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Foliar Diseases of Corn Developing Across Nebraska (Jul 11, 2011)
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Foliar Diseases of Corn Developing Across Nebraska


Bacterial Diseases

Goss’s Bacterial Wilt and Blight


Goss's wilt in corn
     Goss's bacterial wilt and blight

Goss’s bacterial wilt and blight has been reported on samples submitted to the UNL Plant and Pest Diagnostic Clinic from across the state. The disease occurred statewide last year 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, can take advantage of wounds for entry into the corn plant. Infection often occurs after hail, however any wound has the potential to act as an entryway into the plant. The severe weather that has occurred across Nebraska over the last few weeks has created wounds in corn plants that predispose the crop to disease development.

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 to avoid making unnecessary fungicide applications. In addition, the disease occurs in most states surrounding Nebraska and has now been confirmed as far north as Ontario and Manitoba, Canada and as far south as the northern panhandle of Texas.

For more information, see the UNL NebGuide, Goss’s Bacterial Wilt and Leaf Blight of Corn, G1675.

Identification and Management.
Accurate identification of the disease is critical for its management. The foliar blight phase often can 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 1) and the bacteria are often secreted onto the leaf surface where their exudate or “ooze” gives the leaf a shiny or glossy appearance once dry. Although fungicides are not effective, numerous seed companies provide ratings for their hybrids’ reactions to the disease. Although there does not appear to be immunity to the disease, resistant hybrids can substantially reduce its severity, resulting in higher yield compared to that from susceptible hybrids with the disease.

Holcus Spot

Holcus spot of corn              Holcus spot of corn



Holcus spot is a bacterial disease (Figure 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 observed with 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. This makes it more likely that its damage with be blamed on a chemical burn or drift since 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 takes advantage of natural plant openings, such as the stomata, where it can gain entry into the plant. Although your fields may not have been hit by the recent hail and high winds, they may still be at risk for this disease.

Active management of this disease is not normally necessary since it is usually not severe. Hybrid resistance is not common. The pathogen reportedly overwinters in crop residue, so tillage, when practical, may help to reduce the overwintering bacteria. Crop rotation, however, is of limited value since the pathogen has a wide host range infecting several other crops (such as sorghum and wheat) and grassy weeds.

Chocolate Spot

In addition, another bacterial disease -- chocolate spot -- has been reported again this year in western and southwest Nebraska. This disease has a similar appearance to Goss’s wilt on the leaves, but has been a minor disease and has historically occurred in Wisconsin fields deficient in potassium. In the past, this disease has subsided with the onset of hot temperatures.

Fungal Diseases


Gray Leaf Spot


Gray leaf spot of corn
          Gray leaf spot of corn

During this past week, gray leaf spot has begun to develop on the lower leaves of some corn hybrids. With the warm conditions and high humidity, we can probably expect the gray leaf spot severity to continue to increase and development of gray leaf spot lesions (Figure 3) further up the plant.

This disease occurs to some extent each year in Nebraska corn. It is caused by the fungus Cercospora zeae-maydis which overwinters very effectively in the infected crop residue from previously years. The fungus causes the development of rectangular gray lesions on the lower leaves first. Fungal spores, called conidia, are produced in the crop residue and splash up to the lower leaves of plants.

If rain or irrigation water is available or there is high canopy humidity (over 90%) for at least 12 hours, these spores may germinate and infect the plant. The optimal temperature range for infection is 70-90ºF, which is consistent with our recent weather conditions in Nebraska. After infection, lesion development and further spore production on the leaf takes 14-28 days, depending on weather conditions and hybrid resistance. The infection cycle repeats as long as favorable conditions persist and new spores are splashed further up the plant, causing new infections. Severe infections can eventually result in severely blighted leaves compromising grain fill by as much as 50% in severe cases.

Management


Fungicides. Since the ear leaf and those above it contribute the most to yield (approximately 70%), it is especially important to protect those leaves. Fungicide applications may be necessary to slow disease progression and should be seriously considered where the disease has reached the ear leaf, especially in susceptible hybrids. Corn should be scouted regularly to monitor for the development and progression of gray leaf spot and other diseases prior to fungicide applications. Accurate diagnosis is critical, as other diseases are also currently present in Nebraska that cannot be managed with fungicides, such as Goss’s bacterial wilt and blight and holcus bacterial leaf spot. Care should be taken to prioritize the fields in the greatest need of fungicide applications, considering disease severity, hybrid susceptibility, weather conditions, disease history, and other high risk factors, such as continuous corn.

As always, read and follow product label guidelines prior to fungicide use and consult the most current application recommendations from the manufacturer to minimize the risk for potential phytotoxic effects to corn.

Resistance. Resistant hybrids and ratings for gray leaf spot reaction are available from more than 60% of the companies in Nebraska. Although resistant hybrids may still develop disease, resistance slows the progression of the disease by limiting lesion development and slowing advancement up the plant. Resistant hybrids may provide enough protection to delay fungicide applications to some extent or even to avoid them all together.

Cultural. Crop rotation and tillage, when practical, can help reduce the overwintering inoculum and reduce disease severity in subsequent corn crops.

For more information about gray leaf spot and management with foliar fungicides, see the UNL Extension NebGuide, Gray Leaf Spot of Corn (G1902).

Eyespot


Eyespot of corn
              Eyespot of corn

Eyespot of corn, caused by the fungus Kabatiella zeae, recently was reported and confirmed in southwest and northern Nebraska. Prolonged periods of high humidity and cool temperatures earlier in the season favored the disease. Damage is seldom severe, but when severe, it can cause plant death. If it develops early in young plants, it can cause yield losses up to 9%. This pathogen requires cooler temperatures, which is why we normally see it as a short-term early season disease in Nebraska.

Symptoms of this disease are characterized by the development of small circular tan lesions (1-4 mm in diameter) that are surrounded by a brown or purple margin and a yellow, translucent halo that can be seen when held up to the sunlight (Figure 4). This coloration gives lesions the appearance of eyespots.

The appearance of these lesions will vary by hybrid. Lesions generally develop on lower leaves first, and can fuse to cover the entire leaf. The pathogen survives on corn debris, so reduced tillage and planting continuous corn creates a high-risk environment for development of eyespot, especially if there is heavy residue and a history of it in that location. Lesions can develop quickly under favorable weather conditions, in as little as 4-10 days. (In comparison, it may take two to three weeks for visible lesions to develop in gray leaf spot.) Producers and consultants should continue scouting to monitor the development and spread of this and other diseases.

Eyespot can be effectively managed with fungicides, but are usually not necessary. This disease is favored by cool, humid weather. Recent development of hotter weather will help slow its progression and eliminate the need for a fungicide. Hybrids with resistance to eyespot are available and should be considered when you plant corn in a location with a history of substantial disease.
Plant and Pest Diagnostic Clinic

If you are unsure about the identity of these or other diseases, you can submit samples to the UNL Plant and Pest Diagnostic Clinic for analysis. Instructions and sample submission forms can be found at Plant Disease Central.

Source: University of Nebraska–Lincoln


 
 
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