SW MN IPM STUFF 2017 Issue 14

8/18/2017

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This newsletter and the advice herein are free. You usually get what you pay for.

Crop weather

Rainfall, air and soil temperatures, degree-days, soil moistures, and other current and historical weather data for the University of Minnesota Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN, can be found at http://swroc.cfans.umn.edu/weather.

Figure 1. Rainfall Amounts

As of August 14, the ROC was at 14.08” of precipitation accumulated from May 1 compared to a 12.05” historical average. Cool weather has slowed degree-day accumulations for the same period with 1757 degree days accumulated in 2017 compared to an 1830 degree-day historical average. August 16-17 saw areas of heavy rain and high winds, including tornadoes occur. The SWROC received 1.55 inches but some areas north received six inches of more. In the areas hardest hit by this weather event, some soybean fields were lodged and tangled and sweet corn fields were prone to lodging. 

As always, some had too much rain, some could have used more and I heard a rumor that the rainfall on a few farms was just right.

Corn

Figure 2. Leave Discoloration

The effects of earlier hail and moisture stress continue to show.

Check for reduced kernel set or abnormal ear development where maroon or purple areas present on the leaves near the ear.

Image 3. Crazy Corn Top

The color is the result of anthocyanin accumulation. Back in 1979 or 1980, I could have probably recalled the physiological and biochemical processes involved for the accumulation of this pigment. Unfortunately, like many things, this knowledge has been misplaced in my messy intra-cranial filing system. A much abbreviated explanation is the sugars produced by photosynthesis cannot be translocated normally because of a lack of kernels to receive them. Injury to the corn plant’s vascular system, corn borer tunneling for example, can create similar pigment accumulations. An alternative, but less likely, hypothesis for the cause of the maroon plants is some plants are anticipating Golden Gopher football season.

The common smut fungus, Ustilago maydis, often produces spore-filled structures in place of the reproductive tissues of corn plants that been exposed to hail or moisture stress.

Although much less common than common smut, another fungus that produces a systemic infection and bizarre symptoms on corn reproductive tissues is Sclerophthora macrospora. This soil borne fungus produces the disease crazy top. Corn plants can be infected when soils are flooded shortly after planting.

Soybean

Moisture will help with soybean seed fill and yield. Where soybeans have lodged, late season scouting will be difficult.

Pathogens

For the most part, soybean disease has been minor in the SW part of the state. Symptoms of Sudden Death Syndrome (SDS) are starting to appear.  As soybeans reach the R6 stage, foliar symptoms of Brown Stem Rot will become apparent.  Splitting stems and examining the pith for brown internodes will reveal disease in infected plants that do not show foliar symptoms.

Expect to see increased bacterial blight symptoms where storms have occurred. I am curious to see how Septoria brown spot, currently confined to senescing lower canopy leaves progresses or downy mildew develops if cool, wet weather persists. 

White mold is the disease to watch in areas with high fertility, high plant populations and dense canopies. In many areas most of the flowering period was dry and less favorable for infection.  Keep track of fields where this disease is severe as the sclerotia can persist for years.

Marty Lovrien mentioned SCN populations dramatically increased in parts of Central MN this season. The symptoms in the more southerly areas of MN I have been traveling through are subtler than many years. It doesn’t mean we have run out of nematodes though.

Image 4. Defoliation

Late season defoliators. Most insects prefer drier conditions than we are currently experiencing.

Currently you can find thistle caterpillar, green cloverworm, and early instar yellow woolly bear. I am seeing a few yellow-striped armyworm moths and their larvae should be joining the party soon. With the exception of thistle caterpillar, the late season defoliating insects are at low levels. In southern Minnesota many, if not all, populations of thistle caterpillars have started to decline as the older larvae have begun to pupate. 

Soybean Aphid Watch 2017

The wet, windy weather may have impacted some soybean aphids directly. However, at this time of the growing season, most aphids are protected and lower in the canopy.

Figure 5. SBA

Corey Sinn called early this week to discuss the slowing of growth in soybean aphid populations. He was one of the first of several agriculturalists reporting inconsistent populations in the southern part of the state.  Aphid population increases appear to have slowed or declined in many fields. In some, but not all, experiments at Lamberton, aphid numbers have declined by as much as 50 percent over the past week. There are several possible explanations for this: 1) Many fields have lost large number of aphids to migration, 2) Unlike parts of northwest Minnesota, early-season populations of aphids were very low and area-wide aphid populations are still relatively low, 3) Cool weather has increased the time for a generation to complete, 4) The abundant moisture in many areas has likely reduced the nutritive quality of sap, 5) Predator numbers are typical but based on the number of mummies, parasitoid wasp species are now taking a much greater toll on aphids.

The other topic of conversation was when can scouting stop and should the threshold change? The simple answer is use the 250 aphid threshold until the R6 (full seed) stage. Aphids can potential reduce yield even during R6. At the time you start to see any yellow pods or the fields begin to turn, walk away from pest management in general. Until that time, it would be advisable to continue to scout at least a few representative fields.  Early-planted, short-season beans will drop off the scouting schedule first.

Some things to consider about R5 soybeans and aphids in 2017:

  • Many soybean fields are behind in development compared to most years.
  • Soybean aphid reproduction is often reduced on early R5 stage soybeans. On the other hand, population increases can be very rapid during late R5 and early R6.
  • Smaller white aphids lower in the canopy can produce offspring that develop into normal, larger adults. Include these in your assessment.
  • At this time, I’ve not seen evidence that aphids have started to move to buckthorn. Even once this movement begins, aphids do not leave all fields at the same time.
  • Large numbers of predators, parasitoid mummies, or numerous aphids with symptoms are fungal disease can influence your late season decisions. Today, in research plots at the SWROC, we started seeing more numerous lower canopy aphids infected by entomopathogenic fungi. 
  • Scouting heavily lodged soybeans can be strenuous.  A thorough whole-field scouting may not be possible. Be careful and creative when assessing these fields.

Happy trails,

Bruce