Crop Express – August 2024: Tar Spot in Corn, White Mold and Goss's Wilt
Seven updates from the field this edition: tar spot in Midwest corn, white mold in soybeans, Goss's wilt arriving in southern Wisconsin, July tissue tests showing nitrogen dropping, new Satchur 8 trial data, Ultra Fluent on soybeans, and year-to-date growing degree days. Jump to any section below.
Jump to any section below.
Tar Spot in Corn: How to Identify and Manage It
Tar spot has become a serious concern for Midwest corn growers since it was first confirmed in the United States in 2015. It is caused by the fungus Phyllachora maydis, which infects only corn, and it has spread across states including Illinois, Indiana, Iowa, Michigan, Minnesota, Nebraska, Ohio, South Dakota and Wisconsin.
How to identify tar spot
Small, raised black spots called stromata form on the leaves and, in severe cases, on husks and leaf sheaths. They look like flecks of tar and measure about 1/16 to 1/4 inch across.
Tan to brown "fisheye" lesions with darker borders can form around the black spots.
The spots can't be rubbed off the leaf, which separates them from insect frass or soil. Late-season rust can look similar, so get a lab confirmation if you're unsure.
Conditions that favor tar spot
Tar spot favors cool temperatures of about 60 to 74°F with repeated moisture events. In my opinion, hours of leaf wetness matter most, and eight hours or more is key to infection.
The fungus overwinters on infested corn residue. Spores move by rain splash and wind, including between fields, so crop rotation and tillage play only a minor role in management. New stromata can form in about 12 to 21 days, which lets the disease cycle several times in one season.
How much yield tar spot can cost
Moderate epidemics can cut yield by about 10 to 30%, and severe epidemics by up to 50%. Losses are greatest when disease develops before or during tassel to milk stage (VT to R3). The black stromata reduce the leaf area available for photosynthesis, which lowers kernel fill and ear weight. Heavy infections can also kill leaves early and raise the risk of stalk rot and lodging.
Managing tar spot
Scout regularly, especially as corn moves into reproductive stages. Early detection drives every other decision.
Choose less susceptible hybrids. Every hybrid has some susceptibility, so check seed company ratings and university trial data.
Time fungicides carefully. A single application between VT and R3 has given the most consistent protection in university trials. Multiple applications may be needed under high pressure.
Use risk tools. Apps such as Tarspotter, developed in Dr. Damon Smith's lab at the University of Wisconsin, help assess infection risk and time fungicide applications.
Watch the biological research. Scientists are studying fungi and bacteria that parasitize tar spot stromata, and some have shown promise.
Tar spot remains a challenge, but research keeps improving what we know about risk and timing. If you would like help diagnosing tar spot or other plant diseases, reach out and we will travel with you to the farm.
White Mold Has Become an Annual Threat in Soybeans
White mold, also called Sclerotinia stem rot, is caused by the fungus Sclerotinia sclerotiorum. Across the North Central United States and Canada, it has moved from an occasional problem to a yearly threat for soybean growers. A University of Wisconsin research summary put white mold losses in the US and Ontario at more than 101 million bushels, worth about $1.2 billion.
Infection weakens plants and can lead to wilting, lodging and plant death. White mold can also reduce seed quality, so the cost goes beyond bushels lost.
White mold mycelium on a soybean stem.
White mold apothecia, small cup-shaped fruiting bodies, on the soil surface.
Sclerotia forming on an infected soybean stem
What raises the risk
Weather: cool temperatures (59 to 68°F) and high humidity during flowering and canopy closure.
Field history: fields with past white mold carry more sclerotia in the soil.
Canopy: dense canopies, high plant populations and narrow rows hold moisture under the canopy.
Variety: no soybean variety is fully resistant, but some tolerate white mold better than others.
How to reduce losses
Rotate: two to three years of non-host crops such as corn or small grains can lower sclerotia levels in the soil.
Adjust the combine: set it to keep sclerotia out of the clean grain tank and avoid grain quality discounts.
No single practice controls white mold, and results vary with the weather and disease pressure each year. An integrated approach gives the best odds.
Year-to-Date Growing Degree Days
Growing degree days (modified base 50°F, used to track corn development, codling moth and European corn borer) were running ahead of 2023 at every location, by 85 to 236 GDDs. Where a 30-year average is available, 2024 was 105 to 287 GDDs ahead of it.
Location | 2024 GDDs (mod. base 50°F) | 2023 GDDs | 30-year avg. (1991–2020) |
Dubuque, IA | 1,934 | 1,797 | 1,647 |
Lone Rock | 1,763 | 1,645 | – |
Beloit | 1,871 | 1,668 | 1,618 |
Sullivan | 1,706 | 1,582 | – |
Madison | 1,720 | 1,635 | 1,615 |
Juneau | 1,650 | 1,542 | – |
Racine | 1,704 | 1,468 | – |
Waukesha | 1,645 | 1,510 | – |
Milwaukee | 1,688 | 1,544 | 1,474 |
Goss's Wilt Arrives in Southern Wisconsin
Goss's wilt has arrived in southern Wisconsin. This bacterial disease, caused by Clavibacter nebraskensis, can cut corn yield and quality, and how much depends on the hybrid, the timing of infection and how much leaf area is lost during grain fill.
Timing matters most. Infections before V6 often become systemic and can kill plants. Later infections cause leaf blight, and late-season infections usually cost far less yield.
Hybrids differ. Susceptible hybrids infected during vegetative growth can lose more than 50% of yield. Resistant hybrids lose much less, though none are immune.
Look for freckles. Dark, water-soaked spots inside the lesions look translucent when held up to the light. In humid weather, lesions can show a shiny bacterial ooze that dries like varnish.
Fungicides won't help. Because Goss's wilt is bacterial, resistant hybrids are the main tool, backed by rotation to non-host crops, residue management and grassy weed control.
Tissue Tests Show Nitrogen Dropping
Nitrogen levels have been dropping as rain has driven nitrogen out of the root zone. Potassium is also dropping in some fields, as a result of compaction and shallow root systems.
This year has also produced some very low boron levels in corn as we approach R1, lower than I'm used to seeing. About 90% of our tissue tests have come back deficient in calcium and magnesium, which is also very unusual. The weather continues to surprise us, and we continue to learn.
Satchur 8 Trial Results
Four replicated trials show how Satchur 8 performs as a tank-mix partner with insecticides and fungicides, including on rainfastness and wet leaves.
Cotton aphid control on cucumbers, San Joaquin Valley, California
This replicated trial by TRACS (Ron Kukas) compared an untreated check, Brigade 10WP (bifenthrin) at the standard rate of 0.1 lb ai/a, and Brigade plus Satchur 8 at 0.1%. Fourteen days after application, leaves treated with Brigade alone averaged about 19 aphids, compared with about 4 where Satchur 8 was added. Bifenthrin, a pyrethroid, typically struggles to hold aphid populations down for more than 5 or 6 days.
Gray mold on blueberries with Switch fungicide, Visalia, California
This replicated trial on 'Emerald' blueberries tank-mixed Satchur 8 at 0.1% v/v with Switch fungicide at 11 oz/A. Applications were made with a CO2-powered experimental sprayer at 96 gallons per acre, directed from both sides of the row until plants were completely wet. There was no crop injury on leaves, buds or developing berries, and no mixing or spraying issues. The tank mix significantly reduced the number of shoots infected with Botrytis compared with Switch alone, with no infected shoots recorded at day 11, 21 or 35.
Rainfastness on corn: Cornell University trial
This replicated trial was led by Prof. Jed Sparks, Ph.D., Director of the Cornell Isotope Laboratory (COIL) and Professor and Chair of the Department of Ecology and Evolutionary Biology at Cornell University. Isotopically enriched (2H) bifenthrin, the active ingredient in Brigade, was applied to corn leaves with and without Satchur 8 at a 1:1000 dilution. The leaves then received half an inch of rain in 3 minutes every day, for the equivalent of 30 days of precipitation. Satchur 8 provided a 35 to 45% difference in bifenthrin retained on the leaf.
Spraying wet crops: Cornell University trial
A second Cornell trial applied (2H) bifenthrin with and without Satchur 8 to wet and dry corn leaves, then exposed the leaves to a series of artificial mist events. Bifenthrin with Satchur 8 applied to a wet leaf gave statistically similar adhesion to bifenthrin alone applied to a dry leaf for the entire experiment.
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Ultra Fluent on Soybeans Performing Well in 2024
We have had excellent performance from Ultra Fluent on soybeans for the past two seasons. Ultra Fluent Soybean is a soy-based seed fluency agent that also supports biological nitrogen fixation. The photos below show nodules on soybeans treated with Ultra Fluent this season.
Year-to-Date Growing Degree Days
Growing degree days (modified base 50°F, used to track corn development, codling moth and European corn borer) were running ahead of 2023 at every location, by 85 to 236 GDDs. Where a 30-year average is available, 2024 was 105 to 287 GDDs ahead of it.
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