What’s Happening in Those Fields? Take Your Pick, It’s Out There

By Jane Fyksen, Crops Editor
Wednesday, June 25, 2008 12:47 PM CDT

Wisconsin’s crop situation is anything but homogenous at the moment. There’s lots to report, and crops are all over the board in terms of development and challenges. This week, while the water drains away in flooded areas, Agri-View summarizes some situations to watch.

Wisconsin Department of Agriculture, Trade and Consumer Protection entomologist Krista Hamilton says “a definite potential exists” for damaging populations of armyworms in corn, small grains, peas and other crops, particularly in the southern counties. Surplus rainfall and strong winds this month delayed the application of herbicides, and as a result numerous fields have developed dense grassy foliage favorable to larval feeding.

Small grains lodged during storms in previous weeks are also at risk for localized outbreaks.

Moderate infestations of nine to 11 larvae per 100 plants were detected in the margins of Columbia County cornfields June 19, indicating armyworms have started to move into fields in appreciable numbers. Close surveillance this week is strongly advised, cautions Hamilton.

Bryan Jensen with the UW’s Integrated Pest Management (IPM) Program also highlights reports of armyworm damage on corn planted after a small grain cover crop or in dense weed growth. “This is not out of the ordinary for this time of the year. But it does make you wonder what is happening in wheat,” he notes. “I’ve noticed some lodging of wheat from recent storms. These lodged areas are some of the first places I would monitor for signs of armyworm feeding. We normally think of armyworms feeding on leaves but once the small grain has headed, armyworms can (and will) clip the stem off just below the head.”

Jensen also notes reports of slug feeding and a few indicate severe injury. Cool, rainy weather favors slug survival, especially in crop residue or weed growth. “Given the value of corn it could be worth the time and effort to spot check likely areas. Although there is not a lot of threshold information available for slugs, I would suggest that you take into consideration the size of the crop and future growing conditions before treating,” Jensen states. “I would have a hard time suggesting treatment if feeding is concentrated on the lower leaves of corn that is growing rapidly - especially if good growing conditions are predicted and there are no other reasons to think the crop is stressed.”

“If the reverse is true I could feel more comfortable treating if weather conditions were to remain cool and cloudy,” he adds.

“Remember that slugs are not controlled by insecticides. The use of appropriately labeled slug baits is sometimes an economical - if not temporary - solution,” he says. “Control may be reduced if rains break down the bait, and cool growing conditions (that are favorable to slug survival) persist.”

“The best control method is warm, sunny weather that not only hastens corn growth but also increases slug mortality,” Jensen points out.

Jensen also mentions that with first-crop harvest delayed in many areas because of rainy weather, growers should be wary of alfalfa weevil; this pest’s development has been slowed because of cooler than normal weather. “The best decision, of course, is to cut as soon as possible. Larvae which survive and feed on second crop is more difficult to assess. The established threshold for second crop damage is to treat when 50 percent of the stems have feeding symptoms. However, you should take this recommendation to the next level and assess the age structure of the weevil population,” he states.

Ask yourself how much damage can I prevent? Southern Wisconsin weevil degree days suggest that weevils have started to pupate. Alfalfa weevil larvae go through four instar stages. As fourth instars, weevil larvae are approximately 3/8-inch long and only require 91 degree days before pupating. At daily high and low temperatures of 75 and 60 degrees respectively, this takes approximately five days,” he explains, advising growers to watch weevil degree day accumulations at http://www.soils.wisc.edu/wimnext/crops.html for their area and assess the age structure of your fields to see potentially how much damage you can prevent.

Potato crop eyed

Alvin Bussan in UW-Madison’s horticulture department, reports potatoes in central Wisconsin are well into early bulking, and tubers are five to seven days off last year’s pace. He says the largest Russet Norkotah tubers at the end of last week were about 1 inch in diameter by 1.5 inches long, or 75 percent of the size compared to last year. Red Norland at Hancock were 1 inch in diameter or about half the size of tubers compared to last year. Snowden, FL1867 and FL1879 tubers were 3/4 to 1 inch in diameter last week at Hancock.

Heavy rains have lead to some crusting issues in coarse and medium-textured soils. At Hancock, Snap beans planted June 12 were having a difficult time emerging through crusted soil, and germinating seeds show symptoms of being impacted by compaction, says Bussan. Sweet corn is also slow to emerge. On medium textured soils, crusting is severe in some locations and preventing emergence of snap bean and sweet corn. If irrigation is available, you may want to apply 1/4-inch of water to soften the ground and facilitate emergence. On heavier textured soils, rotary hoeing may be necessary to break up the crust on sweet corn. Do not rotary hoe snap beans as it will break stems and cotyledons, he cautions.

“Crusting may be a substantial issue for fresh market growers as well,” he notes. “Investigate stands of vegetable crops planted before June 12 to determine how well crop is emergence through crusted soils. Lightly irrigate soils to soften the crusted layer to promote emergence. Rotary hoeing or working the soil lightly with tools that mimic the action will improve the establishment of crops through these soils. If you decide to till and replant areas where stands are poor pay close attention to replanting instructions for any herbicides you may have applied.”

Russell Groves, UW-Madison vegetable entomologist, says his lab is performing assays of adult Colorado Potato Beetle populations collected in Wisconsin to investigate the potential for increasing levels of insensitivity to the neonicotinoid class of insecticides. Preliminary assays on selected populations of overwintered, adult CPB again document resistance ratios in the 10-fold range. It is important to note, however, that some populations can have as high as 20-fold levels of insensitivity and still be controlled with the compound in question on a field scale.

Walt Stevenson, long-time plant pathologist at UW-Madison, noted as of late last week that there were still no reports of late blight anywhere in the nation. He says growers should be treating their earliest emerging fields with protectant fungicide. For some fields, this will be at least their second to third fungicide spray of the season. Since growers haven’t seen the first late blight symptom yet, he’s still suggesting protectant fungicides be used at this time (maneb, mancozeb, chlorothalonil, metiram, fixed copper).

“We’ll be seeing early blight pressure heat up in the weeks ahead, particularly since we are approaching 300 P-Days when we normally focus our attention on early blight management. Once we hit the magic 300 P-Day threshold, we can begin thinking about adding a strobilurin fungicide to the mix and working a spray of Endura into the program, usually about 400 P-Days. Also for early blight management, you need to carefully consider the crop’s nutritional needs…” says Stevenson. “The plant susceptibility to early blight increases on N-deficient plants, particularly as the plants begin to bulk, thereby making early blight more difficult to manage with fungicide sprays.”

Fields of peas should be examined for symptoms of root rot and epicotyl decay since many fields have been sitting in overly wet warm soil for extended periods. As the soils dry and plants begin to mature, they are going to be under a great deal of stress. Yields will likely take a hit as foliage begins to show the effects of below ground decay.

The same situation holds true for the early planted snap beans. Cultivation can often scuff enough soil against the stem to encourage the development of adventitious roots. These roots will often keep the plant growing and help the plant produce a marketable yield, although it may be a yield below what was anticipated at planting, says Stevenson.

Stevenson says as soils remain wet and growers push the envelope on planting, he anticipates a banner year for Pythium and Phytophthora root rots. “Growers planting into fields with a history of root rots might want to consider application of mefenoxam fungicide at planting. Check the label carefully before application to be sure this material is registered for the crop you intend to plant,” he notes.

Assessing flood damage to soybeans

Shawn Conley, UW-Madison soybean and wheat specialist, and Paul Esker, field crop pathologist, says severe flooding has had many low-lying soybean fields under water. As the water dissipates yield potential questions will arise. Flooding can be divided into either water-logging, where only the roots are flooded, or complete submergence where entire plants are under water. Soybeans can generally survive for 48 to 96 hours when completely submersed. If the soil was already saturated prior to flooding, soybean death will occur more quickly as slow soil drainage after flooding will prevent gas exchange between the rhizosphere and the air above the soil surface. Soybeans often do not fully recover from flooding injury, they say.

Injury from water logging is difficult to assess. Water-logging can reduce soybean yield 17 to 43 percent at the vegetative growth stage and 50 to 56 percent at the reproductive stage. Yield losses are the result of reduced root growth, shoot growth, nodulation, nitrogen fixation, photosynthesis, biomass accumulation, stomatal conductance, and plant death due to diseases and physiological stress.

The UW experts say a significant amount of genetic variability for flooding tolerance among soybean varieties occurs in maturity groups II and III and likely exists for maturity group I soybeans as well.

Increased disease in surviving plants may also occur and limit yield potential. The main culprit will likely be phythophthora given the warm wet weather. Phythium, rhizoctonia, or fusarium may also occur.

The average yield potential for soybeans planted in late June is in the 30 to 35 bushel range. For yield potential and harvestability (a combine may not be able to pick up the lower pods) a grower should plant if possible a mid maturity group soybean instead of an early maturity group for his area.

To maximize yield potential in late planted soybeans, a minimum of 180,000 plants per acre is required in a drilled system as yield potential in rowed beans would be significantly reduced due to decreased canopy development. To achieve 180,000 plants per acre a grower may have to seed as many as 225,000 seeds per acre.

Timely weed control has been difficult this season in both corn and soybean fields. It's nearly impossible to offer any "one size fits all" suggestions given the broad range of crop sizes.

Aaron Hager with the University of Illinois says where no preemergence herbicide was applied, it’s impossible to know with certainty how much yield potential already has been lost, but a range of 6 to 10 percent might be close.

“Using very simple math and an assumption of a 150-bushel corn crop, a 10 percent loss of yield potential equates to 15 bushels; that value multiplied by $6 a bushel comes out to $90 per acre loss because weeds were allowed to compete too long with the crop. If the plan at the beginning of the season was to save money by reducing or eliminating a preemergence herbicide, the amount of money seemingly saved up front - perhaps $20 to $30 an acre - might have been far exceeded by the loss of yield potential,” he says.

Hager says reports of crop injury following the application of postemergence herbicides have been common thus far in the season. The adverse growing conditions encountered soon after corn emergence persisted into the beginning of the postemergence herbicide application window, and he speculates that these conditions probably resulted in a relatively thin corn leaf cuticle that would allow for more rapid herbicide uptake, especially when postemergence treatments included additives such as crop oil concentrates or tank-mixes that included herbicides with an oil-based formulation.

“Also, saturated soils can place the corn crop under additional stress. Crop injury is often more common when postemergence herbicide applications are made to plants under stress,” he notes.

Be sure to scout cornfields before applying postemergence herbicides to accurately determine the crop's growth stage. Adverse environmental conditions, such as prolonged periods of cool air temperatures or wet soils, can result in corn plants that are physiologically older than their height would suggest, so be sure to accurately assess plant developmental stage (i.e. leaf/collar number) in addition to plant height. “Pay close attention to the maximum corn stage listed on the respective herbicide label, and do not apply the product if corn exceeds the labeled stage,” he says.

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