Several Corn Diseases Developing Across Nebraska

 

Several diseases, as well as other problems that look like diseases, have been confirmed in corn samples from around the state. Several diseases are difficult to differentiate from each other and from abiotic causes, so it’s a good idea to familiarize yourself with them and confirm your diagnoses before making treatment decisions.

 

 

Figure 1. Bacterial leaf streak lesions.  Look for narrow wavy interveinal streaks that may appear more yellow when backlit.

 

BLS has been confirmed in samples from several counties across Nebraska, Kansas, and Colorado.  Diagnosis can be difficult as the narrow interveinal streaks (Figure 1) look similar to some other diseases, such as the fungal disease gray leaf spot, and some abiotic damage that is also common right now.

 

 

 

Although the disease has been scarce in Nebraska in recent years because of widespread use of disease-resistant  hybrids, the pathogen is still present and taking advantage of wounds created by recent hail and other severe weather events. The large lesions can look similar to the fungal disease northern corn leaf blight, so look for dark freckles near lesion edges and shiny bacterial exudate on the leaf surface (Figure 2). 

 

 

Figures 3a and b. Holcus spot lesions are elliptical and may occur singly (a) or as several (b) and is more common this year in some hybrids.

 

Another bacterial disease, Holcus Spot (Figure 3), has been more common, and sometimes more severe, than in previous years. This disease is rarely a threat to yield and doesn’t require treatment. Some hybrids may be more sensitive than others. Symptoms can resemble those caused by drift of a gramoxone herbicide such as Paraquat.

 

 

Figure 4. Rectangular gray leaf spot lesions usually have smooth, well-defined margins, in contrast to those of bacterial leaf streak (Figure 1).

 

GLS has been confirmed in a few locations in southern Nebraska. The rectangular gray lesions (Figure 4) caused by GLS begin on lower leaves and move up the plant with continued moisture and high relative humidity. Consider its severity and height on the plant (primarily its proximity to the ear leaf and above), as well as hybrid ratings, when making fungicide treatment decisions.

 

 

 

Brick-red to brown rust pustules can develop on the top or bottom leaf surfaces (Figure 5). The common rust fungus can grow well at lower temperatures than southern rust and so it is more common during the first half of the growing season (now). This can lead to confusion because of its similar appearance to the more yield-threatening southern rust that develops mid- to late-season.

 

 

 

The fungus causing NCLB prefers moist, cool conditions. The disease is most common in eastern Nebraska counties and occurs sporadically in central Nebraska. Large, cigar-shaped lesions can be one to several inches long (Figure 6). Susceptible hybrids can benefit from timely fungicide applications if disease becomes severe as it can lead to blighted leaves and yield loss.

 

 

 

NLCS, which causes oval to brown lesions on leaves or in the midribs, is rarely a serious problem in hybrid dent corn. However, it can be very serious in some susceptible inbred lines.  In hybrids the disease is rarely severe enough to treat with fungicides; however, it may be necessary in some seed corn production.

 

 

This disease is normally not severe, except on susceptible hybrids in wet conditions that favor infection. The yellow lesions on the leaf blade may be quite widespread across entire leaves or in bands (Figure 7). The pathogen requires standing water on plant parts, so lesions can develop in a banding pattern across the leaf after they emerge through the plant’s whorl during alternating wet/dry periods. Lesions may develop in both the midrib and on the leaf blade with appear very different. On the leaf blade, the small yellow/brown lesions may be alarming and appear similar to southern rust pustules, but they will lack the colored spores on the surface that can be wiped away. Lesions that develop in the midrib are often larger than those on the leaf blade and are usually black in color. Management of Physoderma brown spot is rarely necessary, although infection can be concentrated at a node, weakening it and leading to later stalk breakage.

 

 

 

 

The common smut fungus is probably present in every field and infects corn through silks or wounds, such as those created by recent hail storms where the disease is developing now on leaves.  The fleshy galls are smaller, wart-like and more difficult to recognize on leaves (Figure 8a), but can occur anywhere on the plant. Smut most commonly occurs on ears (Figure 8b).  Fungicide treatments are not effective in controlling common smut.

 

   

 

 

Other symptoms have also been quite common in fields now, such as those caused by abiotic (non-living) causes.  Many of these symptoms are consistent with scald or sunscald that can develop in some scenarios. For example, during dry soil conditions with high temperatures and high wind scalding can occur on leaves that first become discolored and then bleached (Figures 9a-d).  The affected areas may look like patches of interveinal white striping, too, but lack any pathogens. Tassels may also be affected and loss of functional leaf area may result in more stalk rot later in the season in the same areas of the fields.