Features Agronomy Diseases
On the lookout for disease threats

“Every year is different environmentally,” says Tenuta. “Warm and dry, hot and wet, or vice versa. Individual field management – more residue, less residue, tillage, no tillage – all of those things are factors that contribute to or decrease the risk of many of these foliar leaf diseases.”

After a long and harsh winter, Tenuta says not to worry as much about the pathogens, like Asian soybean rust, that overwinter in the deep south of the United States. Some, though, will thrive in an especially wet and warm spring. Here are seven diseases to watch out for this year – four in corn, and three in soybeans.

Anthracnose leaf blight
Often the first disease that corn farmers see in their fields, anthracnose can be severe, especially in warm, wet years. Anthracnose can be especially severe in tillage systems where infected debris is left on the soil. Before moving its way up the plant, the disease is first noticeable on the plant’s lower leaves. Once the plant hits the rapid growth stage, the symptoms often disappear entirely, only to return in the form of anthracnose stalk rot. For this reason, it is important to scout thoroughly and return to the sites where anthracnose is first detected.

The fungus that causes anthracnose overwinters in corn residue, which is why the disease can be especially bad in second-year corn crops. Managing for anthracnose starts with choosing a resistant hybrid. To lower disease risk in conventional tillage fields, it’s a good idea to remove corn residue. In minimum- or no-till situations, crop rotation is key.

“Anthracnose is not one that we normally would recommend a fungicide for, particularly because of the timing of the disease,” says Tenuta. “An early application of a fungicide that early on – there’s really no justification. I can’t really see the economic return to the producer on that type of application.”

Eyespot
In recent years, eyespot has been increasing in Ontario, particularly in high-residue fields and continuous corn systems. The disease prefers cool, wet conditions, and does overwinter in corn residue. Eyespot produces round or oval tan brown spots; lesions that can kill large portions of leaf tissue. To manage eyespot, choose resistant varieties, rotate crops and clear fields of crop debris.

Eyespot is ubiquitous, says Tenuta. “It’s there in most fields and growers will see it. We really wouldn’t recommend a fungicide application strictly for eyespot.

“Eyespot is a good example of why it’s important to be out in your fields, scouting, knowing and identifying the diseases,” he continues. “Proper management requires proper identification or accurate identification.”

Northern corn leaf blight
Northern corn leaf blight is the most economically important leaf disease in corn in Ontario. “In the province, anywhere we go, our surveys find that 95 per cent of the fields have some degree of Northern leaf blight – ranging from trace to severe,” says Tenuta. “A lot of this is driven by hybrids, genetics and resistance or tolerance. Northern leaf blight is a great example of a disease organism or pathogen that has multiple different races in existence in both Ontario and globally.” In Ontario, indications are new races are starting to develop, suggesting a decline in tolerance levels.

Northern corn leaf blight prefers prolonged periods of humid or rainy weather. The disease appears as greyish-green streaks, which often begin on the plant’s lower leaves. When the leaves above the ear are infected, losses can be severe.

Tillage will reduce inoculum levels in surface residue, as will crop rotation. In reduced tillage systems, be sure to choose resistant hybrids. If the leaves above or below the ear leaf start seeing disease at tassling or earlier, says Tenuta, a fungicide application is probably warranted and in most cases will provide an economic return especially when corn prices are good.

Grey leaf spot
Both destructive and economically important, grey leaf spot – the number one disease in corn in the mid-western United States – has been increasing in the area surrounding the Great Lakes in the past 10 years. The disease prefers wet conditions, and is especially prevalent in high residue, continuous corn systems. The good news is many new corn hybrids with good tolerance to grey leaf spot are available. Spores that are produced are dispersed by both rain splash and wind.

“If you see it working its way up the plant or starting to challenge the two leaves below the ear, then that is definitely a concern and a fungicide would be warranted for that,” says Tenuta.

Soybean cyst nematode
Soybean cyst nematode is the most economically important disease in soybeans in Ontario, says Tenuta, but it can be managed quite effectively. Soybean cyst nematodes are microscopic organisms that damage the plant’s root system, preventing it from taking in water and nutrients. More often than not, by the time symptoms are visible, significant yield has already been lost. In Ontario, losses range between five and 100 per cent.

There are several steps farmers can take to reduce economic losses due to soybean cyst nematode. “It requires a commitment on the producer’s part in terms of growing resistant varieties, using crop rotation – corn, wheat, for instance, in the rotation – and also, it’s important to be scouting in terms of knowing your populations and getting a soil test done every four to six years, just to get an idea of what the populations are doing.”

Closely associated with soybean cyst nematode is sudden death syndrome (SDS) of soybean, says Tenuta. SDS likes cool, wet conditions early on in the season. “Anything that inhibits root development will allow for the colonization of the roots,” says Tenuta, who suggests that producers plant later if those conditions are present. “One of the problems with sudden death syndrome is that management strategies are choosing more tolerant varieties, but we’re limited to the number of tolerant varieties available to us.” Seed treatments targeting the disease look very promising and hopefully will be available soon.

White mould
White mould ( Sclerotinia sclerotiorum ) is probably one of the most frustrating diseases out there because of its incredible sporadic nature, says Tenuta. It thrives in wet conditions, but will suddenly die off if the weather gets hot and dry.

There are some tolerant varieties available to producers, but Tenuta says that none of them offer true genetic resistance.

He suggests choosing varieties that are resistant to lodging and have a more vertical architecture to them.

Another management technique for white mould is to make the environment less favourable for the disease. “Some of the management practices for high yields are also ones that are increasing the risk of disease as well,” says Tenuta, who suggests planting wider rows, decreasing plant populations, and using longer rotations including wheat and corn.

There are different types of products available – biological, foliar and chemical, for example. Application timing is important, says Tenuta. With foliar applications, for instance, fungicides should be applied in and around the R2 stage or even at almost full bloom. “We need to protect those flowers because that’s the infection source. Once those flowers are infected, the disease can build from there,” says Tenuta. “If you wait too long, your fungicide is not going to be as affective on the white mould fungus.”

Frogeye leaf spot
Frogeye leaf spot is more prevalent in southwestern Ontario, particularly in high-residue fields with shorter rotations. “It can cause significant economic injury if it occurs earlier on a susceptible variety,” says Tenuta. “But the real key with frogeye leaf spot is what we’re seeing in the changing of the pathogen.” It is particularly problematic in the United States, where strobilurin resistance is becoming an issue.

While we often think of these resistance cases as isolated and the result of management techniques, Tenuta says they’re actually the result of natural mutations in the genome of the pathogen. Mutant strains of this pathogen have been found from the Gulf states to the mid-western United States.

“It’s an important reminder that these tools that we have available to us, whether it’s fungicides or germplasm, that things change and we always have to use them in the best possible way so we can maintain their longevity as a useful tool in our toolbox,” he concludes.

It’s a three-step process
1. Scout early and scout often.
2. Practice management techniques that reduce the risks, including choosing disease-resistant varieties and practicing longer rotations.
3. If warranted, use a fungicide.