plant. Genetics and minerals and leaf diseases, but also plant density affect leaf area.

Grain sink size is determined by number of kernels and carbohydrate per kernel. Kernel number is affected by:

  1. Genetics
  2. Moisture availability 4 weeks previous to and 2 weeks after flowering
  3. Minerals, especially nitrogen

Carbohydrate per kernel is determined by

  1. Genetics for translocation rate
  2. Genetics for duration of grain fill
  3. Minerals especially nitrogen,
  4. Water

An individual corn plant performs with these complex interactions of genetics and environment. Grain farmers attempt to maximize the light energy capture system of corn with their best guess of planting rates, fertilizer applications and tillage systems. Seed companies and agronomists advise them with their knowledge of these areas. It is the attempt to maximize productivity that leads to the occasional stalk rot disasters.

The single most common environmental factor associated with occurrence of stalk rot is high plant density. Each hybrid has an optimum density for a specific environment in which it maximizes grain yield and maintains stalk integrity. Not only do we rarely know that specific density for a hybrid for "average" environments, we never are sure of the summer environment at planting time in the spring. We do know, however, that all hybrids do not have the same optimum density.

Stalk rot in central Illinois in the summer of 2000 was about the worst I have seen over a large area in my 30 years of studying stalk rot. There were fields along highway 47 south of Dwight Illinois that were nearly 100% lodged with rotted stalks. Northern Illinois, however, had many fields with very little stalk rot. Why the difference? There were common environmental between the two areas, but one major difference that I noted. Both had limited early moisture but there was probably a little more reserve in the North. Common Rust caused by Puccinia sorghi, was prevalent from early June throughout the summer for most of the state, but perhaps a little more in the Central section. During August and early September, however, Northern Illinois received more rain than the Central area of Illinois.

Rust diseases have a unique interaction with stalk rot. The pathogen is dependent upon living tissue to obtain nutrition, including energy sources. Each infection establishes it's own sink, effectively competing with the host carbohydrate sinks in the kernels and roots. Multiple infections, such as occurred in 2000, must have added considerably to the reduction in carbohydrate availability to the root tissue.

Plant densities were also contributors. High densities limit the photosynthate per plant by limiting light and water per plant. Moisture stress during August and September n Central Illinois was clearly a major contributor. With less than optimum photosynthate produced, and increased competition for carbohydrate by the rust fungus, the roots senesced early and stalk rot followed. As always, hybrids that tended to establish smaller yields in optimum environments were the ones that had less stalk rot.

Stalk rot in 2001 was much less than in 2000 in Illinois. There were pockets of unacceptable stalk quality. The year did not feature the rust factor. There was sufficient water in the early season to produce high kernel numbers but the late season water stress probably was the greatest cause of early root death. Again, hybrid genetics and plant densities were significant factors.

What about this summer? Again there will be uncontrollable weather factors. Plant breeders, agronomists and seed company representatives can influence also by recommending the optimum plant density for each hybrid. Corn growers can also help themselves by acknowledging that each hybrid, and each field, has a different plant density optimum and adjusting accordingly.

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