AgronomyPest Watch 2012

Pest Watch 2012

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Potato pests can vary across Canada’s potato-growing regions. Whether they are new or notorious, their reach and severity depends upon Mother Nature’s whims.

Each year potato growers across Canada must diligently scout their fields for insects. Some pests are familiar to growers, although there are some unfamiliar pests that could show up in Canadian potato fields this year. The following is an overview of some potato pests and the damage they can cause, what to watch for when scouting fields as well as control options for this growing season.

New Invaders for 2012?

The potato psyllid has been migrating farther north within the United States over the past decade, and Canadian scientists are concerned that it’s only a matter of time before it breaches our borders.

According to Eugenia Banks, a potato specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, studies in the United States have shown that controlling the vector of zebra chip disease–the potato psyllid–is the best strategy to reduce the incidence of this disease. Zebra chip affects tubers by forming dark stripes in the potato flesh when they are cut and fried to make chips or french fries. The discolouration renders the chips unsaleable despite the fact that there is no health risk connected with their consumption.

“Potato psyllids have not been detected in potato fields in Ontario as our winters are too cold for them to overwinter here,” explains Banks. “However, American researchers have indicated to me that psyllids carrying the zebra chip pathogen can arrive in Canada, carried by winds.”

Zebra chip was found last year in Washington State, Oregon and Idaho. Potato psyllids were also found last year in North Dakota, but they were not vectoring the zebra chip pathogen.

“This shows that zebra chip is just around the corner,” says Banks. “We need to have a strategy in place to deal with zebra chip. Monitoring fields on a regular basis to detect the presence of potato psyllids is essential in the future to avoid potential crop losses.”

North American Alert

There is also an alert in North America for the emerging strains of potato virus YO (common mosaic virus). Aphids are efficient vectors of PVYO and its strains.

One of the new strains is PVYntn, which causes necrotic rings on tubers, rendering the potatoes unmarketable. The incidence of this new strain has been increasing in North America, and scientists recommend that growers pay more attention than ever to their aphid management strategies to avoid crop losses.

“Aphids that colonize potatoes such as the green peach aphid and the potato aphid are efficient vectors of viruses,” explains Banks. “Unfortunately, visitor aphids—those that do not colonize the crop but fly in, probe plants and fly out—can also be efficient vectors.”

Seed testing is a valuable tool to determine the levels of PVYO and PVYntn infection in seed lots. PVYO generally causes mosaic symptoms in the foliage, helping seed growers inspecting their fields to identify infected plants, which are then rogued. Roguing and the Canadian Food Inspection Agency’s certification seed testing have kept PVYO incidence at low levels. However, the new strain of PVYntn does not cause obvious symptoms on leaves, thus roguing is not an effective tool to eliminate PVYntn-infected plants from seed fields. At harvest, infected tubers may or may not have necrotic rings on their surfaces; the necrotic rings usually develop in storage.

Defence

Insecticide applications at planting or as foliar sprays are not effective in reducing the incidence of PVYO and PVYntn. “Aphids acquire and transmit the viruses very quickly, thus, insecticides cannot kill the aphid before the virus is transmitted,” says Banks. “Mineral oils are effective in reducing the spread of viruses—they seem to act as aphid deterrents.”

Potato Leaf Aphids

According to Gilles Boiteau, an entomologist with Agriculture and Agri-Food Canada’s Potato Research Centre, producers should continue to guard against this pest. “It is important to keep a watchful eye out for aphids as they can cause viruses and diseases, in particular potato virus Y, which in turn can really hurt a grower’s bottom line.” For seed growers, it’s especially important to stop vectors of viruses in their potato crops.

Defence

Mineral oil sprays have proven to be very effective in plant protection, but thorough coverage of the whole plant is required, says Boiteau. “In field trials [mineral oil sprays] have shown they prevent transmission of viruses from the aphid to the plant.”

Additionally, the use of certified seed, non-host crops such as soybeans or wheat used as field borders where aphids can lose the viruses they carry and roguing should be part of an aphid vector/plant disease management strategy.

European Corn Borer

A localized pest, European corn borer can cause tremendous damage to a potato crop. The larvae bore through and eat the stems of plants, thus weakening and ultimately breaking the stems, while the bore holes provide access for disease to infect the plants. The weakened stems will often break during heavy rains and wind. European corn borer is very difficult to control—once the larvae enter the stems they are protected from insecticides, and the damage may not be noticeable until a month or two after infestation.

“It’s difficult to time the spraying of products for producers,” explains Boiteau. “It is hard to catch the larvae before they enter the stems, so it is important to trap and monitor adult moths to develop systems and/or models to calculate when egg-laying may occur.”

Defence

Insecticide application should occur after the majority of eggs hatch and before larvae begin to bore into the stem. Egg development monitoring is very important for successful insecticide application because this application window is narrow at only a few days. European corn borers overwinter as larvae within the discarded stems of the potato plants following harvest. Control of these overwintering larvae decreases the population of these pests the following year.

Potatoes should be planted away from corn fields and kept as weed-free as possible. Volunteer populations along field boundaries should be removed. A three-year rotation away from host crops can also help prevent high European corn borer population levels.

Potato Leafhopper

Its appearance hard to predict, the potato leafhopper may be present in Canadian fields some years and not in others. Adults and nymphs feed on potato plant leaves with piercing and sucking mouthparts. Toxins are injected into the leaves as the pests feed, causing the tips of the leaves to yellow, eventually curling and turning brown and brittle. This foliar damage is called hopperburn. Control of the potato leafhopper is not necessary in most years; however, economic losses can be considerable when populations are high, causing early plant death and reduced yields. The pest does not overwinter in Canada, as it originates in the United States and is carried to this country each year on wind currents.

Defence

Registered insecticides should be used when the pest population reaches economic thresholds. Producers should avoid planting alfalfa or cover fields near potatoes. When nearby forage crops are harvested, a flush of leafhoppers may appear in potato fields; this is a critical time to scout for potato leafhoppers.

Colorado Potato Beetle

This insect is the most common and dominant pest species across Canada, with the exception of Newfoundland, most likely because it is an island. All life stages, from larva to adult, feed on potato plants, which is why Colorado potato beetles can be so destructive. The beetles feed on potato foliage, but can also attack the stems, and it’s this defoliation that decreases the plants’ ability to take in nutrients, eventually reducing crop yields.

The adults can overwinter in the soil of hedgerows around the field, as well as in previous potato fields, and then emerge in the spring. There is generally one generation per year, but in Ontario and Quebec up to two generations can emerge, according to Boiteau.

“The larvae will dig into the ground of the potato fields to pupate,” he explains. “There are insecticides available to combat these pests; however, producers must be aware that Colorado potato beetles can develop resistance to these insecticides. Producers should only spray when their fields are at economic threshold levels.” Repeated use of the same active ingredient can result in the insecticide becoming ineffective against the pest, which is why Boiteau encourages producers to use all available control methods to ensure a prolonged lifespan of the insecticide products that are available to Canadian producers.

Defence

Rotating the new year potato crop up to 1.5 km away from the previous year’s crop remains the best method of controlling this pest, says Boiteau. “It will go a long way in reducing the colonization of the new crop by the spring emerging adult Colorado potato beetles, thereby reducing or eliminating the need for insecticides.” Beetles and larvae are easy to find if producers scout their fields regularly, he explains. Adult Colorado potato beetles can be trapped by potato plants planted around the field’s boundary a week or two prior to planting the rest of the field. The early emergence of the outside rows will attract most of the spring adults.

Effective Control

According to Banks, a researcher at the 2012 Potato Expo conference said, “A good-looking crop in the field does not translate into disease-free tubers.” Banks agrees.

Effective insect control depends on a combination of both cultural and chemical practices. Good control of weeds and volunteer potatoes within and around potato fields removes alternative food sources for many insects. Proper field rotation will also help to reduce the numbers of many insects, particularly Colorado potato beetles. Potato fields are more severely affected by insects when they are suffering from other stresses such as inadequate moisture and fertility. Effective disease and weed management will also help minimize yield losses from insects.

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