At the Jan. 8 Ag Research Update session at the Southwest Research and Outreach Center at Lamberton, Dean Malvick, University of Minnesota Extension plant pathologist, told Minnesota crop consultants there’s a new disease needing attention.
Goss’s bacterial leaf blight and wilt was first discovered in Nebraska in 1969 and confirmed this 2009 season in Minnesota.
“We need to watch for this. We’ve detected it only in a few fields near Morris and Hastings. It’s likely in other locations but not yet discovered. Yield losses up to 50 percent have been measured in Nebraska. Wet weather with sand blasting and hail would be ideal environments for this disease.
“Goss’s is still very limited but increasing in other states as well, especially Nebraska. At this stage, accessing hybrids with good disease resistance scores, plus tillage that buries most of the plant residue from infected fields is our suggested control.”
At this Lamberton event, Malvick and Ken Ostlie, Extension entomologist, covered a variety of plant diseases and insect disease issues. Ostlie pointed out an unexpected culprit in 2009 was the corn leaf aphid.
He said scouting for this critter should begin three weeks before tasseling, and spray if more than half the corn plants have 100 or more aphids. He also said corn rootworm pressures were up last season, likely the consequence of extended diapauses, which is generating every-other-year corn rootworm problems. Drought conditions in many 2009 cornfields also exacerbated the issue.
On the multiple benefits of SmartStax trait technologies, Ostlie said it gives slightly better CRW protection than with just the single trait. He questioned if the rush to refuge protection in the bag products might lead more quickly to potential resistance issues.
“We’re seeing some problems with rootworm-resistant corn, with both the western and the northern beetle. And we’re seeing lodging problems in triple-stack hybrids. I think we still need a separate refuge area. Growers might be tweaking the 20-percent refuge compliance issue but we could be challenging the long-term reliability of genetic traits in the process,” Ostlie said.
Bruce Potter, SWROC crop specialist, said, “the question I have is how quickly are corn rootworms going to adapt to the various transgenic hybrids being introduced? The resistance examples we saw in 2009 hopefully were just a fluke, but if not, this is a cause for great concern.
“We don’t know the mechanism triggering this reaction. We don’t know if it’s a lower expression of the proteins. We don’t know if it’s the later hatch because of cool weather. It could be real resistance in the corn rootworm population, or it could be environmental factors. Also we have some issues with western corn rootworm getting around Herculex in southwest Minnesota, northerns in south central, and also in southeast Minnesota in mixed populations (both northern and western CRW).”
Potter hesitated to blame continuous corn and higher corn populations for triggering some of these resistance examples. He did suggest, “with higher corn populations you tend to have higher rootworm populations so maybe the constant ‘nibbling’ of these rootworms could be overwhelming the trait.”
However these “events” make Potter somewhat leery about lower refuge requirements.
So in view of the environmental stresses occurring in Minnesota cornfields in 2009, why were yields so good?
“We had cool weather which compromised the crop to some extent. But the dry weather compensated the other direction by really suppressing a lot of disease development. We would have had a lot more white mold for example if we would have had more rain,” Malvick said.
In his four years of Minnesota Extension work, he’s very much noticed the drier July weather. “Also there are some very large regions with severe moisture deficits for the past two years. If that continues we’ll be set up for some other problems, some other diseases such as charcoal rot.”
With corn populations trending up, is there a linkage to increased disease problems? “You could relate it to more stalk rot perhaps, especially under droughty conditions. But with good growing conditions, and being particular about your hybrids, I think you can crowd your populations without being concerned about more disease.”
Malvick also doesn’t see a direct link to more disease with continuous corn. He admits that the more residue in a field, the more risk for a given disease especially foliar diseases such as gray leaf spot, anthracnose. He does surmise that with higher populations and a wetter growing season, corn foliage could stay wet longer, which could potentially induce more foliar diseases.
“We’re not seeing any pattern,” said Malvick, “just suggesting it as a possibility.”
Cloudy weather can also trigger leaf disease problems because with higher humidity in a cornfield you have a more favorable environment for the disease fungi. Also stalk rot partially occurs because of cloudy conditions.
The plant is robbing the stalk of nutrients in the process of filling the kernels of corn. “So as we produce less photo synthates in the leaves, we’re mobilizing sugars out of the stalk and that could lead to stalk rot,” Malvick said.
With corn stalk bedding more and more common with cattle producers, is the hauling of this “manure” to fields transferring many of these disease fungi from field to field?
“Yes, that definitely is the case. Most of these pathogens are pretty hardy. So if they are in the stalk residue baled up for bedding, they’ll still be there when that litter is hauled to the field.”