Intruder Alert: Wheat, Pulse and Soybean Growers Beware

Friday, April 17, 2020

Better Farming connects with industry experts to learn the latest on pesky pests and dreaded diseases.

by Geoff Geddes

For city dwellers, the greatest threat from insects could be a bed bug infestation or a mosquito bite. But when farming is your livelihood, insects and plant diseases can bite into your bottom line in a hurry.

The key to winning any battle is knowing your enemy. The more you learn about these unwanted guests and their effects on pulses, soybeans and wheat, the better your chance of winning this year's war.

Insects targeting pulses

The two insects that pose the highest risk to pulses are cutworms and grasshoppers, many industry stakeholders say. Scouting your fields and using economic thresholds to make spraying decisions are your best ways to address these pests.

"Cutworm populations tend to go in cycles, and they have been building for a few years now on the Prairies," says Dr. John Gavloski. He's an entomologist in the primary agriculture branch of Manitoba Agriculture and Resource Development. Gavloski is based in Carman, Man.

"We've also seen growth in grasshopper numbers over the last three years because of hot, dry summers." These pests love these conditions, he adds.

Apart from weather, natural enemies also influence grasshopper populations. In 2019, Manitoba had high levels of bee flies, which are predators of grasshopper eggs. After tracking the places where grasshoppers lay their eggs, bee flies deposit their eggs nearby. The bee fly larvae feed on the grasshopper eggs.

Though the presence of bee flies has somewhat mitigated grasshopper damage in pulses, both natural enemies and conducive weather conditions are necessary to eliminate the threat of grasshoppers.

For years, farmers in Alberta and western Saskatchewan have worried about pea leaf weevil. Recently, the pest spread to eastern Saskatchewan and, in 2019, appeared in Manitoba, Gavloski says.

"The larvae of pea leaf weevil feed on pulse root nodules, preventing the plant from fixing nitrogen," says Gavloski. "I am trying to educate agronomists and pea growers on what to look for when surveying their fields so we can track the location and levels of this pest."

Lentil growers can use a sweep net to gauge insect levels, while pea growers should examine plant tips for signs of the pest.

Insects attacking soybeans

Cutworms and grasshoppers are also two of the greatest enemies of soybeans.

Agronomists encourage growers to scout their fields in late May or early June for both cutworms and grasshoppers. Producers should only spray if needed, using patch spraying if populations are isolated. Since both pests are nocturnal, growers should spray as late in the day as possible. (While grasshoppers are active during the day, they also feed at night.)

"Regardless of the pest, producers should consult a provincial guide such as Manitoba's Guide to Crop Protection for information on registered insecticides and economic thresholds to aid in spraying decisions," says Gavloski.

In 2019, the thistle caterpillar was another common threat in dry areas of the Prairies. In these conditions, the pest thrived and soybeans grew poorly, making it harder for the plants to resist the attack.

"Thistle caterpillar tends to be hit and miss," says Gavloski. "Most years, the levels are low enough that it's not an economic issue. But, when you have high populations combined with drought conditions, (thistle caterpillars) can be a problem for growers. While no insecticides are registered in Canada for thistle caterpillar, soybeans are good at compensating for defoliation brought on by this pest."

Spider mites – another soybean pest – also do better in drier conditions. These insects can often be controlled by spraying the edges of fields where the pests tend to congregate.

Insects homing in on wheat

One of the leading wheat enemies on the Prairies today is the orange blossom wheat midge, a tiny mosquito-like orange fly that lays eggs in developing wheat flowers. Adults emerge during a four- to six-week period that coincides with wheat head emergence and flowering. The wheat midge deposits its eggs on florets or developing kernels just prior to flowering and generally hatch in four to seven days.

Wheat stem sawfly – which belongs to the wasp group – is common in southern Alberta and parts of Saskatchewan. The adult looks like a wasp and lays eggs on growing wheat stems. The eggs hatch into larvae and live inside the plant.

"This wasp can be devastating to yields," says Dr. Haley Catton. She's a Lethbridge-based research scientist at Agriculture and Agri-Food Canada and focuses on cereal crop entomology. Wheat stem sawflies "mine the inside of the stem out and make the plant fall over, so it lodges and can't even be harvested."

Control methods for wheat stem sawfly may be warranted when 10 to 15 per cent of last year's field is cut. However, effective chemical control does not exist for this pest. The most effective management system is a combination of cultural and biological control. Cultural control consists of crop rotation away from cereals and using resistant (solid-stemmed) varieties.

Some of Catton's research focuses on wireworms. As the distribution of this pest tends to be patchy, farmers must know the field's history to determine the likelihood of a problem. But, if left unchecked, wireworms are another pest that can cause great damage to wheat.

A relative newcomer, the cereal leaf beetle appeared on the Prairies in 2005. This leaf-feeding pest only warrants spraying if you find at least one larva in every plant.

"There is a beneficial insect called T. julis that controls cereal leaf beetle quite well, so we don't want producers spraying for it unless it's widespread," says Catton. "We are asking people to send us a sample of the larvae, and we can tell them if there are beneficials in the field that need to be preserved."

For many crop pests, risk maps are available through the Prairie Pest Monitoring Network, a group of field crop entomologists who aid crop protection through research and tracking.

"Controlling insects takes a multi-pronged approach that includes risk maps, crop scouting, resistant varieties and proper rotation," says Catton.

Diseases threatening pulses

One disease common to all legume crops is Sclerotinia or white mold. While this disease is most common in dry edible beans, its host range extends to other broadleaf crops, such as canola, grown in rotation. Sclerotinia also affects sunflowers, which are an important crop in Manitoba.

Both canola and sunflowers can leave a lot of resting bodies in the soil, so the fungus survives apart from the living host. The leftover fungus is the greatest threat to all pulse crops, which don't generally have plant-to-plant contact. Sclerotinia can move quickly this way and cause considerable damage.

Several fungicides are registered for this disease in beans, but all products must be used preventatively and applied before spores of Sclerotinia are airborne in the area. The application window would usually be when the bloom begins and continues through the bloom period for up to two to three weeks depending on the weather.

But when two applications are warranted in a season, which occurs when conditions favourable to spore release persist beyond the effective protection period of the fungicide, other factors come into play.

"Any time you apply two fungicides consecutively in a crop, you must be concerned about resistance management," says Dr. David Kaminski. He is a field crop pathologist in the Government of Manitoba's primary agriculture branch. He focuses on agriculture and resource development and is based in Carman, Man.

Growers should also be aware of pre-harvest intervals and how late in a crop's development a product can be applied. Because people eat many of these crops, the fungicides must break down and be undetectable at the time of harvest.

Pulse crops are often grown under contract, so producers should check with their buyers to confirm which fungicides or pesticides are acceptable to the end user.

Since pulse crops are often grown for the food market, eye appeal is important. As a result, any disease that visibly stains the seed could be an issue for producers as it can cause dockages for lower grading. In wet years, for example, anthracnose – a fungal disease of edible beans that can transmit through the air or crop stubble – may pose a problem.

Bacterial diseases such as common blight and halo blight often occur in bean crops. These blights require minimal moisture to establish themselves and begin growing.

"If these blights are serious enough, they will discolour pods, and not many products are effective in controlling them," says Kaminski. "The most common fungicides for this purpose are copper-based, and they have a short residual lifespan, so they may have to be applied repeatedly to get results."

Diseases affecting soybeans

Septoria leaf spot and bacterial blight are regular visitors to Prairie soybean fields, especially in wetter periods. White mold and frogeye leaf spot may also pose challenges, and seed staining can result from a range of canker diseases that attack the stems.

One of the more destructive diseases for soybean yield is root rot, which thrives in wet conditions. Soybean cyst nematode (SCN) can also devastate yields, and industry experts pay close attention to this disease in Manitoba.

"Nematode is firmly on our radar, as many areas of the province rely heavily on soybeans in their rotation," says Kaminski. SCN "survives apart from the plant as a cyst in females that can move with the soil via many modes including water, wind and equipment. Growers should be cautious when moving equipment from field to field and focus on management techniques to minimize the disease, just as with club root in canola."

While many fungicides now have broader registrations for use in many conditions, growers must consider the specific disease-host combination when they select their strategies.

Diseases striking wheat

Fungal leaf spot complex is a constant threat to wheat. Depending on the weather, rotation and the variety's resistance, this complex can range from trace amounts to levels that require fungicide application on the flag leaf to prevent losses.

"You need to scout for cereal leaf spot every year and look for symptoms on the leaves that often appear as light green or yellow spots or streaks," says Dr. Michael Harding. He's a research scientist and plant pathologist at Alberta Agriculture in Brooks.

Other diseases such as seedling blight and root rot are harder to spot as they exist underground, so growers must selectively sample some plants to determine prevalence. These diseases can cause problems with stand establishment, as new plants either fail to emerge or die soon after emergence.

To avoid these interlopers, use good-quality seed with high vigour and time planting to minimize the number of days to emergence. Growers should seed into warm soils and avoid seeding more deeply than necessary.

"The more time your germinating seed spends underground struggling to get up, the more time there is for those pathogens to attack and cause problems," says Harding.

A good news story of late is the lower incidence of Fusarium head blight (FHB) in Alberta. In 2016, the disease caused substantial damage, so the reprieve is welcome. FHB does best in wet soil, so the drier weather through much of the province in recent years has kept the disease at bay. Even so, growers should never let their guard down.

"The problem is that the FHB pathogen doesn't just attack the wheat head," says Harding. "It grows happily at the crown of the plant and in the roots, just hanging around and waiting for an opportunity to cause head blight again. Extended periods of precipitation or high humidity when flowering are both conducive to FHB, so don't assume it has gone away."

Strategies for combating FHB include growing cultivars with the best levels of resistance, using healthy seed with a fungicidal seed treatment, and employing at least a two-year break between host crops.

Knowing your enemy – pest or disease – is no guarantee of success, but it's a good start. BF