Rolling Out the Welcome Mat to Tiny TerrorsIt’s tempting to plant corn – even soybeans – as soon as soil temperature reaches the 50º F threshold for a couple days, but pluses may not outweigh minuses, risk-wise, according to Illinois-based cropping consultant Ken Ferrie.

“While the gain from early planting can be five to 10 bushels per acre, we’ve seen sudden (seed) death syndrome (SDS) turn an 80-bushel soybean crop into a 40-bushel crop in a hurry,” Ferrie said in a recent podcast. “Furthermore, if your crop is infected by SDS, you likely won’t even be able to confirm it until weeks from now.”

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 said that both seed chilling and “spike down” loss have the possibility of a 7% – 10% stand reduction, so fighting both could already result in a 14% – 20% reduction, if everything else is perfect. He stressed that growers concerned with soybean germination scores might want to bump up population by 10%.

With SDS, the microscopic fungus Fusarium virguliforme infects soybeans in their early growth stages, but foliar symptoms won’t appear until it’s too late to protect against the disease. Growers can experience as much as 80% yield loss from SDS, according to Douglas Jardine, Extension plant pathologist and professor emeritus, Kansas State University.

Soybean SDS has become a major soybean disease throughout North America. Disease symptoms caused by F. virguliforme are most identifiable during late vegetative or early reproductive growth stages; however, initial infection usually occurs shortly after germination. Jardine explained that the late-season foliar symptoms appear as yellow, chlorotic blotches that form between soybean leaflet veins. The blotches expand into large, irregular, chlorotic patches that become brown and eventually die. Leaflets usually drop off the plant, leaving the petiole attached to the stem.

Moving back to corn (and plant diseases affecting this summer annual), we find a comparatively new kid in the plant disease community called tar spot. Purdue agronomist Dan Quinn, Ph.D., wrote that due to its relatively recent U.S. discovery, and ability to threaten corn productivity, tar spot is a very worrisome topic amongst Indiana corn farmers.

A severely infected field can incur yield losses upwards of 60 bushels/acre. Such losses often result from reduced photosynthetic capacity of the corn plant during grain fill, causing kernel abortion, or at least reduced weight. In addition, severe infection can reduce corn stalk integrity and cause significant lodging later on.

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 in that state in 2021. It’s caused by the fungus Phyllachora maydis and can be identified by small, raised black and circular spots present on corn leaves, stalks and husks.

Quoting Quinn again, “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.”

Gary Bergstrom, Ph.D., plant pathologist at Cornell, discusses tar spot further. He 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 Bergstrom: “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 of his statement, we glean that failure to introduce non-susceptible crops between years of corn doesn’t break the generation cycle of P. maydis. Also, leaving trash in the field gives pathogens a convenient place to hide. Sadly, so far, no seed treatment is highly effective against corn tar spot.

Bergstrom weighs in on soybean pathogens, specifically soybean cyst nematode, or SCN, (Heterodera glycines). SCN has been identified, in low numbers, in most of New York’s agricultural counties where soybeans (and other beans) are grown on large scale. He noted that for over 20 years 95% of SCN-resistant varieties have been developed, centering on one genetic resistance strain (through selective breeding). Sadly, he said, as a result “nematodes are becoming resistant to the resistance.”

Thus, for low SCN infestations, he recommends choosing high-yielding SCN-resistant varieties and rotating with non-host crops. Quoting him further: “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 these tiny terrors is key to beating them. With planting corn, I recommend that daybreak soil temperature be at least 50º before planting – and limit that early planting to 5% – 10% of the grower’s intended corn acreage. When that planting emerges and looks good, plant the rest.

More than 10 years ago, Eli called me out to examine corn crop failures on May 10 of that year. His farm was organic, so there was no seed treatment. I 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. Unusually cold soil temperatures stalled out the slowly germinating corn seed, offering prime fodder to these nematodes. Eli replanted half of his corn originally planted May 2. I asked him if I hadn’t recommended to him that 5% – 10% limit. He said that I did, but that he decided to ignore me.