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A print-friendly version of this newsletter is available to download here: SW MN IPM Stuff 2019-08
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.
As of July 15 we are still behind (about 4-5 days) on growing season temperature with 1134 degree-days compared to a 1213 degree-day long-term average. Maybe I could have squeezed another hyphen or two into the previous sentence! At 10.99 inches of precipitation since May 1, we are now only 1.7 inches above long-term average. On your farm, your mileage may vary.
We see tassels in some early-planted corn (very few fields of this near Lamberton) and corn developmental stage declines from there.
Soybeans are beginning to bloom and some fields are now at R2. Early planted and narrow-row soybeans are starting to close rows. Late planted 3-inch row soybeans not so much. Open canopies might present some weed challenges later this summer. The calendar date means fomesafen applications should cease to avoid carry-over injury to corn.
Once again, there are some reports of armyworm in corn planted with a rye cover. Nothing economic reported at this time, but just like dense grassy weeds, a live rye cover appears to increase attractiveness to egg-laying moths.
European corn borer (ECB)
ECB degree-day development models can be found at VegEdge Degree-day Models & Forecasts. Scouting should be focused on univoltine stage corn borer now (Figure 1). Focus on non-Bt hybrids first. While the 1st generation of the multiple-voltine (generation) ECB biotype is often most abundant on the largest whorl stage corn, univoltine corn borer usually lays eggs on corn at or near tassel. Scouting of corn larger than whorl stage means focusing on leaves near the ear leaf for egg masses and larvae. NDSU has a nice web publication for scouting and determining economic thresholds for univoltine corn borer.
Figure 1. Univoltine corn borer development based on degree-day accumulations.
Figure 2. Corn blotch leaf miner adult and leaf scars produced from egg laying
Corn blotch leaf miner (CBLM) appears to be causing some angst for growers and their advisors this year. Populations in some areas are high this year. Higher populations of CBLM may be related to wetter weather. The adult is a fly resembling a small house fly in shape and color. The female creates a small flap in the corn leaf and lays eggs within leaf. This egg laying creates distinctive scars. The larvae feed and tunnel between the leaf surfaces. The injury caused by this insect’s egg laying (Figure 2) and larval feeding (Figure 3) can be confused with some diseases.
Figure 3. Corn blotch leafminer injury to corn leaves. The injury can be confused with disease but is presumed mainly cosmetic.
The larval tunneling in leaves (mines) is greatest on the lower leaves where any impact on photosynthesis would be minimal. Control of the larvae inside leaves would be difficult and the long period of adult activity and multiple generations of this insect mean it would be difficult to time adult control with foliar insecticides. Additionally, since the injury is typically confined to leaves below the ear, control would not be likely to provide economic benefit.
In a corn date of planting study at the SWROC, the earliest planted corn is more heavily infested at this time, presumably based on the size of corn plants when first generation flies were active.
In any event, this injury cannot be helped by applying a fungicide. The presence of larval leaf mines and the small elongate scars produced by adults do, however, make it harder to identify any disease issues on lower leaves.
At this time, populations appear to be generally low with early planted soybeans most likely to harbor aphid populations now. However, there is quite a bit of time left in aphid season.
Please let me know if you see populations approaching economic threshold.
Those darn defoliators
There are several species of defoliators out there now and it is not likely worth the effort trying to seperate which one is doing what.
Figure 4. Red-headed flea beetle adult. Larvae feed on roots. Seldom enough defoliation to be economic on corn or soybeans.
Most commonly observed now are green cloverworm, flea beetle and thistle caterpillar. As soybeans reach the reproductive stages and, assuming the defoliating culprits are still present, the action threshold for insecticide treatment is 20%. Often the upper, newer leaves are most heavily fed upon but the threshold is based on leaf area of all photosynthetic leaves, not just the worst. Average defoliation of upper, middle and lower leaves and do not make a decision based on field borders alone. Figure 5. Shows computer generated representative defoliation for several insect species. With field practice you may be able to look at the feeding injury and know which insect is present before you see it.
Figure 5. Representative defoliation for several insect species. Source: R. Koch, UMN
Wheat, Oats, Barley
There are some nice looking wheat fields in SW MN. Hopefully, recent heat will not seriously impact grain fill.
Some have found crown rust in headed to dough stage oats. Unfortunately, the oats are past fungicide labeled stages. Fortunately, late-season rust infections will have less impact on yield.