Corn Disease Update - Tamra Jackson-Ziems

 

The 2018 crop season got off to a slow start with cold, wet conditions delaying planting in much of the state. Some seedling diseases developed in early planted corn, but with ample heat units the corn caught up quickly and most of the crop finished ahead of schedule. High relative humidity and rainfall drove the development of several leaf diseases mid-season. Severe weather, including heavy rain and hail, delayed harvest and threatened stalk quality and damaged ears (as well as heavy western bean cutworm pressure). Below are some of the most common diseases observed in Nebraska corn in 2018 and some management recommendations.

 

 

Bacterial leaf streak was confirmed on seedling corn plants in early June 2018 following wet conditions. During the same time period, bacterial leaf streak of wheat, caused by Xanthomonas translucens pv. Undulosa, also increased due to favorable weather conditions. It’s important to remember that the disease on wheat is a different species of bacterium than that on corn, X. vasicola pv. vasculorum, although their symptoms are similar. The incidence and severity of disease appeared to be more severe than that of 2017 and similar to 2016.

 

Figure 1. Confirmed distribution of bacterial leaf streak from corn samples from 74 Nebraska counties (Terra Hartman, UNL).

 

Since 2016, the disease has been confirmed on corn samples from 74 Nebraska counties (Figure 1) at varying incidence and severity. Results from greenhouse experiments on 54 plant species identified 16 crop, weed, and native species that were susceptible to the bacteria, Xanthomonas vasicola pv. vasculorum. Thirteen species developed visible symptoms, including two developing disease in subsequent field experiments. Three additional species did not develop visible disease lesions, but allowed bacterial reproduction internally, potentially serving as sites for inoculum increase and overwintering for later disease development (Table 1).

Gray leaf spot developed earlier than usual, too, being confirmed on samples in the UNL P&PDC in late June following warm and wet conditions that favored the fungus, Cercospora zeae-maydis, causing the disease. Having gray leaf spot develop early and during an outbreak of bacterial leaf streak makes it more difficult to differentiate the two diseases, which is important since fungicides won’t control bacterial leaf streak.

 

Southern rust was confirmed on samples from southeast/south central Nebraska July 20, 2018. The national website http://ext.ipipe.org/ is now being used to monitor the distribution of southern rust in the United States. The pathogen does not overwinter in Nebraska so its spores (urediniospores) must be blown into the area by wind from the south.

 

The fungus needs moisture to germinate and infect, so high relative humidity, rainfall, etc. are expected to hasten disease development. Warm temperatures also favor southern rust development, especially temperatures in the upper 70s to lower 80s F, which are optimal for the fungus, even if they occur during the overnight hours. Cooler and dry conditions may slow disease spread. Identification of the disease in July was somewhat of a surprise since it had not been reported in the southern states yet. This is an important lesson that even low levels of disease in the South can create enough inoculum to potentially be a threat here. Cooler temperatures in the subsequent weeks likely kept the disease severity low in most areas making foliar fungicide applications unnecessary to manage the disease in most areas.

 

 

Stalk rot diseases became evident early following periods of heavy rain. More rain further delayed harvest and compromised stalk quality as stalks further degraded. Various stalk rot diseases were confirmed in samples from cornfields across Nebraska. Stalk rot diseases and the pathogens that cause them are common in cornfields. They can weaken corn stalks leading to stalk lodging, breakage, and harvest difficulties for producers, but can also potentially impact yield if they prematurely kill corn plants.

 

Ears lost due to lodged corn may create volunteer corn issues in the following year(s). It’s important to be aware of stalk rot incidence in fields to better understand the risk they create for harvest difficulties if plants lodge. In addition, knowing which diseases are present in their fields can help producers manage, prevent, and anticipate them in subsequent growing years.

 

Unfortunately, the symptoms that stalk rotting pathogens cause can be misleading and make it difficult to diagnose. For example, some of the earliest symptoms of stalk rot disease may be the discoloration of leaves and rapid plant wilting and/or premature death.

 

Numerous stalk rot diseases  occur in Nebraska and other states. Crop stress anytime during the growing season can lead to the development of some stalk rot diseases. Pay special attention to fields that have one or more risk factors for stalk rot diseases and lodging:

Fusarium Stalk (and Crown) and Gibberella Stalk Rots were the most common ones observed in most areas. Fusarium stalk rot is especially common during damp conditions, but may occur anywhere. One of the most common symptoms is the development of crown rot inside the lower stalk, below the soil line, near where roots attach to the stalk. The pathogen, Fusarium verticillioides, can sometimes be visible as white fungal growth on the outside of stalk nodes.

 

Eventually, the disease may cause discoloration of the inside of stalks to light pink or salmon. Gibberella stalk rot is also a common disease in Nebraska, caused by Fusarium graminearum (syn. Gibberella zeae). Pink to red discoloration inside the stalk is a common symptom of the disease that may also be accompanied by tiny black fungal structures on the surface of the stalk and may be scratched off. This and other stalk rot diseases also may lead to weakening and breakage of stalks at nodes.

 

Charcoal rot, caused by the fungus Macrophomina phaseolina, is one of the few diseases that are more common during drought conditions, and so, is more likely to affect corn in non-irrigated fields or pivot corners. The disease is characterized by the presence of many tiny black, round structures, called microsclerotia, inside the stalk that can give it a gray to black appearance, like charcoal dust. In addition, the fungus has a wide host range and can cause the same disease and symptoms in several other crops, including soybean, sorghum, sunflower, and alfalfa.

 

Anthracnose stalk rot is caused by the fungus Colletotrichum graminicola, which can also cause a leaf disease and is a common cause of top rot in corn. In more advanced stages, the disease can cause the development of black lesions visible on the outside of the stalk and is caused by the fungus.

 

 

Similar to the stalk rot diseases, Fusarium and Gibberella ear rots were the most common ear rot diseases observed during fall 2018. Much of the affected grain experienced wounding earlier in the season caused by hail or insect damage, especially western bean cutworm. Some cornfields had ears with one or more ear rot diseases. Ear rot diseases may be evident by the discoloration of kernels (or sometimes cobs underneath), evidence of fluffy fungal mycelium or spores that may be white, pink, red blue, or green and can be a clue as to which ear rot disease is present. For example, Fursarium and Gibberella ear rot were the most common diseases encountered, although others have been observed, as well.

 

Fungi causing ear rot diseases can continue to grow in storage, adversely affecting grain quality. Drying corn to less than 15% moisture and cooling it will help to slow mold growth for short-term storage, but will not kill the fungi or prevent future growth in the bin. Long-term storage into the warmer months of next year is risky and not recommended for grain affected by ear rot diseases. Long-term storage of grain is best for the highest quality corn because grain molds develop and will likely worsen in the bin with fluctuating temperatures and development of condensation. Continue to closely monitor stored corn for development of crusting and “hot spots” of mold that can occur during times of fluctuating temperatures and condensation in the bins.

 

Some of the common fungi causing ear rot diseases also may produce mycotoxins in the grain that can be harmful to some livestock. Which mycotoxins are present and in what concentrations will determine the risk to various livestock and storage considerations may need to be made when they are present in grain. If you are concerned about mycotoxin concentrations you can collect a sample and submit it to a laboratory for analysis.