June 25, 2010

The severe weather that Nebraska has experienced during recent weeks presents special challenges to crop production. Specifically, there are numerous corn diseases that we can anticipate showing up after the flooding (Figures 1 and 2), damaging winds, and hail. Several diseases are already being reported around the state.

Goss’s Bacterial Wilt and Blight

Corn that was wounded during recent hail and high winds may have been infected with Goss’s bacterial wilt and blight. The disease has re-emerged statewide during recent years and has become the worst disease of corn in some areas of the state, particularly the Panhandle and southwest Nebraska where we are already receiving unconfirmed reports of its development.

It is important to make an accurate diagnosis of the disease because foliar fungicides will not control it. Currently, our most effective management strategies are crop rotation, resistant hybrids, and tillage to promote the breakdown of residue where it overwinters.

Two key diagnostic features are the presence of dark green to black “freckles” near the edges of lesions and eventually a shiny ooze (Figures 3 and 4) on the surface of the plants. However, if the disease develops early, it may spread systemically throughout the plant and kill it quickly (Figure 5). The disease is also already suspected in southcentral Nebraska in fields where plants were damaged during recent high winds. For more information, please see the UNL NebGuide, Goss’s Bacterial Wilt and Leaf Blight of Corn.

Anthracnose Leaf Blight

Reddish brown anthracnose leaf blight lesions are common on the lowest leaves of corn plants. At this time, the disease does not present a serious threat to the plant or yield, as most of these lowest leaves will be lost, but the same pathogen may cause more extensive damage later in the season as a foliar disease or stalk rot if it continues to progress. Warm conditions and high humidity will support further infection.

Diseases in Flooded Conditions

Saturated soils and flooded conditions promote a number of diseases in corn that producers and crop consultants should watch for now and later in the season. One easily recognized disease is crazy top of corn, which causes a leafy proliferation of the tassel (Figure 6) and sometimes the ear. The pathogen requires standing water for spores to swim and infect plants systemically. There are no rescue treatments, but it rarely causes substantial yield loss.

Stalk rots (including bacterial stalk rot) and lodging are common in areas that were flooded. Revisit these areas throughout the season to monitor stalk quality and prioritize fields for harvest to avoid losses due to lodging. One reason why stalk rots tend to develop in corn that was previously flooded is because of the loss of nitrogen during heavy rain and flooding (See Unknown Soil N Losses: What to Do?). Nitrogen deficient plants often utilize more resources in the stalks during grain fill, leaving them weakened and prone to disease and lodging. For more information about stalk rot diseases, see the UNL Extension Circular, Common Stalk Rot Diseases of Corn.

Other Diseases

We have received unconfirmed reports of bacterial chocolate spot and eyespot in southwest Nebraska, neither of which are expected to cause serious problems considering the short-term weather forecast for hot, dry conditions that will limit the spread of these diseases.

Holcus bacterial leaf spot is also developing in southern counties and shouldn’t be confused with some of our more damaging fungal leaf diseases, such as gray leaf spot.

Tamra Jackson
Extension Plant Pathologist