by Elizabeth A. Tomlin
“Are corn leaf blight epidemics the new normal?” asked Cornell University’s Plant Pathologist Dr. Gary Bergstrom.
Bergstrom was addressing identification and control of corn diseases at the 2014 CNY CCE Corn Day, held at the Otesaga in Cooperstown.
“Looking back over some of the trends of corn diseases over the last three years, we’ve been seeing more and more leaf blight, especially late in the growing season. More leaf blight and new races of fungus,” Bergstrom confirmed.
Northern corn leaf blight (NCLB) has been observed most frequently and the disease has been found throughout New York State and in large areas of New England and Eastern Great Lake states.
“Northern corn leaf blight caused by the fungus Exerohilum turcicum,” Bergstrom said. Lesions are ‘canoe shaped’, usually 1 to 6 inches long, run parallel to leaf margins and may cover the entire leaf. The disease begins on lower leaves and moves upward to canopy of the plant. “Spores are produced on the underside of the leaf below the lesions giving the appearance of a dusty green fuzz.”
NCLB causes yield loss when it develops early in the season, before or during tasseling and silking phases.
Bergstrom said NCLB fungus lives in corn debris, and is prevalent in fields planted in continuous corn, especially in low or no till situations. The fungus survives the winter on infected residue at soil surface and produces new spores when temperatures rise in spring and early summer. The fungus thrives in wet, humid and cool weather when temperatures are 65-80 degrees. Symptoms are frequently seen following periods of heavy dew and overcast days, especially in valleys and fields adjacent to wood lots.
“When you have continued wet weather, you can have multiple generations of this fungus,” stated Bergstrom.
Lesions form one to two weeks after contamination. Fungal spores causing the disease are transported long distances from infected fields by wind and quickly spread throughout neighboring fields. Hot, dry weather restricts fungal development and spread.
Management of NCLB includes using the appropriate fungicide at the right time in conjunction with the use of resistant hybrids — although Bergstrom suggested that new races of the fungus might have developed, making it more resistant.
“I want to point out some particular vulnerability that we have been seeing for folks that are growing silage corn — particularly BMR varieties. It appears that there is quite a bit of susceptibility, particularly to NCLB, but also to some of the other diseases in the BMR line.” New genes are being bred into hybrids making them more resistant.
NCLB can be mistaken for other foliar fungal diseases such as Southern Corn Leaf Blight or Goss’s Wilt; therefore accurate diagnosis is important.
“There is a hot spot area of Grey Leaf Spot down along the Pennsylvania border,” Bergstrom reported. “In that case there’s a 30 to 40 bushel per acre difference in yield.”
Bergstrom advises looking at your field’s history and to consider rotating crops. “But,” he concluded, “probably the most important decision is in your hybrid selection.”
Cornell University Entomologist Elson Shields reported on the emergence of Western Corn Rootworm (CRW) resistance in Central New York State.
“In 2013 we had a Bt failure in Cayuga County,” Shields said. “I didn’t think we would have failure in New York. I was wrong.”
Shields showed photos of the cornfield with severe goose necking, lodging and extremely pruned roots, all classic characteristics of CRW damage.
Shields reported the field’s history included nearly 10 years of continuous corn and no legal refuges for many years, either in the field or in adjacent fields. Furthermore, the field had high beetle populations and had also received unreported damage over the previous two years.
Shields pointed out that if legal refuges had been planted as required, Bt resistance would not have developed as it had.
Corn producers using Bt corn hybrids are required to follow Insect Resistance Management (IRM) guidelines for effective management Bt hybrid technology.
Shields provided charts depicting trial results proving the impact corn refuges have on resistance development. The size of the refuge acreage in the field helps determine the amount of resistance produced in the Corn Rootworm.
“Don’t layer a soil insecticide over a failing Bt event,” Shields added, explaining that soil insecticides only work in a 7-inch zone around the root, still allowing damage in the upper plant to continue.
“We’re producing a lot of beetles,” Shields stressed. “If we allow them to inter-mate without refuge; that’s where our resistance is developing. The planting of untreated refuges produce large numbers of unexposed beetles to dilute any genetic resistance, thereby keeping the frequency of the resistance very low.”
Whereas not planting Bt-free refuges allows surviving beetles to mate producing an even more resistant strain of beetle.
The Corn Rootworm lays eggs in existing cornfields coinciding with pollination time; therefore, first year corn has an advantage. “That’s why rotation works so well,” Shield explained.
A high rate of seed treatment is recommended for second year corn, which has about a 30 percent chance of risk. Third year corn has about a 70 percent chance of risk and at that point planting Bt Rootworm corn along with refuge corn is recommended.
Begin scouting fields mid to late July to detect CRW early. Report and document problems immediately when they are first detected. For fields in continuous corn, rotate crops every 3rd or 4th year to some other non-host crop such as alfalfa, soybeans or small grains, or don’t use the same Bt in each consecutive year. Plant hybrids containing different Bt traits for CRW.
“The longer you’re in continuous corn, the more you need something to withstand Corn Rootworm,” said Shields.
For more information on corn disease contact Central New York Cornell Cooperative Extension Field Crops Specialist Kevin Ganoe at firstname.lastname@example.org.
2014 Central New York CCE Corn Day addresses current corn crop issues (Part II)
by Elizabeth A. Tomlin