Dryland Corn Newsletter - CO Corn Diseases

Update on Prevalent Colorado Corn Diseases

Proper variety selection, seed treatment, and rotations can prevent most serious disease problems.

Corn diseases are not normally a problem under Colorado corn growing systems. In recent years the occurrence of gray leaf spot (Cercospora zeae-maydis) has increased in eastern Nebraska and eastern Kansas and is being monitored. As of now, this disease poses no problem for Colorado growers. Other foliar diseases are not a problem, and, if found, usually appear so late in the season that, for all practical purposes, they cause no significant loss of yield or quality.

In the past, virus and virus-like diseases have rarely been found. In the case of the new High Plains Disease, little to no threat to field corn production is posed. If it does occur, it is easily managed with variety selection and other cultural management tactics.

There are three groups of problems that on occasion can cause concern and some damage in corn in Colorado. These are: plant emergence failure (damping-off), stalk rots, and smuts.

In some years, poor stands have resulted from failure of plants to emerge or seedling death. While frequently diagnosed as damping-off, such problems are not always a problem caused by damping-off fungi. Both soil moisture and temperature can be involved, as well as compaction and/or soil surface crusting. Damping-off fungi can have an effect if seed is planted early, and exceptionally wet and cool weather develops. Usually the water mold type fungi, Pythium species, and/or Fusarium fungi are associated with seed rot and damping-off. Some of this can be prevented with appropriate seed treatments. But a seed treatment is not the only solution. Proper seedbed preparation, moisture and time of planting are critical. Planting into a dry seedbed and then trying to irrigate the corn up can also cause problems with crusting and, in some parts of fields, rotting due to excessive water and or poor drainage.

There are actually several stalk rot fungi and at least one bacterial stalk rot found in Colorado. In some instances, any one of these can cause severe damage and yield loss. The most important stalk rot diseases and their causal agents are:

Fusarium stalk rot
[Fusarium moniliforme, Fusarium subglutinans]

Gibberella stalk rot -
[Gibberella zea (Fusarium graminearum)]

Charcoal stalk rot
[Macrophomina phaseolina]

Bacterial stalk rot
[Erwinia spp]

Goss's wilt
[Clavibacter michiganense subsp. nebraskense (syn. Corynebacterium nebraskense)]

While resistant varieties are available for most of the stalk rots, there are times when a particular situation gives raise to a stalk rot problem that was not expected.

	Fusarium stalk rots are always found to some degree. Usually the amount of damage is associated as much with cultural practices as with varieties. Crop stress is a critical factor in predisposing corn to damage from the Fusarium stalk rot fungi. In many instances, corn can support a certain level of Fusarium invasion without showing a significant loss of yield or quality.
	Gibberella stalk rot is potentially the most damaging under Colorado conditions. Fortunately, it does not consistently develop and, therefore, is not an annual problem. While proper rotations, variety selection and stress management will minimize damage, current corn growing practices do not utilize rotations as effectively as possible.
	Charcoal stalk rot is generally found only in the southern part of the state, but, in some years, it has been found in dryland corn on the plains. It is a heat and stress driven disease, and appropriate stress management practices will help to minimize damage.
	Goss's wilt is always associated with highly susceptible varieties, lack of rotation and low to minimum tillage. In the last couple of growing seasons, this disease is being reported on the increase and, in some instances, in varieties that historically have been considered resistant to the pathogen. Stalk rot management depends on variety selection, rotation and stress management. No pesticides are currently labeled for this problem.
	There are two smut diseases of corn in Colorado. Common smut caused by Ustilago maydis.
	Head smut caused by Sphacelotheca reiliana.

Bacterial stalk rot is only found when temperatures are high during the night and fields are heavily fertilized or heavy manure applications have been made. This disease has only been found in a few fields a couple of times over the last 15 years and is not a major problem over a wide area.

The two smut diseases are very different, both in the way they attack the plant, and in the way the plant is affected. Common smut infections come from wind borne spores and infect the plant locally through wounds. Common smut is not systemic or seed borne. Head smut infections in contrast, come from soil borne spores and develop systemically through the seedling into the mature plant.

Common smut infects any part of the plant, while head smut shows only in the tassels and the ears. Head smut, if infection occurs early, will also cause considerable stunting of the plant.

Smut management generally depends on resistant varieties. Certain cultural practices will help to minimize damage but are not always successful.

In 1997, a unique fungus stalk rot disease was found in the southeast part of the state. Pyrenochaeta stalk rot, caused by Pyrenochaeta terrestris, causes shallow, dark brown, blotchy lesions that blend with reddish areas as the plant matures. These lesions commonly are found at the base of the stalk and frequently below the soil line. Mature lesions will have very small dark pepper-grain-sized fungal bodies. The disease is considered of little importance, and no specific management tactics are recommended.

Corn diseases under Colorado conditions are readily managed with rotations, variety selection, clean seed selection, seed treatment, tillage, and stress management.

In most instances, conservation tillage poses no major disease problem to corn production under Colorado conditions. Proper variety selection, seed treatment and rotations can prevent most potentially serious problems.

Bill Brown and Tamla Blunt
Professor of Plant Pathology & IPM Coordinator and Graduate Student