January 10, 2007 | By: Laura Skillman
PRINCETON, Ky.

With higher prices leading farmers to grow more corn, many fields in Kentucky will be planted in corn for subsequent years. However, farmers need to be aware of the increased pest pressures caused by continuous production.

“We don’t want to discourage people from doing what the market tells them,” said Ric Bessin, an entomologist with the University of Kentucky College of Agriculture.

But, he said, pest specialists do want farmers to be aware of potential problems and to protect their crops when possible. That means watching for corn rootworm. Rootworms chew off the roots damaging the plant’s productivity. In some cases it can completely kill the plant. The rootworm is a potential problem in continuous corn production and the risk increases with each subsequent year, Bessin said.

Farmers using continuous production need to be scouting fields in June looking for the rootworm beetle. If an average of one beetle per plant is found, a producer needs to rotate that field into another crop for one year or use a soil insecticide, a Bt corn variety with rootworm control or a seed treatment.

Several diseases can be more active in continuous corn production as well, particularly diseases caused by pathogens that survive on crop residue or in the soil. These include gray leaf spot, diplodia ear rot, anthracnose stalk rot and top dieback, pythium seedling diseases, and northern leaf blight.

Gray leaf spot is a significant threat wherever corn is grown year after year, said Paul Vincelli, UK plant pathologist. It is spread by wind and rain to leaves of new corn from residue of the previous growing season. Tillage practices can reduce levels of inoculum, but crop rotation is a key management practice as well. In the absence of rotation, susceptibility to gray leaf spot should be carefully considered when selecting a hybrid. Hybrids have differing levels of partial resistance.

“When growing continuous corn, always select hybrids with as high a level of resistance as you can against gray leaf spot,” Vincelli said. “This is particularly important in fields under conservation tillage, in which corn residues provide high amounts of inoculum.”

Diplodia ear rot does not cause damage in most corn fields but it can occasionally cause severe epidemics, rotting as many as 50 to 75 percent of the ears in a field. The fungus only attacks corn and survives on residue. Therefore, continuous production allows the pathogen to build up to potentially destructive levels.  Whenever planting corn after corn, Vincelli recommends farmers check with their seed dealers to determine if the company specifically breeds hybrids with partial resistance to the disease. Some companies do, while others do not.

There was quite a bit of anthracnose top dieback in 2006, so Vincelli said he suspects inoculum levels are “rather high” in many fields. This disease can affect yield, and the lower stalk rot phase of anthracnose can affect harvestability.  Farmers need to plant hybrids with resistance in fields where they noticed the disease.

Farmers also should select hybrids with resistance to northern leaf blight. This disease also survives in crop residue and has re-emerged in the past several years as a serious limitation to yields in Kentucky fields where susceptible hybrids are grown. Also, pythium seedling diseases can cause seed decay and seedling death as well as damage root hairs and young rootlets, causing reduced vigor and ultimately impacting yields. Seed treated with fungicide targeting pythium are a low-cost means of combating the disease.

Corn itself can prove to be a nemesis in continuous corn production.

“Volunteer corn traditionally has not been a problem in Kentucky, but the past two seasons it has been especially in the Green River area and likely will be in the 2007 growing season,” said Jim Martin, UK weeds specialists.

It can be a particular challenge if the volunteer corn is a glyphosate-resistant hybrid because it reduces the options farmers have to kill it, he said. Volunteer corn is likely to sprout and grow in clumps, and these clumps can crowd out and compete with other seedlings. One clump per 8 square feet that is not controlled within 10 weeks of emergence can cause 25 percent yield loss.

In fields with severe volunteer corn, the best option for control is to rotate that field into another crop, Martin said.

Contact: 

Ric Bessin, 859-257-7456, Jim Martin, 270-365-7541, ext. 203, Paul Vincelli, 859-257-7445, ext. 80722