This newsletter and the advice herein are free. You usually get what you pay for.
A print-friendly version of this newsletter is available to download here: IPM STUFF 2019-09
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 SWROC Weather.
Adequate soil moisture in most fields is allowing corn to tolerate high temperatures well.
Quite a few fields with tassels emerging now. Later planted fields have been able to take advantage of the heat and their potential is looking more optimistic. Watch for tasseling as black layer (physiological maturity) occurs approximately 60 days later.
Soybeans are blooming and pod set has begun in some fields. Septoria on lower leaves and bacterial blight are the diseases I am seeing most often.
They are not a crop pest issue but the 2nd generation of red admiral butterflies and monarchs are unusually abundant in some areas of SC MN this year. Painted ladies are members of the same family of butterflies as the red admiral.
Physoderma brown spot
Figure 1. Foliar symptoms of Physoderma brown spot. Note bands of tiny yellow spots and purple on leaf bands.
Finding a warm, wet environment can make things happen. Reid Olson called to mention that foliar symptoms of Physoderma maydis (Figure 1) have started to show up in SC MN. Corn plants are infected when spores are splashed from the soil into the whorl of young corn plants. The zoospores are mobile and swim through water in the whorl to meristem tissue. The yield concern for this disease occurs when a rot and stalk breakage develop in a lower stalk node of one of the plants.
It is only relatively recently that this disease has been causing yield concerns in southern MN. I looked at one of these fields with a consultant yesterday to see if it might work as a study site. It has enough disease that SW MN IPM will put out a small fungicide timing study. Always willing to help out plant pathologist colleagues.
A change in diet seems to work out sometimes
On June 26, I came across some unfamiliar beetles (Figure 2) feeding on corn leaves in a study at the U of MN West Central Research and Outreach Center in Morris. The small (3/16“), dark bronze, active beetles appear to have shifted to corn from Canada thistle killed by herbicide. Recently, these were determined to be Rhabdopterus picipes by the retired but very active, Dr. John Luhman. The U of MN entomology museum (a valuable resource) had a specimen of this species that was collected in 1948 from corn near Rochester, MN. A common name for this beetle is the cranberry rootworm. It may be hard to believe, but even I am younger than many of the specimens in the museum’s collection.
Figure 2. Rhabtopterus pecipes and its feeding injury on corn leaves
Soybean aphid watch
I appreciate those who have responded over the past week with their field observations. Soybean aphid populations remain generally low with early planted soybean fields most likely to harbor aphid populations. Cold winter temps may have reduced egg survival higher in the buckthorn canopy. This spring, winged aphids moving from buckthorn likely had few emerged soybean fields to deposit nymphs. Some were able to colonize volunteer soybeans, but many of these were killed when late-planted corn fields were tilled. Those that did find a soybean field have struggled with high soil moistures and small soybean canopies providing little protection from rain and wind.
I would not waste any time feeling sorry for the aphids though. There is quite a bit of time left in aphid season. As protective soybean canopies close, host quality of early reproductive stage soybeans improve, and upcoming temperatures cool, aphids should fare better. Drier conditions would also favor aphids.
The unknown is long-term weather and whether there are any areas of Minnesota with sizeable aphid populations that are able to produce winged aphids to infest new areas. Although many fields of soybeans were planted late, these fields may be numerous enough that aphids may not be concentrated as usually happens with isolated, late planted fields.
Please let me know if you start to see fields where soybean aphid populations are easy to find and the percent of plants with aphids begins increasing.
That’s my hypothesis and I’m sticking to it…at least for now.
Figure 3. Imported longhorn beetle
More on those darn defoliators
I have been getting numerous calls and texts on imported longhorn beetles (Figure 3) and their distinctive feeding damage on the edge of soybean leaves (Figure 4). The beetles cannot fly, and damage typically remains isolated on field edges. We have a few soybean fields at the SWROC that have chronic infestations from an adjacent grassy area.
I had to puzzle a bit and open a few cranial filing cabinets before I was able to identify the Colaspis beetles that I was seeing in some Rock County soybeans. I had not seen these for a while and others have reported seeing these as well.
Figure 4. Imported longhorn beetle damage to soybean leaves.
Green cloverworm larvae are common in some areas right now. I was seeing quite a few of the dark adult moths a couple weeks ago and it appears they had some success in reproducing. While the larvae can reach economically damaging populations large numbers of moths showing up around Twin Cities homes can get folks real excited.
Figure 5. Green cloverworm. Four sets of prolegs.
Most butterfly and moth caterpillars have five pairs of fleshy abdominal prolegs. Green cloverworms (Figure 5) have only four pairs and other loopers have only three. There are a few forage/clover looper caterpillars (Figure 6) out there. These gray to tan, slender, twig-like caterpillars have three sets of prolegs. The less frequently encountered, green and more robust soybean, alfalfa, celery, and cabbage loopers (Figure 7) are not easy to identify in the field and also have three sets of prolegs.
Figure 6. Forage looper. Three sets of prolegs.
Painted lady butterflies are on the increase. These adults are the offspring from spring migrants from the south as well as reinforcements of recently migrating butterflies and their larvae. Thistle caterpillars (Figure 8) can attack crops, notably soybeans, dry beans and sunflowers.
Figure 7. Looper species. Three sets of prolegs.
Dave Willis, an old NW Minnesota crop consulting colleague and friend, reported numbers of thistle caterpillars in soybean and painted lady butterflies. He mentioned the butterflies were very abundant in canola, most likely for the nectar. Some of Dave’s butterflies may have come from southern Minnesota, maybe further south. Painted ladies like to migrate and we had spares.
Figure 8. Thistle caterpillar. These, alfalfa caterpillars, cutworms and woolly bears have five sets of prolegs.
For more information on assessing soybean defoliation and action thresholds see 2019 SW MN IPM Stuff issues 6 and 8.
WHEAT, OATS, BARLEY
The chronic rains and warming temperatures have created disease issues in some fields. A grower from WC Minnesota called and then stopped in at the SWROC with some wheat from a field he described as the plants having only a few leaves. The wheat had a severe bacterial streak/black chaff infection (Figure 9) of both leaves and glumes. Because its caused by a pathovar of the bacterium (Xanthamonus transluscens), this disease is not controlled by fungicides. Wheat varieties vary in their susceptibility. Planting clean, certified seed can help as this disease can be seed borne.
There are a large number of prevent plant acres in SW MN that are being planted to cereal cover crops, with some seed originating outside MN. Unfortunately, this might be a good way for a field to find some new disease and weed issues to deal with.
Figure 9. Bacterial streak of wheat.
Potato leafhoppers are the main pests to watch on this cutting.
Matt Benson just reported a cutworm species defoliating an alfalfa seeding under rye. This has implications for feeding treated hay.
DRY EDIBLE BEANS
Still hearing of economic potato leafhopper populations. The adults are very mobile and can easily reinfest fields from neighboring soybeans or alfalfa.
John Mahoney called on a WC Minnesota dry bean soybean field where green cloverworm larvae mortality was delayed after a generic lambda-cyhalothrin application but feeding on bean leaves appears to have stopped. I did not find any references to pyrethroid resistance in this species and suspect high temperatures and humidity during application as a possible cause.