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Corn disease update: NCLB, rust, and a disease impostor
Author: Travis Faske, Extension Plant Pathologist

This year’s crop was planted later than in 2012, which has many folks wondering if corn diseases will be a problem.  Overall, the corn disease pressure is relatively low across the state, but there are a few diseases that may be of concern depending on individual planting dates.  Thus, producers and consultants need to be aware of current disease issues, potential disease threats from other regions, and what is being mistaken for a disease.

Northern Corn Leaf Blight (NCLB) has been relatively easy to find in several early planted corn fields in SE Arkansas.  Most of these fields have a disease incidence and severity of less than 1% (less than one lesion per plant and less than one lesion among several plants in an area); however, there have been a few isolated cases with a moderate amount of the disease observed in the field.  NCLB is relatively easy to identify by the large (2 to 4 in.) gray green, cigar-shaped lesions (Fig. 1).  The fungus that causes this disease overwinters on corn residue so disease severity is typically higher in corn followed by corn cropping systems, such was the case in the fields mentioned above with a higher disease severity.  Some hybrids have good resistance and are highly recommended in these cropping systems.  Fungicide are effective at managing NCLB, but determine when to apply depends on corn maturity, weather forecast, risk of other diseases (see southern rust below) in addition to current disease severity in the field.  A 3% yield loss can occurs with a 10% loss in total leaf area.  For a reference, given the average size of a NCLB lesion and the total leaf area of a corn plant, if there was one lesion on each leaf the disease severity would be ~1.5%.   NCLB is favored by moderate temperatures (64 to 81° F) with frequent light rainfall or heavy dew events.  Typically, the hot summer conditions in the state are unfavorable for NCLB, so the threat of NCLB was likely higher on earlier planted corn when conditions were cooler rather than later planted fields.

Figure 1.  NCLB lesions on upper (top) and lower (bottom) surface of a corn leaf.

Figure 1. NCLB lesions on upper (top) and lower (bottom) surface of a corn leaf.

Common rust can be found in almost every corn field.  Common rust pustules are a darker reddish-brown whereas southern rust pustules are lighter reddish-orange in color (Fig. 2).  Common rust is frequently found below the ear leaf whereas southern rust is initially found on leaves near or above the ear leaf. Southern rust pustules develop on the upper leaf surface; in contrast, common rust pustules develop on upper and lower leaf surfaces. Common rust tends to slow down in the hot summer months and resistance is common in hybrids; thus, treatments are typically unnecessary in field corn as the threat of common rust to impact yield is very low.

Southern rust was found last year in mid-June along the I-40 corridor, but as of this update, there has been no report of southern rust in Arkansas.  However, southern rust has been reported in Mississippi and Louisiana (near Morganza) so this disease does have the potential to be of concern, especially if it occurs in the field at or before tasseling.  Spores progress northward throughout the growing season, so pay close attention to future updates.  Conditions that favor disease development consist of high temperatures (80 to 90’s) and frequent rainfall or heavy dew events.  Like all corn diseases, determining when to apply a fungicide depends on corn maturity, weather forecast, hybrid susceptibility, and yield potential.

Figure 2.  Southern rust compared to common rust on corn.

Figure 2. Southern rust compared to common rust on corn.

Purple sheath stain is one disease impostor that is easy to find after pollination.  This impostor has a close resemblance to sheath blight in rice with a large (2 to 4 in.) light brown lesion and a darker purplish-brown margin (Fig. 3).  This discoloration or staining occurs when non-pathogenic bacteria and fungi develop on pollen lodged in the collar and/or sheath.  There is no treatment as this is not a disease.

Figure 3.  Purple sheath stain on sheath and stalk of corn. Photo courtesy of John Smith.

Figure 3. Purple sheath stain on sheath and stalk of corn. Photo courtesy of John Smith.



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