Update On Bacterial Leaf Streak Of Corn In Nebraska

 

 

 

Bacterial leaf streak disease of corn is caused by the bacterium Xanthomonas vasicola (syn. X. campestris pv. zeae). It was previously only reported on corn in South Africa, and had not been reported in the United States until August 2016.

 

 

This bacterium is most noted for causing gumming disease, leaf blight, and a vascular wilt that affects sugarcane. The vascular wilt phase has not been observed in corn.

 

Bacterial leaf streak has been confirmed in Nebraska, Colorado, Kansas, Iowa, Minnesota, South Dakota, Illinois, and Texas. A 2016 survey funded by the Nebraska Corn Board confirmed that bacterial leaf streak was present in at least 51 Nebraska counties (Figure 1) across much of the state. The pathogen was not detected in samples from some extreme eastern and western counties. These results don’t necessarily indicate its absence, but rather may be due to a lack of representative samples from those areas, especially considering the disease has been confirmed in several adjacent counties in neighboring states.

 

 

 

Symptoms include yellow, tan or dark to light brown striped lesions with wavy margins that can appear on the leaf blade (Figures 2 and 3) or be concentrated around the midrib (Figure 4). These lesions can appear yellow when backlit (Figure 5). Symptoms have been observed on the lower leaves as early as the beginning of June in Nebraska on V7 corn, and tend to move up the plant as the infection progresses. While this seems to be the normal pattern for disease development, there have been cases where symptoms first appeared in the mid-canopy or upper leaves during mid-season.

 

Bacterial leaf streak and gray leaf spot are often mistaken for each other due to their similar appearance and behavior. There are two key differences to look for when trying to distinguish between these diseases:

 

First, consider the time of year when the symptoms develop. If the symptoms first developed in June, it is likely bacterial leaf streak, as cooler weather conditions in Nebraska are not often favorable for gray leaf spot development.

 

Second, examine the margins of the leaf lesions. Bacterial leaf streak typically has a wavy margin, whereas gray leaf spot lesions usually have smooth, linear margins (Figures 6 and 7).

 

While these considerations can be useful, neither can be used exclusively to diagnosis these diseases every time, especially when both diseases are present on the same leaf.

 

Be sure to observe many lesions on several plants and always submit a sample for diagnosis if you’re unsure of a diagnosis or if you suspect the bacterial leaf streak has developed in a new area.

 

 

 

Xanthomonas vasicola is capable of overwintering in infested residue, where it can survive until environmental conditions become favorable for it to infect the next year’s crop. This bacterium does not require wounds to establish an infection, and it is believed to enter the plant through natural openings, such as the stomata.

 

Center pivot irrigation and wind-driven rain may increase the severity of infection by creating conditions that allow the pathogen to enter the plant’s natural openings, such as stomata. Bacterial leaf streak has been confirmed in dent (field) corn, popcorn, seed corn, and sweet corn.

 

 

This pathogen is favored by warm, humid conditions, and is thought to be spread by wind-driven rain.

 

 

When making management decisions, it is important to verify the diagnosis. Bacterial leaf streak and the common fungal disease, gray leaf spot, can appear very similar in some hybrids and the fungicides used to treat gray leaf spot are not expected to be effective against bacterial leaf streak.

 

Resistance to the bacterial leaf streak pathogen has not been confirmed in corn, so resistant hybrids are not available. Crop rotation to a non-host crop may help reduce disease severity in future corn crops, however disease development has been observed after one-year rotations to soybeans, wheat, and after fallow. Rotation alone won’t prevent bacterial leaf streak.

 

Tillage may reduce disease severity by promoting degradation of the infested crop debris source, but it will not eliminate the disease. Cleaning equipment when moving from an infested field to a disease-free field may slow the spread of the disease. These are standard practices for mitigation of bacterial diseases, but none of them will eliminate the pathogen and some may be impractical in some production systems.