Which pest is going to strike where next and how hard ranks right up there with weather forecasting for jobs that are difficult to get right.
But extension agronomists say these are some of the yield robbers on their watch list.
Soybean cyst nematode
This pest hasn’t been found in Manitoba yet, but it could already be here, said Holly Derksen, a plant pathologist with Manitoba Agriculture, Food and Rural Initiatives (MAFRI). If it isn’t, it’s almost certain to show up soon.
Soybean cyst nematodes have been working their way north in the United States arriving on Manitoba’s doorstep in Pembina County, North Dakota in 2011.
“One thing to keep in mind is that wherever soybeans have been grown soybean cyst nematode has followed,” Albert Tenuta, an extension plant pathologist with Ontario’s Ministry of Agriculture, Food and Rural Affairs at the Manitoba Special Crops Symposium in Winnipeg last year.
“It is the No. 1 soybean (pest) in North America and the yield losses on average are probably twice those of what we’d see with… seedling-root rot diseases. In Ontario we probably lose $10 million to $30 million a year, or have in the past, to soybean cyst nematode. Those (losses) have declined over time.”
Soybean cyst nematodes are very small parasitic roundworms (Heterodera glycines) that attack soybean roots, robbing plants of nutrients, disrupting water uptake and making it easier for root diseases to infect plants.
“The most common symptom of soybean cyst nematode, in the field above ground, is no symptoms whatsoever,” Tenuta said.
That is until the nematode populations build. Then stunted and chloric plants show up and yields decline.
Soybean cyst nematodes are soil-borne and spread via wind, water, machinery or plant transplants.
The key to managing it is early detection. To that end the Manitoba Pulse Growers Association is funding Mario Tenuta (Albert’s brother), a professor of soil microbiology at the University of Manitoba, to search for soybean cyst nematodes in the province.
It’s akin to trying to find a needle in a haystack. Soil samples from 50 higher-risk fields are being collected, he said. Fields include those close to the border that have grown soybeans and also may have been flooded by the Red River.
Growing tolerant soybean varieties, Derksen said, can control soybean cyst nematodes. While none are commercially available in Manitoba yet, it’s presumed seed companies are working on developing them. Control is easier before populations explode.
“Hopefully we can learn from Ontario and North Dakota, which has been dealing with this, and figure things out before it gets to that point,” Derksen said.
This unusual and complex disease, which damaged canola crops across Western Canada in 2012, is not new, but outbreaks have been relatively uncommon. However, the gaps between infestations are narrowing, said Chrystel Olivier, an entomologist with Agriculture and Agri-Food Canada in Saskatoon.
“It seems to be on the increase,” she said.
Previous outbreaks occurred in 2007, 2000 and 1957.
“The problem is we have those south winds coming from the U.S. earlier almost every year,” she said.
“Ten years ago they started coming in at the beginning of May and now we have them coming in at the beginning of April. The winters are warmer so the chances of the leafhopper surviving are higher.”
Aster Yellows phytoplasma are wall-less bacteria and obligate parasites of plant phloem tissue spread mainly by infected leafhoppers.
Leafhoppers feed on infected plants and then blow north into Canada infecting a wide range of plants, including canola, wheat and carrots.
There’s no chemical control. Fungicides don’t work and spraying leafhoppers isn’t economic in most crops because they come in waves.
“When the leafhopper comes they need less than a day to inoculate the disease and once the disease is in the plant there is nothing you can do,” Olivier said.
The best answer is to develop resistant varieties.
Even though the DNA of this disease was found in two unrelated Manitoba fields in 2011, Derksen still considers Manitoba clubroot free — for now.
“I’m honestly surprised we haven’t found it yet,” she said. “I do think it’s going to show up eventually.”
Clubroot, which cuts yields in cole crops, including canola, was first found in the Edmonton area in 2003. Since then it has spread to more than 600 fields and as far south as Lethbridge.
In 2011, two infected canola fields in north-central Saskatchewan were identified.
Clubroot (Plasmodiophora brassicai), is a soil-based pathogen known as a protist — an organism with plant, animal, and fungal characteristics.
The best control is keeping it out of fields in the first place. The disease, which damages canola roots, is soil-borne so farmers are urged to clean equipment after being in infected fields, Derksen said.
Although clubroot seems to prefer acidic soils it’s believed it can do just fine in Manitoba.
Farmers can control the disease by growing clubroot-resistant varieties.
This bacterial disease, which attacks corn, was first suspected in Manitoba in 2008 and confirmed in 2009 in two Roland-area cornfields. Now it’s ubiquitous throughout Manitoba’s Corn Belt, Derksen said.
Goss’s Wilt overwinters on corn stubble as well as several weeds, including barnyard grass and green foxtail, but the bacterium can survive on corn residue in the soil for 10 or more years.
It’s spread by wind and rain and enters corn plants through abrasions — even microscopic ones — caused by hail or wind.
It normally first shows up in patches and then throughout a field in subsequent years.
“In the odd field where there has been lots of moisture there have been yield losses of up to 30 bushels an acre,” Derksen said.
“We know on a windy day in fall corn stubble moves so if can definitely move from field to field.”
Once a field is infected, the best control is to plant corn hybrids resistant to the disease. Because Goss’s Wilt is a bacterium, fungicides are ineffective.
It’s possible that a new race has developed overcoming the tolerance in current varieties of winter wheat, Derksen said.
It could also mean this rust, which normally appears when temperatures are cooler, might be able to do well under warmer conditions.
Like other rusts, striped rust normally blows in from the United States, but the fungal disease is believed to have overwintered in Alberta and Saskatchewan in 2010-11.
Striped rust, so named because it deposits postures in stripes along wheat leaves and stems, reduces plant photosynthesis and ultimately yield.
Fungicides will control it, but only if applied before the plant is infected.
“Once it shows up it’s too late,” Derksen said.
This insect has been in Manitoba for a while, but only in recent years has it sometimes been an economic pest, said MAFRI entomologist John Gavloski.
Alfalfa weevil larvae, green legless worms with a white stripe down their back, eat alfalfa leaves. Where their numbers are high they can reduce hay yields and seed production.
One option for hay producers is to cut the alfalfa when it reaches the bud or early-bloom stage, Gavloski said. Then they usually starve or desiccate to death.
“It’s good to have a look just before it (alfalfa) starts to bloom,” he said. “Then farmers have time to do something.”
There are insecticides registered to control alfalfa weevils. Check MAFRI’s Guide to Field Crop Protection. Some products are more effective than others.
“They are a tough insect to kill it seems,” Gavloski said.
Gavloski doesn’t know why alfalfa weevil populations have risen to damaging numbers in some fields.
“Sometimes when a pest is new to an area it takes awhile for the biocontrol to catch up and stabilize,” he said. “We’re hoping eventually the natural enemies will help keep them down but that remains to be seen.”
That’s why farmers should avoid applying insecticides if they can because they risk killing insects that can control the weevil.
They’re also harmful to honey- and leafcutting bees. Both are important to seed production, but leafcutters are essential.
Cereal leaf beetle
This is a relatively new pest in Manitoba, which to date, hasn’t caused any economic damage to wheat crops. But farmers should be watching for this insect, Gavloski said. Spray only when economic thresholds are reached.
“You don’t want people out there spraying at levels that aren’t really economical because they’re taking out parasites, which have been successfully introduced to control cereal leaf beetle and they risk creating a situation where it becomes a big problem because it doesn’t have its most important natural enemies,” he said.
Cereal leaf beetles were first found in wheat south of Minitonas and near Swan River in 2010.
Parasitic wasps were introduced and it appears they’re controlling the beetle, whose larva feeds on wheat leaves. High populations can reduce wheat yields.
Cereal leaf beetle larvae are small. They coat themselves in their own feces and resemble a small drop of oil. They don’t eat right through the leaf, leaving a small white trail behind.
Cereal leaf beetles can be moved around easily in cereal straw.
“My gut feeling is it will get moved around,” he said. “I wouldn’t be surprised if we start seeing it in other parts of Manitoba, especially western Manitoba.”