By Paul Vincelli, Extension Plant Pathologist
University of Kentucky



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.



Several diseases can be more active under continuous corn, particularly those caused by pathogens that survive in crop residue or in the soil.



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.



Diplodia Ear Rot



Although most corn fields do not experience much damage from this disease, Diplodia ear rot can occasionally cause severe epidemics, causing rot on as many as 50 percent to 75 percent 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 percent to 3 percnet of ears with Diplodia 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.



Anthracnose Stalk Rot and Top Dieback



The anthracnose fungus survives in corn residue, and causes a variety of symptoms including leaf spotting and blighting. The leaf blight phase only affects plants at the one- to four-leaf stages, but it alerts growers to scout fields later in the season for the lower stalk rot phase and "top dieback" (upper stalk rot) phase, which can affect yield or harvestability. There was quite a bit of anthracnose top dieback in 2006, so I suspect inoculum levels are rather high in many fields. Be sure to give consideration to selecting for anthracnose resistance if the field goes back into corn in the next couple of years, especially if the disease was observed in the field.



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 post-emergence 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 fungicides which specifically target Pythium, such as metalaxyl or mefanoxam.



Given the relatively low cost of seed treatment fungicides and the enhanced Pythium risk when monocots are cropped without rotation, sowing seed treated with metalaxyl or mefanoxam would make sense where corn follows corn. This is especially important under conservation tillage, where soils warm up and dry out more slowly, conditions which favor Pythium diseases.



Other Diseases



The fungi that cause northern leaf blight (NLB) and southern leaf blight (SLB) survive in corn residue. NLB has re-emerged in the past several years as a serious limitation to yield in fields in Kentucky where susceptible hybrids are grown, so I would consider the susceptibility of the hybrid to NLB when growing continuous corn. SLB 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. Since inoculum levels of NLB are so much higher than those of SLB, selecting resistance to NLB takes a much higher priority than SLB.



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 direct impact on the severity of stalk rots. Stalk rot incidence is influenced by high plant populations, excessive nitrogen, leaf diseases, and other factors.



However, increasing acreage of continuous corn could result in an extension of the harvest season simply because fields may stand unharvested while other corn acreage is being combined. This could result in occasional increases in lodging risk and marginal increases in mycotoxin risk while mature ears stand out in the field for longer periods than normal.



SOURCE: University of Kentucky Kentucky Pest News 1/16/2007.