According to Katelyn Miller, Western New York field crops and forage specialist, a relatively new disease threatens NY corn production – tar spot.
This plant disease originated in Central America and was first reported in the U.S. in 2015. The first significant yield-reducing tar spot event was observed in 2018. In 2021, NY’s first tar spot case was identified. Thus far, 12 NY counties reported this pathology, with no signs of it slowing down.
A foliar disease of corn caused by the fungus Phyllachora maydis, symptoms include small, round to irregular-shaped, raised black structures (called stromata) appearing on upper and lower surfaces of corn leaves. In severe cases, stromata are seen on husks, leaf sheaths and tassels.
P. maydis requires living hosts to grow and reproduce. Normally distributed through wind and rain splash, spores are also spread within the field. After infection, new stromata form within a few weeks.
Cool temperatures (60º – 70º) and high relative humidity favor tar spot. Extended periods of leaf wetness (at least seven hours) from rain, fog, irrigation or high relative humidity favor disease prevalence.
Yield loss from tar spot depends on several factors: time of disease onset, weather conditions and susceptibility of hybrids. Losses mostly consist of reduced ear weight and poor kernel fill in grain corn. Stalk rot and lodging occur in fields with high severity. Severe infestations hurt corn silage quality by reducing moisture, decreasing digestible components and reducing energy.
Resistance to tar spot is being increasingly bred into corn varieties. Growers should contact seed corn dealers regarding tar spot resistant varieties.
Despite temptation to plant corn – even soybeans – with soil temperature at 50º for a couple days, minuses may outweigh pluses, risk-wise. “While the gain from early planting can be five to 10 bushels per acre, we’ve seen sudden (seed) death syndrome (SDS) turn 80-bushel soybean crops into a 40-bushel crops in a hurry,” said Illinois-based cropping consultant Ken Ferrie 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 the following: “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 both seed chilling and “spike down” loss have the possibility of a 7% – 10% stand reduction, so having to fight both could already result in a 14% – 20% reduction – if everything else is perfect. Growers concerned with soybean germination should consider increasing population by 10%.
With SDS, the microscopic fungus Fusarium virguliforme infects soybeans during early growth, but foliar symptoms won’t appear until it’s too late to protect against disease. Growers experience as much as 80% yield loss from SDS, according to Douglas Jardine, Ph.D., Extension plant pathologist, Kansas State.
Soybean SDS has become a major North American 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 said late-season foliar symptoms are yellow, chlorotic blotches between soybean leaflet veins. Blotches expand into large, irregular, chlorotic patches that become brown, eventually dying. Leaflets usually drop off the plant, leaving the petiole attached to the stem.
Returning to corn diseases, Purdue agronomist Dan Quinn, Ph.D., wrote that due to its relatively recent U.S. discovery, tar spot concerns many Indiana corn farmers – first confirmed there in 2015. Severely infected fields can suffer yield losses approaching 60 bushels/acre.
Losses often result from reduced photosynthetic capacity of the corn plant during grain fill, causing kernel abortion (or at least reduced weight). Severe infection reduces corn stalk integrity, thus significant lodging later on.
Gary Bergstrom, Ph.D., Cornell plant pathologist, addressed tar spot: cool temperatures and high relative humidity – along with leaf wetness for at least seven hours – create ideal environments 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 (spores), but there’s still some left. Farmers planting corn on corn, and reducing 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 also fails to break the P. maydis generation cycle. Leaving trash in the field gives pathogens convenient places to hide. So far, no seed treatment is highly effective against corn tar spot.
Bergstrom changed gears to target soybean pathogens, specifically soybean cyst nematode (SCN, or Heterodera glycines (HG)). SCN has been identified, in low numbers, in most of NY’s agricultural counties where soybeans (and other beans) are grown large scale. For over 20 years, 95% of SCN-resistant varieties have been developed, centering on one genetic resistance strain (through selective breeding). As a result, “nematodes are becoming resistant to the resistance.”
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.” (Mixed mostly-grass sod fits here nicely.)
Uninviting these miniature menaces 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 your intended corn acreage. When that planting emerges and looks good, plant the rest.
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