Wet weather or dry, the pests are with us, though which ones are troublesome varies with the weather. Weather conditions in 2012 and the spring of 2013 stressed plants in extreme ways and those who study these pests and diseases. The results have raised questions about efficacy of existing controls and the pests themselves.
Last year’s heat and drought and the extremely heavy and prolonged rains in many areas this spring created textbook opportunities for studying insects and pests as corn and soybean planting stretched out into mid-June and in some cases later. Fields of newly emerging corn and soybeans sit alongside fields with seedlings inches, or in some cases, a foot and half to two feet taller.
“Last year’s heat speeded planting and the development of insects,” said Matt O’Neal, field crop entomologist, Iowa State University (ISU). “Growers were concerned by defoliation from Japanese beetles on plants in May and June, a pest that is normally seen in July and August. The early emergence of this pest was worrisome because it is not controlled by seed treatments. We also had the worst spider mite outbreak since 1988. Spider mites do very well in dry, hot conditions. This year the cold weather slowed the development of these pests, likely reducing their impact in 2013.”
When interviewed in late June, O’Neal was still waiting to see if the wet soils would increase insect mortality. He was surprised to hear reports of early and high levels of soybean aphids. Because of the extreme weather, everything is up for grabs. In a normal year, predator insects tend to emerge with prey. This year, O’Neal suggested, that may not be the case.
“The delays in planting and emergence may delay a predator response as well,” he said. “Predators need insect prey to fl ourish. How strong their response will be is the question.”
THE MYSTERY OF SOYBEAN APHIDS
In 2012, one pest that was hard to measure was aphids temperatures,” said O’Neal. “This year appears to be the opposite of 2012, with cooler spring temperatures allowing soybean aphids to establish and later colonize soybean fields. The evidence is they are out there, and we may see populations we haven’t in the last couple of years.”
He suggested that heavy populations in areas that have a history of problems with the pest might encourage farmers to consider aphid-resistant soybean varieties. Work done at his lab at ISU emphasizes the benefits of multiple or pyramided resistant genes. He noted that plants carrying a single aphidresistant gene benefi tted from a seed treatment. However, plants with two resistant genes were so aphid-clean that no benefit in aphid control could be seen with a seed treatment.
“A single resistant gene can reduce aphid populations from thousands to hundreds,” said O’Neal.
“When two genes are combined, survivors drop from hundreds per plant to tens. What we don’t know is how this works, whether by producing a toxin or simply allowing the plant to ignore the aphids.”
While other pests such as corn rootworm have developed resistance to gene-based protection, work in the O’Neal lab is focused on preventing this from happening with the soybeanaphid.
Although single and pyramid aphid resistance is available, it is not found in many varieties. Through his website, O’Neal reports the commercial sources of soybean aphid resistant varieties (http://www.ent.iastate.edu/soybeanaphid/files/SBAresistance2012corrected.pdf).
“The limited use of the aphid-resistant genes contributes to preventing soybean aphids from becoming resistant to these genes,” O’Neal said. “By using a pyramid line, farmers are employing a strategy that can also limit soybean aphids from becoming resistant to these genes as well.”
O’Neal’s lab is exploring other tactics that may be necessary to prevent resistance from developing if more farmers use soybean aphid-resistant varieties. “With funding from the Soybean Checkoff program, we are developing Insect Resistance Management (IRM) plans for the long-term use of aphidresistant varieties. We are exploring if a refuge-in-a-bag strategy that is used for corn rootworms can be used for soybean aphids.”
What is made clear by such extreme weather years is how much is yet to be learned about crop disease. Dean Malvick, Extension plant pathologist, University of Minnesota, reported he will be keeping a close eye on phytophthora, rhizoctonia and white mold diseases of soybean. The wet spring with prolonged
planting set the stage for problems with them as well. Smaller plants in wet but warm, high organic soils are also favored by rhizoctonia. He cites cases with soybeans planted following peas in central Minnesota where half the field was killed. This year, similar conditions are much more widespread. The warm and wet field conditions in many fields also increase the risk of phytophthora rot, which is being seen in a number of fields.
The wet weather pattern may increase the risk of white mold if it continues into flowering and pod set, although hot weather during that time will tend to decrease this disease.
How well the crops are protected by seed treatments and resistant varieties, where applicable, may be telling. For example, there are many different races of phytophthora in Minnesota that render the common resistance genes in soybean ineffective in some fields. Malvick also reported that some seed treatments appeared to miss their target.
“Pythium is generally controlled by the seed treatments targeted at a few previously identified species,” he said. “But pythium seemed to overcome the seed treatments in some areas.”
What is yet unknown is whether it was the environmental conditions or the species that may not be very sensitive to the seed treatments. Malvick reported finding more than 20 different pythium species while sampling soybean fields. He admitted that much is unknown about them.
“We don’t know about their relative aggressiveness or pathogenicity, when they thrive or under what conditions,” he said. “Pythium is generally considered to thrive in cool and wet conditions; however, some of these species may prefer warmer soils.
He cited the loss of several thousand acres of corn in south central Minnesota in 2012 as likely being caused by pythium root rot. Although drenching rains caused crusting, it appeared to be the root rot that killed the seed-treated corn.
“Based on work in other states, it appears pythium species are not all equally susceptible to seed-treatment fungicides,” said Malvick. “We need to understand what these different species are doing in Minnesota.”
More information is needed on fusarium as well. Malvick noted that while it has long been known to be a problem in soybeans and corn, no highly effective treatments have yet been found. Although soybean varieties have been developed with resistance to the fusarium species causing SDS, that is not the case with other species. His concern is that the problem is more complicated than previously thought and may be getting worse.
“We have found 12 different fusarium species while looking at root rot in corn and soybeans,” said Malvick. “The issue is whether we look at enough fields under the same conditions to really know if things are changing. It is difficult to get enough information, but fusarium appears to be thriving and possibly changing to be a little more effective on corn and soybeans. That may be the case with pythium as well.”
Malvick pointed to new tools on the horizon, including new seed treatment fungicides and biological seed treatments, some of which are already included in seed treatments. The wet conditions in the upper Midwest and resulting disease pressure are providing ample challenges to current control methodologies. These new controls may well prove extremely valuable.