Plant Pathology
Factsheet

PPFS-AG-C-1

DISEASES OF CONCERN IN CONTINUOUS CORN

Although most corn in Kentucky is planted following a rotation to other crops, individual producers are often interested in planting corn following corn. In these situations, one of their main concerns is increased pressure from diseases, and rightfully so. Crop rotation is one of the most fundamental disease control practices available. Rotating to other crops deprives pathogens (disease-causing microorganisms) of a food source and exposes them to starvation. Furthermore, as infested crop residues decompose, pathogens are exposed to antagonism by native soil microbes. These mechanisms have the effect of naturally eradicating many pathogens from the soil.

Numerous diseases can be more active under continuous corn, particularly those caused by pathogens that survive in crop residue or in the soil. Three are important enough under Kentucky conditions that they deserve specific management attention.

Gray Leaf Spot

This disease is a significant threat wherever corn is grown after corn. The fungus survives between growing seasons in residues of corn leaf blades and sheaths. From there, it is spread by wind and rain to leaves of the new corn crop. Tillage practices can reduce levels of inoculum (spores or other pathogen structures that initiate disease), but rotation is a key management practice, as well. Thus, in the absence of crop rotation, susceptibility to gray leaf spot should be carefully considered when selecting a hybrid.

Hybrids exhibit differing levels of partial resistance to gray leaf spot. Partial resistance is the most common type of disease resistance in field crops. Varieties with partial resistance are those that, under uniform conditions, exhibit less disease than some standard cultivar or host line. Gray leaf spot will still develop on a partially resistant hybrid, but it typically is slower to develop and less severe. For example, compared to a fully susceptible hybrid, a hybrid with partial resistance will usually have fewer lesions for a given spore load, and those lesions will be smaller. This reduces the impact of the disease on crop development and yield.

When growing continuous corn, always select hybrids with as high a level of resistance as you can against gray leaf spot. This is particularly important in fields under conservation tillage, in which corn residues provide high amounts of inoculum. Susceptible hybrids growing in a no-till, continuous corn situation will sometimes benefit from application of a fungicide. For a susceptible hybrid, a single application sometime between the V10 (about head height) and V14 stage will often be beneficial, if scouting indicates the disease is present at least halfway up the plant. Specialty corns susceptible to gray leaf spot are especially likely to benefit from a fungicide application in a no-till, continuous corn situation, because of their inherently higher crop value. Although products containing mancozeb (Dithane, for example) are labeled for gray leaf spot, best results can be expected with Quadris (azoxystrobin) or Tilt (propiconazole). Expect best results from a fungicide when applied using a ground rig fitted with a single nozzle over the row and two drop nozzles straddling each row.

Diplodia Ear Rot

Although most corn fields do not experience notable amounts of this disease, Diplodia ear rot can occasionally cause severe epidemics, causing rot on as many as 50-75% of the ears in a field. The fungus that causes the disease only attacks corn, and it survives between seasons in residue of corn stalks, cobs, and fallen kernels. Thus, continuous corn production-especially under conservation tillage--allows the pathogen to build up to potentially destructive levels.

It is a good practice to scout fields for Diplodia ear rot as the crop matures, especially if under conservation tillage. Pull back the husks of 50-100 plants selected at random as you walk the field. Look for white, cottony mold growth between the kernels, which usually progresses upwards from the base of the ear. Suspect samples can be confirmed through your county Extension agent. Producers growing no-till corn who find more than 2-3% of ears with Stenocarpella ear rot should consider implementing some management practice, such as rotation to another crop, tillage, or both.

Whenever corn is planted following corn, check with your seed supplier to determine if the seed company specifically breeds hybrids with partial resistance to Diplodia ear rot; some companies do, but not all. Also, avoid hybrids that have experienced serious outbreaks of Diplodia ear rot, since this may indicate unusually high susceptibility to the disease.

Pythium Seedling Diseases

Pythium microbes commonly present in agricultural soils can cause a variety of symptoms on corn seed and seedlings. Seed decay, pre-emergence damping off, and postemergence damping off are the more striking problems caused by Pythium. However, Pythium can also infect root hairs and young rootlets, causing reduced vigor of developing plants, which can ultimately affect yields. Studies with a variety of monocot crops, including corn and sorghum, have shown that Pythium diseases can significantly reduce stand, vigor, and yield in continuous cropping situations. These studies have shown a significant advantage to using seed treated with metalaxyl fungicide, which specifically targets Pythium. Metalaxyl is found in several products, including Allegiance and Apron. Given the relatively low cost of seed treatment fungicides and the apparent enhanced Pythium risk when monocots are cropped without rotation, sowing seed treated with metalaxyl would make sense where corn follows corn.

Other Diseases

Diseases caused by the anthracnose fungus, including top dieback (a rot of the upper stalk during grain fill), can be enhanced in continuous corn. Since corn is the only important host of this pathogen, inoculum levels may build up in continuous corn. Attention should be paid to hybrid susceptibility to anthracnose when growing continuous corn. Substantial levels of resistance would be especially important in areas with a history of top-dieback or other anthracnose diseases. Southern leaf blight generally occurs at low levels in Kentucky, because past breeding efforts have led to high levels of resistance in most of the hybrids currently available. Yield-limiting levels of northern leaf blight are also uncommon, because of our generally warm summer conditions as well as sound breeding for resistance. Be aware, however, that the risk of both diseases is higher under continuous corn because, like the gray leaf spot pathogen, the fungi that cause these diseases pathogens survive in corn residue. Thus, it may be wise to consider hybrid reaction to these diseases when growing continuous corn.

Concern is sometimes expressed that stalk rots might be worse in a continuous corn situation. In reality, studies to date indicate that rotation has little to no impact on the severity of stalk rots. Stalk rot incidence is influenced by high plant populations, excessive nitrogen, leaf diseases, and other factors.

Additional Information

The following publications provide more information on several of these subjects. They can be accessed on the web at www.ca.uky.edu/agcollege/plantpathology/PPAExten/pppublin.htm#Corn and Sorghum.

Ear Rot of Corn Caused by Stenocarpella maydis (=Diplodia maydis), UK Extension Publication PPA-43.

Gray Leaf Spot of Corn, UK Extension Publication PPA-35.

Reviewed by DEH, July 2001

Of course, ALWAYS READ AND FOLLOW LABEL DIRECTIONS FOR SAFE USE OF ANY PESTICIDE!

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