September 6, 2013

Survey fields now and prioritize harvest and storage accordingly

Corn in parts of Nebraska has been damaged during recent weeks by hail and and stressed by very hot, dry conditions. These events and conditions can potentially lead to serious problems with grain quality during harvest and storage. Corn fields have sustained varying levels of damage. Plants that were impacted by hail likely have wounds on leaves, stalks, and ears where pathogens can infect the plant and cause diseases (Figure 1).

Some of these diseases and mycotoxins might have been exacerbated during the recent heat wave. The pathogens that cause stalk and ear rot diseases (Figures 2a and 2b) are common in fields and have probably already infected wounded plants. The severity of these diseases will depend on the extent of wounding and infection, as well as weather conditions and hybrid genetics.

As farmers approach harvest, the development and extent of these diseases should be determined in high risk fields so decisions can be made regarding field harvest order and how to best handle grain to minimize potential impacts of these diseases. Scouting is recommended prior to harvest to determine the extent of damage caused by stalk and ear rot diseases.

Stalk Rot Diseases

You can determine the risk for lodging and stalk rot disease incidence in fields using the “Push or Pinch Test.” While walking through fields, randomly select at least 100 plants that represent most of the field area and either:

• Push the plant tops approximately 30° from vertical . If plants fail to snap back to vertical, the stalk may have been compromised by stalk rot and is at risk for lodging prior to harvest.
   
• Pinch or squeeze the plants at one of the lowest internodes above the brace roots. If stalks crush easily by hand, their integrity has been reduced by stalk rots, making them more prone to lodging.

Fields with elevated risk for lodging should be harvested before less affected fields to minimize the chance of lodging and other complications during harvest.

Ear Rot Diseases

Ear rot pathogens will likely continue to grow during storage of infected grain and lead to grain mold that can seriously reduce grain quality and lead to major deductions at the elevator. Even under the best growing conditions, grain molds will usually continue to grow and, in some cases, may spread through the entire bin. Losses of up to 30% have been reported in south central Nebraska in recent years when grain was removed from storage, especially after a mild winter. Even during cold winters, remember that conditions inside the bin may be very different from its surroundings. Temperatures inside the bin may take several weeks to stabilize and condensation may develop, adding unwanted moisture that promotes fungal growth.

Steps you can take to minimize losses due to ear rot diseases and grain molds:

• Remove old grain from empty bins because it harbors fungi that can infect new grain.
• Avoid further damage to kernels during harvest and handling.
• When possible, avoid storing grain from fields with a high incidence of ear rot diseases, or at least avoid mixing affected grain with healthy grain.
• If storage is necessary, store for a minimum amount of time.
• Cool grain as quickly as possible after harvest.
• Dry grain to less than 15% moisture within 48 hours of harvest to slow further growth of fungi, especially with longer term storage.
• Continue to stir and aerate grain bins during storage to prevent the development of hot spots.

Aspergillus Ear Rot

Aflatoxin is the best known mycotoxin in Nebraska and is produced by the fungus that causes Aspergillus ear rot. At this time there have not been any reports of aflatoxin in the 2013 crop. Hot, dry weather during the latter half of the growing season after pollination especially favors aflatoxin production. Drought-stressed corn, such as that in non-irrigated fields and non-irrigated pivot corners, is especially vulnerable to the accumulation of aflatoxin. Aflatoxin is toxic and carcinogenic to humans and livestock. The FDA has suggested action levels for aflatoxin concentrations, ranging from 0.5-300 parts per billion (ppb), depending on its end use.

Fusarium Ear Rot

Fusarium ear rot is a common disease in hail-damaged corn. Fusarium may infect any part of the ear and take advantage of wounds created by insects or hail. The species that cause this disease also can secrete a mycotoxin called fumonisin into the grain. This mycotoxin is carcinogenic, but is not measured at concentrations as low as that of aflatoxin (up to 50 ppm). Fusarium ear rot is favored by a wide range of environmental conditions and can be recognized by its scattered tufts of mold on the ears that may be white to pink in color and may be accompanied by starburst patterns on the kernels.

Diplodia Ear Rot

Diplodia (also called Stenocarpella) ear rot is a common disease in the Corn Belt. The fungus that causes this disease does not usually produce a mycotoxin in the United States, but can significantly reduce grain quality. Extensive fungal growth usually begins at the base of the ear and can overtake the entire ear, creating a lightweight mummified ear. In addition to these symptoms, it produces small raised, black fungal reproductive structures on infected kernels and stalks that give it a rough feeling when touched, similar to sandpaper.

Penicillium Ear Rot

Another grain mold, Penicillium, can cause discoloration of the embryo known as “blue eye” and produce penicillic acid, which is not usually a toxigenic concern. This disease is particularly a storage problem and is favored by high moisture levels in grain bins. The disease is managed by reducing insect wounds in the field and maintaining low moisture while the grain is in storage.

Grain Handling and Management During Storage

Ear rot pathogens will usually continue to grow to some extent during grain storage. However, taking extra steps can minimize fungal growth and contamination of unaffected grain and potential deductions at the elevator. Drying grain to less than 15% moisture (for longer storage) or at least to less than 18% (for shorter term storage) is necessary to slow mold growth.

In addition to storage in bins, storing the grain in bags for silage, earlage, etc., to promote fermentation will only hasten the growth of these fungi when grain moisture is high and temperatures are still warm. Eventually, once oxygen is depleted and fermentation begins, it may indeed slow mold growth, but the process may take several days to weeks to do so, during which time the environment inside the bag can act as an incubator for fungi, allowing them to grow even more rapidly than they would inside a bin.

Identification and Management Resources

Identification of ear rot diseases can be made by the UNL Plant and Pest Diagnostic Clinic. Sample submission forms and more information can be found at Plant Disease Central.

• Common Stalk Rot Diseases of Corn (EC1898).
• Corn Disease Profiles II. Stalk Rot Diseases of Corn (EC1868). Photos and brief management notes. 
• Corn Disease Profiles III: Ear Rot Diseases and Grain Molds (EC1901)Corn Disease Profiles III: Ear Rot Diseases and Grain Molds (EC
• Major Fusarium Diseases on Corn, Wheat, and Soybeans in Nebraska (G2181).

Tamra Jackson-Ziems
Extension Plant Pathologist