Features Agronomy Diseases
Looking back, looking ahead

The cool, wet conditions in many parts of Ontario in 2014 favoured certain leaf diseases in corn and soybeans. Here, plant pathologist Albert Tenuta, with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), discusses several of the key foliar diseases in 2014. He also highlights some of the recent research findings on these diseases, and looks at management options for 2015.

“Why are we seeing more leaf diseases in general? One factor is more corn-on-corn and soybean-on-soybean acres. We’re also seeing more no-till and reduced tillage, which means more crop residues are being left in the field, and many of these foliar pathogens can survive and overwinter on residues. As well, weather conditions in recent years have been favourable for many of the leaf diseases,” Tenuta explains.
“In addition, the pathogen populations for diseases like northern corn leaf blight and soybean cyst nematode are evolving, with new strains or races that are able to bypass the resistance genes in our cultivars.”

Northern corn leaf blight
Northern corn leaf blight (NCLB) continued to be an important corn disease in 2014. “We’ve seen a steady, linear increase in northern corn leaf blight over the past 15 years or longer that OMAFRA and Agriculture and Agri-Food Canada (AAFC) have been doing corn disease surveys,” Tenuta notes.
“Northern corn leaf blight was found in over 90 per cent of the fields in the 2014 survey. In southwestern and central Ontario, almost every field had some degree of the disease. In eastern Ontario, the disease occurred in about 70 per cent of the fields.” 

NCLB is caused by the fungal pathogen Exserohilum turcicum (also called Setosphaeria turcica). “Typical symptoms are long, elliptical, greyish green to tan lesions on the leaves,” he explains. “When the lesions first start, they may be about two centimetres in diameter. They grow to about 15 centimetres or longer. The lesions get a blighting, almost a burn-like look to them.”

According to Tenuta, the disease often results in about a 10 to 15 per cent yield reduction. However, if the infection starts at an early growth stage in a susceptible hybrid, then losses can approach 50 per cent. The disease can also reduce the crop’s feed value.

Tenuta recommends an integrated pest management (IPM) strategy to manage crop diseases, including NCLB. A cornerstone of any IPM strategy is hybrid selection. However, some new strains of the NCLB pathogen are able to overcome a resistance gene that has been used in many hybrids.

“In the past, about 90 per cent of the NCLB isolates in Ontario and in the U.S. Corn Belt were controlled by the Ht1 gene. But in this past year’s work done in Lana Reid’s lab at AAFC in Ottawa, we found 11 different races of the pathogen in Ontario, and 10 of those were able to bypass the Ht1 gene,” Tenuta explains. “So if the resistance package in your corn hybrid relies on that single gene, it won’t be as effective as it used to be.”

Fortunately, other effective resistance genes have been identified. “The main ones are Ht2, Ht3, HtN and HtM. We found Ht2 was very effective, controlling 96 per cent of the isolates we found in Ontario in 2014. With Ht3, 87 per cent of the isolates were controlled by it. HtN controlled about 60 per cent, and HtM controlled about a quarter of the isolates,” he says.

“So Ht2, Ht3 and HtN, either alone or in combination, can be very effective at conferring resistance to northern corn leaf blight. We just need to incorporate them into our hybrids and make sure Ontario growers use hybrids with resistance to the isolates found here.”

Delayed planting tends to favour NCLB, so earlier planting is a useful IPM tool. He notes, “In 2014, the weather delayed planting across much of the province, which meant the main growth stages of the corn crop were later in the season when a lot of foliar diseases increase.”

Crop rotation and residue management are also helpful. “Northern corn leaf blight and a lot of other leaf diseases are residue-borne. If you can reduce the amount of residue in a field to about 30 per cent, then you reduce the disease risk while retaining the residue cover’s benefits for erosion prevention and soil management.”

Fungicides are valuable tools for managing NCLB. “We’re seeing an increase in the number of fungicides available and in their efficacy. There are some subtle differences between the fungicides, but overall, they are all pretty good at managing corn leaf disease, whether it is northern corn leaf blight, grey leaf spot or rust. So a big part of a fungicide decision will rest on the cost of application and the price of corn. When corn is at $3 or $4 per bushel versus $8, you need to get a bigger yield increase from the fungicide application to reach your return on investment,” Tenuta says.

“So, more and more growers should be and are targeting their fungicide applications to fields with the highest disease risk – corn-on-corn, high residue levels, a history of the disease, a susceptible hybrid, disease infection at an earlier crop stage – all those IPM factors.” He recommends scouting just prior to tassel emergence to assess each field’s disease status, considering all those risk factors as well as whether the weather conditions favour disease development, to help decide whether a fungicide application is warranted.

Tenuta and his colleagues have evaluated fungicide timing options for NCLB. “We’ve seen in Ontario and in the regional trials with our U.S. colleagues that traditional application timing from tassel to silking time is best. That timing gives about a seven- to eight-bushel yield increase, regardless of the disease pressure. Earlier applications, like a three-leaf or six-leaf timing, give less than a three bushel increase.”
He adds, “Where a fungicide application really shines is when you have more disease. Even with five to 10 per cent leaf disease, you start seeing a greater response to those fungicides and better return on an investment.” For example, in 2014 Ontario trials with two different hybrids, the researchers inoculated the hybrids with the NCLB pathogen to ensure high disease levels. They found a 14-bushel difference between the plots that received a fungicide application and those that didn’t.

White mould in soybeans
Tenuta notes the cool, wet conditions in 2014 caused white mould problems in much of the province’s soybean growing area, with higher levels in eastern Ontario. Fields with very serious infestations had greater than 50 per cent yield losses, but most fields had only low disease levels. “If white mould infection levels reach about 20 to 25 per cent, the disease will definitely have an impact on soybean yields.”

Also known as Sclerotinia stem rot, this fungal disease is caused by Sclerotinia sclerotiorum. The pathogen overwinters as resting bodies called sclerotia. In the spring, the sclerotia produce little mushrooms that release millions of tiny spores. The spores use soybean flower petals as an energy source to begin infecting the plant’s stem.

“Infected petals of soybean flowers are the primary method by which the disease starts in the plant. The big difference between 2014 and other years was that weather in 2014 across much of the region during post-flowering really favoured the disease, allowing the initial infections to continue,” Tenuta says.
He notes white mould is a disease of high yield potential fields. “The agronomic practices to reach high crop yields also create favourable conditions for many diseases, including white mould. Narrower rows, higher plant populations, earlier planting – those practices create a thicker canopy that closes faster. Those same factors result in a microclimate within the canopy that is ideal for white mould.”
For fields with a history of white mould, IPM strategies could include wider row widths or lower plant populations. 

Sclerotinia sclerotiorum can infect over 400 plant species, including crops such as canola and dry beans. Wheat and corn are not hosts of the pathogen, so a corn-soybean-wheat rotation can help reduce white mould risk.

Tillage practices have implications for white mould inoculum levels. “If the sclerotia are buried in the soil, they’ll survive for five to seven years, and so tilled fields with a history of white mould have a higher risk,” notes Tenuta. “In no-till systems, the sclerotia are left on the surface and that reduces their viability to less than two years. Although no-till or minimum till can reduce sclerotia viability, your risk of white mould can increase substantially under favourable conditions if soybeans are grown back to back, which may be the case for many producers in 2015.”

According to Tenuta, the pathogen’s wide host range includes many weed species, so good weed control is another IPM tool. In the U.S., herbicides that burn the foliage, thus opening the canopy, have been used to reduce white mould, although with mixed results.

Although no soybean varieties are totally resistant to white mould, varieties that are shorter, less bushy and straighter and that tolerate lodging produce canopies that are less favourable to white mould. “To choose a variety, growers can look to company literature and to Ontario soybean variety trials,” notes Tenuta. White mould performance trials are conducted by Tom Welacky, at AAFC’s Greenhouse and Processing Crops Research Centre at Harrow, Ont. In 2014, the susceptible checks in those trials had 25 per cent or greater white mould levels, so the trials generated good comparative data for the varieties.
Fungicide advances are providing biological and chemical options for managing white mould. “Some new biological fungicides are available, like Contans and Serenade. Contans is applied to the soil so the sclerotia can be parasitized. Serenade is foliar product. Both are good options for organic producers,” Tenuta says.

“New chemical foliar fungicides include Acapela, Allegro, Priaxor and Stratego PRO. Just like the biological fungicides, none of these provide total control for white mould. They are all registered as suppression only. Under low to moderate disease conditions, the level of control will be very good. But in epidemic years, don’t be surprised to see the disease which, although slowed down, may require a second follow-up application.”

Timing of fungicide applications is a little different for white mould. “For many other diseases, growers think of R3 timing (beginning of pod development) for fungicides. With white mould, because the petals are driving the early stages of infection, growers should consider R1 (first flower) to early R2 (full flower) timing,” Tenuta says.

He adds, “In research trials this past year, under high disease pressure, the best results tended to be with an R1 application followed by an R3 application.” So he suggests applying a fungicide at R1 and then, if conditions continue to favour white mould, making an R3 application. Research from Ontario and the University of Wisconsin shows that applications after R3 didn’t have much effect on soybean yields.
Because the flowering period in soybeans can last several weeks, fungicide timing can be a challenge. If you spray too early, you could still get a lot of petal infection. If you wait too long, the canopy will get thicker and it will be more difficult to get the product into the canopy where it needs to be. Tenuta emphasizes, “Spray penetration into the canopy and uniformity of spray coverage are critically important to suppress white mould.”

SCN plus SDS
Tenuta reminds growers to watch for soybean sudden death syndrome (SDS), which goes hand in hand with soybean cyst nematode (SCN), a major soybean disease in Ontario.

“Soybean cyst nematode is present in most of the soybean-growing areas across the province, with higher populations in the southwest than in regions like eastern Ontario, and it is now into Quebec. All Ontario soybean growers should know about SCN and be scouting for it,” he says.

“However growers may not be as aware of sudden death syndrome. Its distribution is closely related to the distribution of soybean cyst nematode. SDS is highest in the areas where we’ve had soybean cyst nematode for the longest time, which are the southernmost six or seven counties.”

SDS is caused by Fusarium virguliforme. This fungal pathogen overwinters on crop residue and in the soil as survival structures called chlamydospores. The chlamydospores germinate in spring and infect soybean roots. Root damage caused by SCN makes it easier for the SDS pathogen to enter the roots. As well, research indicates the SDS pathogen moves around in the cyst of SCN.

The SDS pathogen causes root rot, but it also produces a toxin that results in SDS’s foliar symptoms. “The typical symptoms of SDS are interveinal chlorosis – yellowing between the veins. The affected tissue turns brown and then disintegrates, leaving just the veins. Eventually, the leaves fall off, but the petioles, the stalks that attach the leaf to the stem, remain attached to the stem,” Tenuta says.

SDS infection usually occurs within the first few weeks after planting. The disease is favoured by prolonged cool, wet conditions early in the growing season, poorly drained soil, compacted soil, and the presence of SCN. Early planting tends to increase the disease risk.

Tenuta is working with his U.S. colleagues on a major SDS project. One component of the project involves studying isolates of the pathogen so researchers can get a better handle on the strains involved. Tenuta notes, “We’re seeing some diversity in the isolates, and it may be possible to group the isolates by region. So we’re investigating how our isolates relate to those found in other parts of North America.”

Another component in the project is evaluating practices for managing SDS. “We’ve found choosing varieties that have both good SCN resistance – PI88788 and Peking sources of resistance – and good SDS tolerance can provide a substantial yield increase and a reduction in both SDS and SCN symptoms,” he says.

The researchers have been assessing various new seed treatment products, with some good results. “One of the products, ILeVO from Bayer CropScience, has just been registered in the U.S., and has been submitted for registration in Canada. Other companies are developing SDS products, too,” Tenuta says. “In our research, we’ve found, even when planting early, the seed treatments can provide good SDS protection, with about a two- to four-bushel yield benefit.”

He adds, “New SCN seed treatments are also coming, so I think we’ll be able to put together a nice IPM disease management program, with crop rotation, good resistant varieties for both SDS and SCN, and the new SDS and SCN seed treatments.”

April 13, 2015  By Carolyn King

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