As a lifetime ag educator, I try to impart a spirit of patience to corn growers this time of year. It’s tempting to plant corn – even soybeans – as soon as soil temperature reaches the 50º threshold for a couple days (soybeans prefer 60º), but pluses may not outweigh minuses, risk-wise.

Crop Comments: Let’s uninvite tiny, greedy ground gremlinsHere’s my favorite example of the cost of impatience: About a dozen years ago, an organic farmer who powers field operations with real horses called me out to examine a corn crop failure on May 10 of that year. Because his farm was organic, there was no seed treatment. I manually dug under the press-wheel marks, discovering thin corn hulls and minute white thread-like worms. Conducting a layman’s diagnosis, I determined the culprit critters were nematodes who’d stuffed themselves with the kernels’ innards.

Unusually cold soil temperatures stalled out slowly germinating corn seed, offering prime fodder to these nematodes. This farmer ended up replanting half of his total intended corn crop – the part that he’d originally planted May 2. I tried to avoid reminding him that I had recommended that he limit early corn planting to 10% to 15% of total intended corn acreage. The other part of that advice was as follows: when that planting had emerged – and was showing good green color – plant the remaining 85% to 90%.

Illinois-based cropping consultant Ken Ferrie looks at a potential seed failure problem this way: “While the gain from early planting can be five to 10 bushels/acre, we’ve seen sudden (seed) death syndrome (SDS) turn an 80-bushel soybean crop into a 40-bushel crop in a hurry. Furthermore, if your crop is infected by SDS, you likely won’t even be able to confirm it until weeks from now.”

He said folks itching to plant untreated soybean seed in cool soils in early May should observe this caveat: “The longer seeds sit in cold dirt unable to ‘fire’ a plant and emerge from the soil, the higher your risk that SDS will snuff out a chunk of your soybean yield potential later in the season.”

Ferrie further explained that both seed chilling and “spike down” loss each have the possibility of a 7% to 10% stand reduction, making a total possible yield loss of 14% to 20%. He recommended that growers worried about soybean germination bump up population by 10%.

According to Kansas State University agronomists, with SDS, the microscopic fungus Fusarium virguliforme infects soybeans in early growth stages, but foliar symptoms won’t appear until it’s too late to protect against the disease. Growers experience as much as 80% yield loss from SDS, now a major soybean disease throughout North America.

Disease symptoms caused by F. virguliforme are most identifiable during late vegetative or early reproductive growth stages; initial infection usually occurs shortly after germination. Late-season foliar symptoms appear as yellow, chlorotic blotches forming between soybean leaflet veins. Blotches expand into large, irregular, chlorotic patches that turn brown, eventually dying. Leaflets usually drop off plants, leaving petioles attached to stems.

Moving back to corn diseases, we find a comparatively new kid in the pathology community: tar spot. Purdue agronomists write that due to its relatively recent U.S. discovery, and ability to threaten corn productivity, tar spot seriously worries Indiana corn farmers. Severely infected fields incur yield loss up to 60 bushels/acre. Losses often result from reduced photosynthetic capacity during grain fill, causing kernel abortion (and at least reduced test weight). In addition, severe infection reduces corn stalk integrity, causing significant lodging later.

Tar spot was first confirmed in northwest Indiana in 2015. The first significant yield-reducing event of the disease was observed in 2018. Similarly, severe outbreaks and large areas of infection of this disease were observed there in 2021. It’s caused by the fungus Phyllachora maydis, identified by small, raised black and circular spots present on corn leaves, stalks and husks.

Quoting Purdue agronomist Dan Quinn, Ph.D., “These black and circular spots are known as fungal fruiting structures called stromata, each of which can produce thousands of spores. Overall, tar spot infection and severity can vary based on environmental conditions, the total amount of the pathogen present in the field and corn hybrid chosen.”

Cornell plant pathology researchers are weighing in on the subject. They said that cool temperatures and high relative humidity – along with leaf wetness for at least seven hours – create the ideal environment for P. maydis spore germination. Quoting these scientists: “Since tar spot overwinters, any affected residue left in the field will manifest the following year under the right conditions. For those producing silage, you’re going to remove and ferment a large part of the inoculum, but there’s still some left. Farmers who plant corn on corn, and reduce tillage, can expect tar spot to proliferate.”

Reading between the lines, we glean that failure to introduce non-susceptible crops between years of corn doesn’t break the generation cycle of P. maydis. Leaving trash in the field gives pathogens a convenient hiding place. Presently, no seed treatment is highly effective against corn tar spot.

Cornell plant pathologist Gary Bergstrom, Ph.D., weighed in on soybean pathogens, specifically soybean cyst nematode (SCN, Heterodera glycines). SCN has been identified in low numbers in most of New York’s agricultural counties boasting large soybean acreages. Bergstrom noted that for over 20 years, 95% of SCN-resistant varieties have been developed, centering on one genetic resistance strain through selective breeding. Unfortunately, as a result “nematodes are becoming resistant to the resistance.”

For low SCN infestations, Bergstrom recommended choosing high-yielding SCN-resistant varieties and rotating with non-host crops: “For moderate to high infestations and use of resistant varieties, farmers should do an HG-type test, and choose a suitable resistant variety or rotate to a non-host crop.”

Uninviting in-ground gremlins is key to beating them. I recommend daybreak soil temperature be at least 50º before planting corn, limiting early planting to 5% to 10% of the grower’s corn acreage. When that planting emerges and looks good, plant the rest.