Watch for Bacterial Diseases in Corn
June 19, 2009
Holcus Spot, Goss's Bacterial Wilt and Blight
|Figures 1 (above) and 2 (below). Holcus spot has been confirmed in three Nebraska counties and there are other unconfirmed reports in southcentral Nebraska.|
With recent wet, rainy conditions, growers should be on the lookout for bacterial diseases, especially in hail-damaged corn. Holcus spot was recently confirmed in three Nebraska counties — Hall, Polk, and York — and there are unconfirmed reports from several other southcentral counties. While bacterial wilt and blight hasn't been confirmed this season, outbreaks are possible.
Holcus spot is a bacterial disease (Figures 1 and 2) of corn caused by Pseudomonas syringae pv. syringae. The appearance of the spots is similar to that of other diseases, such as the more common fungal diseases, eyespot and gray leaf spot, and can be easily misdiagnosed. However, the spots lack the margins around the lesions typical of eyespot and do not necessarily develop on the lower leaves first, making them different from diseases caused by fungi.
The pathogen also has a wide host range and can develop on many other grassy weeds and crops, making it more likely to be blamed on a chemical burn or drift, because it may be observed on several plant species in the field at once, unlike most other pathogens that have a limited host range.
This disease also may develop in the absence of obvious crop injury because it can also take advantage of natural plant openings, such as the stomata, to gain entry into the plant. So, although your fields may not have been hit with the recent hail and high winds, you may still be at risk for development of this disease.
Holcus spot usually isn't severe and won't require active management. Hybrid resistance is not common. The pathogen reportedly overwinters in crop residue, so tillage, when practical, may help reduce the overwintering bacteria. Crop rotation is of limited value since the pathogen has a wide host range infecting several other crops (such as sorghum and wheat) and grassy weeds.
Goss's Bacterial Wilt and Blight
Although there have been no confirmed cases in Nebraska yet this year, now is a good time to remind everyone of the potential for Goss's bacterial wilt and blight development in corn. The disease occurred in 15 confirmed counties statewide in 2008 to varying degrees of severity leaving behind greater populations of the bacteria that overwintered and are available for infection this season. These bacteria overwinter very well in infected corn residue, and thus increase the risk for disease development this year after last year's widespread occurrence.
The bacterium that causes the disease, Clavibacter michiganensis subsp. nebraskensis, requires a wound for entry into the corn plant. In many cases, infection occurs after a hail storm, however any wound has the potential to act as an entryway. Sandblasting also can create small plant wounds.
After the recent severe weather and the high sustained winds earlier this season over much of the state, the risk for Goss's wilt development is higher than ever.
During the past few years, Goss's Bacterial Wilt and Blight has been the predominant disease of corn in the tri-state region of western Nebraska, northeast Colorado and southeast Wyoming. The disease is still active in these areas, demonstrating the pathogen's ability to thrive in Nebraska. Since both of these diseases are caused by bacteria instead of fungi, we do not expect foliar fungicides to provide effective control, making proper disease identification very important.
In addition, the disease occurs in most states surrounding Nebraska and has now been confirmed for the first time in Indiana in Pulaski County. The disease is likely much more common than we are aware, especially since the pathogen is surviving and active statewide despite the near disappearance of the disease for more than 25 years since the 1970s.
|Figure 3. Goss's bacterial leaf blight with abundant leaf "freckles" (white arrow) and scattered shiny exudates on the surface.|
Accurate identification of the disease is critical for its management. The foliar blight phase of the disease can often be recognized due to the frequent presence of two major characteristics.
Dark green/black "freckles" or flecks often appear near the edge of lesions (Figure 3) and the bacteria are often secreted onto the leaf surface where their exudate or "ooze" gives the leaf a shiny or sparkling appearance once dry. Although fungicides are not effective, numerous seed companies provide ratings for their hybrids' reactions to the disease. Immunity may not be available, but resistant hybrids can substantially reduce the severity of the disease, resulting in higher yields than with susceptible hybrids.
For more information, see the UNL NebGuide, Goss's Bacterial Wilt and Leaf Blight of Corn, G1675.
If you are unsure about the identity of these or other diseases in your field, you can submit a sample to the UNL Plant and Pest Diagnostic Clinic. Instructions and sample submission forms for the Clinic are at Plant Disease Central.
Extension Plant Pathologist
UNL Plant and Pest Diagnostic Clinic