Agronomy •  2023-07-31

9 late season corn & soybean diseases to scout for in August

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northern corn leaf blight

Scouting for late season diseases in August means you’ll have a better idea of what to expect as you make plans for harvest. Healthy, thriving plants hold the promise of full bins, while disease infection in a corn or soybean field could indicate problems ahead. In many cases, if you identify a problem it may be too late to mitigate or manage the impact of an infection, but farmers should remain vigilant in their scouting efforts.

Late season, or August scouting can also guide future variety, fungicide or seed treatment selections. It’s important to identify and record the presence of mid to late season corn and soybean diseases. Here are some common diseases Eastern Canadian farmers should watch for and note.


1. Tar spot

August scouting for tar spot is necessary to determine the presence and severity of the disease. Tar spot can be identified by small black and circular lesions. They are slightly raised bumps that can be felt on affected plant tissue. Lesions can appear on leaves, husks or stalks and are often surrounded by a light tan-coloured halo. Lesions are caused by fungal structures called stromata that cannot be rubbed off affected plant tissues.

Symptoms appear most frequently from silking through late grain fill. Tar spot can seriously impact yield, and under severe infestation, where most of a field shows 50% severity on the ear leaf during grain fill, significant yield losses of 20-60 bu/ac have been reported.1

Notes for next year: If tar spot is identified in your field this year, chances are you’ll need to make a plan to manage it again next year. Tar spot management requires both in-plant tolerance and fungicide application. Corteva research scientists have been studying tar spot and screening hybrids to develop a rating scale to communicate a hybrid’s level of tolerance, and significant genetic response to tar spot looks promising. Select tolerant hybrids next year and scout vigilantly to time fungicide applications wisely.

2. Northern Corn Leaf Blight

Northern corn leaf blight is the most common corn leaf disease in Ontario. Early season infections can have a serious impact on yield. Common symptoms often start on lower leaves and include elliptical, gray to tan lesions on leaves. Lesions can be anywhere from 1” – 6” long. Under high humidity, spores coating the lesions turn olive-green or black, giving leaves a dark or dirty appearance.

Notes for next year: Plant a high yielding corn hybrid with tolerance to northern corn leaf blight, rotate crops and avoid planting corn in successive years, till fields to encourage decomposition of infected residue, and apply a fungicide preventively or, if necessary, curatively.

3. Grey leaf spot

An early infection of grey leaf spot can also see secondary outbreaks later in the season, disease spread and leaf damage — resulting in even greater yield losses. If late-planted corn is exposed to the disease at vulnerable early growth stages, it can suffer higher levels of infection. Significant yield loss can occur if leaf area is destroyed before grain fill is complete.

Grey leaf spot can be identified by long, thin, rectangular lesions, up to 2” in length that appear on leaves. The lesions start out tan and usually turn grey. Grey leaf spot fungus overwinters in corn residues, making preventative crop management practices essential, especially since the threat of infection is greater with continuous corn.

Notes for next year: Choose grey leaf spot resistant hybrids and carefully plan crop rotations, avoiding planting corn in successive years. Till fields to encourage decomposition of infected residue. Fungicides can be applied preventively or, if necessary, curatively.

4. Gibberella Ear Rot

A pink-coloured mould that usually begins at the tip of the ear, Gibberella ear rot thrives in cool and wet summer conditions. Mycotoxin contamination of grain may or may not accompany Gibberella ear rot symptoms. The primary mycotoxin is deoxynivalenol (DON), also called vomitoxin.

Gibberella ear rot can be most readily identified by the red or pink color of the mould and almost always begins at the ear tip and progresses from there. Scouting fields in August and prior to harvest is essential to identify severe disease outbreaks and make informed decisions about harvest timing, postharvest grain handling, storage, and utilization. Affected fields should be harvested as soon as grain moisture allows.

Notes for next year: Selecting tolerant hybrids can help reduce Gibberella ear rot and associated mycotoxins. Plant breeders have been able to select for physical traits that limit disease development, as well as genetic tolerance. Crop rotation and residue management can also be integrated as control measures.


5. White mould

A serious yield robber and growing perennial issue in Eastern Canada, white mould is caused by the fungus Sclerotinia sclerotiorum. Disease symptoms typically show late in the season as grey to white lesions on the stems. When scouting, a tell-tale white mould symptom is wilted, dead leaves that will remain attached to the stem at the end of the growing season. As the growing season progresses, infected soybean plants are generally killed in patches late in the growing season, and the black bodies (sclerotia) of white mould are sometimes found in the seed at harvest.

Notes for next year: Adopting best management practices to minimize the risk of white mould is a farmer’s first line of defense. Early season scouting for infection is essential, and fungicide application(s) can be effective. The most effective fungicide available to combat white mould in soybeans is Viatude™, a new multiple effective mode of action solution for resistance management. Variety selection should also be a consideration. Enlist E3™ soybeans offer enhanced traits that can help with resistance to white mould.

6. Soybean cyst nematode

Left unmanaged, soybean cyst nematode (SCN) population densities and the potential for yield losses steadily increase. Unfortunately, SCN often goes unnoticed in fields because aboveground symptoms of damage and yield loss may not be visible.

SCN can be deceiving, with SCN infested fields and soybeans looking healthy, despite being infected. Farmers should scout for SCN by carefully digging-up plants, gently removing the soil from the roots and examine them for the “pearl white to yellow” cysts. This goes for farmers who are using SCN resistant varieties too, since SCN field populations can adapt to these varieties resulting in increased SCN reproduction and yield losses. Checking roots for SCN can be done 5-6 months after planting and throughout September.

Notes for next year: If SCN is detected in soybean roots, collect soil samples in the fall to confirm SCN numbers. This information can direct the best management strategy to implement. The first line of defense against this disease is selecting a resistant variety, so refer to your scouting notes when selecting varieties for the 2024 growing season. Remember, as nematodes adapt, rotating genetic sources to preserve the effectiveness of SCN-resistant varieties is important. Success also requires diligent scouting and proactive management.

7. Sudden death syndrome

A fellow yield robbing disease, sudden death syndrome (SDS) causes infected soybean plants to lose leaf area and drop leaves prematurely, deteriorates roots to restrict water and nutrient uptake, aborts flowers and pods, and causes smaller seeds and later-forming pods that may not fill or mature. Although infection often occurs during the first 6 weeks after emergence, symptoms generally do not develop or become noticeable until the mid-pod reproductive stage of soybean development. The first leaf symptoms of SDS appear as yellow spots (usually on the upper leaves) in a mosaic pattern. Leaves will also show yellow and brown areas contrasted against green veins, and twist and curl and fall from plants prematurely. Foliar symptoms are a result of the toxin produced by the fungus Fusarium virguliforme following early season infection of soybean roots. A blue colouration may be found on the outer surface of taproots due to the large number of spores produced. SDS is likely to first appear in fields with soil compaction or history of soybean cyst nematode (SCN) infection.

Notes for next year: Currently, the most effective method to manage SDS is the use of tolerant varieties. Consider Enlist E3™ soybeans that offer enhanced traits that can help with resistance to SDS. Seed treatments can also be an effective option, along with best management practices like planting the most problematic, or SDS infected fields last in your planting sequence.

8. Brown stem rot

Brown stem rot is caused by Phialophora gregata, a fungus that survives in soybean residue. Starting at the roots, brown stem rot infects the plant and causes yield loss. The fungus infects roots early in the season, but symptoms of vascular system damage usually appear in mid-summer, during reproductive development. Tell-tale signs of infection are brown to reddish-brown leaves.

Foliar symptoms of brown stem rot can be confused for sudden death syndrome. Be sure to distinguish the difference between the two soybean diseases by splitting stems lengthwise when scouting. The centre of the stem or pith will be white for sudden death syndrome and brown for brown stem rot.

Notes for next year: Since the fungus survives in infected soybean residue left on soil surface and can continue to reproduce throughout the winter, disease management is important. Selecting brown stem rot resistant varieties is essential. Crop rotations, with a recommended two years in between soybeans crops per field and burying infected soybean residue with tillage are also effective practices to reduce the risk of brown stem rot.

9. Phytophthora root and stem rot

Phytophthora root and stem rot can occur throughout the growing season in susceptible varieties, making late season scouting essential. This disease favours extended wet field conditions, so target scouting to fields with poor drainage 1-2 weeks after a heavy rain. Early symptoms include wilting and yellowing leaves. Eventually, a chocolate brown lesion will form, starting below ground to almost mid-height. Roots of infected plants will be discoloured. The disease displays seed rot, seedling blight and root/stem rot phases, but above-ground symptoms may not be evident for several weeks after initial infection. The phytophthora fungus can kill plants at all stages of growth and result in yield loss.

Notes for next year: Variety selection and seed treatments are the most effective ways to manage Phytophthora. Enlist E3™ soybeans offer variety traits with resistance to phytophthora root rot. Field drainage should be another consideration, especially planting soybeans on heavy soils or in no-till systems.  Consider LumiTreo(TM) seed treatment for industry leading protection against phytophthora.

Corteva Agriscience scores seed products for disease tolerance, determining reliable product scores to help farmers select varieties and hybrids best suited to their fields and disease risk.

If you have questions or concerns about crop conditions or diseases, consult your trusted agronomist, retailer, or local Corteva Agriscience™ Territory Manager for advice. For more information on scouting late season corn and soybean diseases, visit the Ontario Ministry of Agriculture, Food and Rural Affairs Agronomy Guide for Field Crops online resource.