Southwest Michigan field crop update – Sept. 10, 2020

Recent rainfall and cooler temperatures will help move crops toward the finish line. Diseases that love these conditions may be too late to ruin the party, depending on current crop development stage.

Precipitation totals
Precipitation totals for the last seven days, most of that falling on Tuesday, Sept. 8. The bar graph on the right depicts rainfall received at MSU Enviroweather stations throughout the region during this time period.


Temperatures this past week were near normal for the region with slightly cooler than normal temperatures to our north. The 6-10 and 8-14 day outlooks are both hinting at a moderation of these cooler temperatures toward normal, but Michigan State University Extension agricultural climatologist Jeff Andresen says the confidence level in these outlooks is currently fairly low. Rain since last Thursday, Sept. 3, has helped to make up for moisture deficits, particularly in counties that missed out on previous rains. The system that moved through earlier this week was widespread with an average of 1.5 inches received at MSU Enviroweather stations in south central and southwest Michigan. The current version of the Drought Monitor does not appear to include impacts from rainfall earlier this week.

Drought Monitor as of Sept. 8
Drought Monitor as of Sept. 8 (released Sept. 10).

The precipitation forecast for the next week calls for between 0.1 and 0.5 inches of rain, all of that expected to fall overnight Saturday, Sept. 13, with little if any chance of rain next Monday through Thursday.

Precipitation forecast
Precipitation forecast for Sept. 10-17 (left) and overnight Saturday, Sept. 13 (right).

Crops and pests

As the 2020 growing season draws towards a close, there are some important things we can learn from field observations before the crop leaves really degrade with maturity. This window has probably already closed for early planted soybeans, but later planted, longer maturity variety fields can still be evaluated for issues.

Tar spot has once again spread widely across southwest Michigan corn fields. The growth stage reached when the lesions infect the leaves and the leaf area impacted are probably the most significant indicator of potential yield loss from corn. There are wide variations between hybrids in terms of susceptibility to tar spot, with some hybrids able to withstand quite high levels of severity before the plants begin to collapse from the disease, while others may collapse quite quickly. Dry conditions helped to reduce infection in dryland fields this late summer, but timing and amount of rainfall has left patterns of infection across swaths of the region that follow the thunderstorm tracks. A lot of the infection we are seeing now is in fields that were scouted regularly and were “clean” up until the first few days of September. A return to rainfall spurred this later infection period. The degree of damage from these late infections will depend upon the plant growth stage, time of infection, and how long the crop will take to reach black layer.

Corn that is at R5.5 (half milk line) at the time of infection most likely will escape with limited yield loss. Corn closer to R5.0 (early dent) will probably have reductions in test weight and are very likely to have decreased stalk strength and lodging issues as we approach harvest. We still have a lot to learn about this disease. MSU research is being focused on trying to evaluate optimal timing for fungicide application, determining critical levels of the duration of leaf wetness that trigger infections, and at what growth stage at the end of the season a profitable application of fungicide can be made to protect the crops.

Martin Chilvers, field crops pathologist at MSU, is conducting a large-scale research project near Decatur, Michigan, in Cass County to help answer some of these questions for our region. MSU field crops agronomist Manni Singh is conducting research investigating crop development under our more current climate conditions. He is looking at yield potential for planting corn earlier as well as yield potential of shorter maturity hybrids that may help corn to reach physiological maturity earlier. This may help corn to evade damage from tar spot because the shorter days, higher humidity and more frequent rainy weather in late August and early September often tend to amplify the spread of the disease widely during this period.

Look over your fields, determine the level of corn tar spot, and see which leaves on the plant show signs of the lesions from the disease. If there is significant tar spot, the field should be evaluated for stalk strength regularly and prioritized for early harvest to avoid excessive lodging.

Sudden death syndrome (SDS) continues to spread across the region. Look for areas that have lost their leaves early, or for later planted or longer maturity varieties, those that turn to mature leaf colors early. There are a couple of things in play this season: spider mites as well as SDS and soybean cyst nematode (SCN). Spider mite infected fields should have some evidence of damage along the edges of fields while SDS/SCN often are in pockets in the interior of fields that may have increased incidence at the field entry points for equipment into fields.

Consider taking soil samples of areas of low yields or stunted plant heights by collecting soil samples through the root masses of soybeans in areas of the field that have reduced growth. Split the samples, send half in for a soil fertility analysis, the other half for SCN analysis. If the field has a history of SDS/SCN, consider having a bio-type analysis for SCN run on the sample. There is increasing evidence that there is spread of nematodes that are able to overcome the Peking source of SCN resistance in fields in southwest Michigan.

We have a limited window of opportunity to look at fields and look for the telltale signs of these diseases. Taking samples, noting when infections are likely to have occurred, and conducting sampling for SDS can really help you to make management decisions on how best to control these challenges in the years to come.

Water use

Forecasted weekly reference evapotranspiration (FRET) through Sept. 16 is significantly reduced from previous weeks at 0.85 inches due to cooler temperatures. With much of the corn crop approaching full dent and the early-planted and lower maturity group soybean in beginning mature pod stage, that amount is close to actual crop need, according to the MSU Soil Water Balance fact sheet.

Turning off the irrigation too soon could lower corn and soybean yields or reduce test weight. However, irrigating beyond the crop’s water need wastes time, energy and money. Applying water only when needed (irrigation scheduling) is environmentally and economically important. September rainfall in most years alleviates the late-season irrigation scheduling questions. The typical crop water usage drops quickly as average rainfall increases making late-season irrigation less important. However, many of the areas where crops were planted late may have substantial water needs well into September, signaling the need for some type of irrigation management either by a scheduling program or crop monitoring to determine timing and volume of irrigation application.

Try to minimize the number of applications needed to finish the crop. Large applications (0.8 to 1.1 inches) every six to eight days is more water-use efficient and has less tendency to increase disease potential than more frequent smaller applications. The goal should be to provide enough water to avoid yield and quality loss but not leave much of the applied water left in the soil profile in the end.

Soybean irrigators should maintain at least 50% of the available soil water-holding capacity until most pods are light yellow. Corn producers trying to maintain test weight in dry, late-summer conditions should maintain at least 50% of the available soil water-holding capacity until the crop reaches black layer.

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