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Late Season Corn Health

This year certainly has come with new and unique challenges in crop production. We started under drought conditions during planting which quickly changed to saturated conditions. We've had new insect and disease problems and extreme weather events. Here are some challenges Ontario growers are facing in the final stretch of the corn crop.

Nitrogen deficiency - Photo courtesy of Lyss Gingras

Abiotic Stresses

Generally, we see the lowest leaves die off as the corn plant ripens and firing from nitrogen deficiency is pretty common to see on lowest leaves, both of which are not a concern. But when we start to see higher leaves firing, especially up to the ear leaf, it is a sign that there isn't enough nitrogen available and the crop is taking nutrients from lower leaves to fill ears. This can be seen in the picture above. On top of existing nutrient deficiencies, a few weeks ago, we had a nasty storm that caused hail and wind damage in several counties in our area. I live in Harriston (North Wellington) and there are several fields in the surrounding area where leaves on corn plants were shredded and there was some stalk breakage, similar to the picture below. The loss of this photosynthetic tissue often leads to the plant taking nutrients from the lower leaves and the stalk to fill ears, which can exacerbate existing nutrient deficiencies and lead to poor standability, increased susceptibility to stalk rots, and reduced test weight.

Hail damage - Photo courtesy of University of Nebraska-Lincoln

Foliar Disease and Subsequent Challenges

Leaf diseases are always a concern and often growers will apply a fungicide around silking for ear diseases, which also helps reduce or prevent leaf diseases as well. Disease pressure in corn has been significantly higher than in recent years. This year, growers are faced with a new disease: Tar Spot. Last year, this disease was only confirmed in a few fields in Southern Ontario, but it overwintered in residue here and has exploded. Now it's being found in fields across Southern, Western and West-Central Ontario. See most up to date distribution map below for confirmed locations as of September 15th. Severe infections can cause significant yield loss. Currently no fungicides are registered for this Tar Spot in Canada, as it is so new, but existing fungicide have also shown activity against this disease and it looks like we will have a couple of promising new options for next year. Anthracnose has been very common this fall and is another one to watch for as it can cause foliar symptoms, top die back and stalk rot. It also overwinters in infected corn residue. We are also seeing Gibberella stalk rot and a lot of Northern Corn Leaf Blight. See pictures below for descriptions of symptoms of these diseases.

Tar Spot Distribution as of September 15th, 2021 - Courtesy of Field Crop News

Northern Corn Leaf Blight

Appears as long, elliptical-shaped, olive-grey spots on leaves. As the disease worsens, lesions will join as and can cover and kill entire leaves. The spores/dust will rub off on clothing.

For more information:

Tar Spot

Appears as though someone took a paintbrush and splattered tar on leaves. Starts out as a couple of dots on leaves and worsens. If you scratch the spots, they will not come off. For more information:

Anthracnose Stalk Rot and Leaf Blight

Appears as large, black, shiny lesions on the exterior and black decaying tissue on the interior. The pathogen also causes top die off and foliar symptoms. the top die off appears as bleached upper stalks and leaf symptoms appear as oval or spindle shaped lesions that are tan or brown with dark margins. Similar to NCLB, lesions can grow together as infection worsens. For more information: +

Gibberella Stalk Rot

Not only does it affect ears with the characteristic pink mould, Gibb will also infect stalks, eating away at the pith tissue. On the outside of stalks, there are small black fungal structures that can be easily scrapped off. It can be mistaken with Fusarium due to the colour, but generally Fusarium will have less pink colour and some browning tissue as well. For more information: +

NCLB and Anthracnose photos courtesy of Crop Protection Network

Tar spot and Giberella photos courtesy of Matt Robinson

Why do These Matter?

Loss of photosynthetic leaf tissue, nutrient deficiencies and foliar disease can all cause plants to cannibalize remaining leaf tissue and stalks in order to fill the grain as much as possible. This can severely impact stalk quality, reducing water and nutrient movement and leads to increased disease susceptibility, stalk breakage, as well as a reduction in test weight and yield.

On the plus side, we have not observed any significant concerns around ear mould so far.

Yield, Quality, and Management Implications

As we've seen, disease and nutrient deficiencies have the ability reduce grain fill in several ways, leading to tip back, lighter test weights and lower starch levels, and even premature death of plants. As mentioned, standability is also affected and the crop is more vulnerable to stalk breakage, which can make harvest more difficult. When deciding which fields to harvest first, standability and disease pressure should be assessed. Conducting push tests in several areas of the field will help. These consist of pushing a plant just below the ear to a 45 degree angle to see if the stalk snaps. Cutting open stalks that break can help diagnose if the issue is a stalk rot or nutrient deficiency, foliar symptoms are also indicators. Fields that do not stand well should be considered for harvest first. Another component to determining harvest order, which we didn't touch on, is the severity of don producing ear moulds in each field. Consider harvesting fields with higher mould levels first before they progress. To reduce the potential for infection in future years, residue management and crop rotation are important. In severely infested fields, consider removing infected residue from the field or more tillage to bury it. Rotate out of corn to decrease inoculum in the field in subsequent years. Additional Considerations 1) Tar Spot has been confirmed on volunteer corn, so controlling it is becoming is increasingly important, as it can absolutely serve as a host. Not only for disease, but problematic insect pests as well *cough resistant corn rootworm cough*. 2) Apply a second, later pass, to reduce those later season leaf infections and keep the leaf tissue green and healthy. 3) With new diseases such as tar spot, later fungicide applications will also be increasingly important in corn management. 4) Selecting hybrids with good stalk strength and disease resistance for your higher risk fields can help. As tar spot is so new to us, research is underway to determine the most tolerant hybrids to the disease. From a scouting perspective, we are seeing differences in infection levels between hybrids this year, so hopefully this information will available at seed ordering time.

With all that being said, corn yields are anticipated to be strong this year. We hope that everyone's harvest goes well. We are looking forward to hearing how everyone's corn and beans do!

- Lyss Gingras 519-321-1454

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