May 15, 2006

Manitoba Insect and Disease Update

Compiled by:    

John Gavloski, Entomologist, Manitoba Agriculture, Food and Rural Initiatives, Phone: (204) 745-5668; Fax: (204) 745-5690; and David Kaminski, Plant Pathologist, Manitoba Agriculture, Food and Rural Initiatives, Phone: (204) 745-5656; Fax: (204) 745-5690;

To report observations of insect and disease activity or control that may be of interest or importance to others in Manitoba, please send messages to the above contact addresses.

To be placed on an E-mail list to be notified when new Manitoba Insect and Disease Updates are posted, please contact John Gavloski at the address or numbers listed above.

Winter Cereals

Wheat Streak Mosaic and Powdery Mildew in winter wheat

In early May, the Diagnostic Centre received a sample of winter wheat from south central Manitoba that showed the yellow striping characteristic of wheat streak mosaic virus infection. It was beginning to appear in patches in the field, however the most seriously affected plants were between the rows – apparently volunteers that had begun to grow after last years canola crop was harvested. A subsequent sample taken from this field and another yet to be seeded revealed that there were wheat curl mites present. These tiny, flightless creatures are the sole vector of the disease.

Figure 1 (Wheat Streak Mosaic)
Click on Image to Enlarge

There are no options for intervention once the disease has taken hold in a crop. The only effective strategy is prevention by destruction – either with herbicide or tillage – of the infected plants and delayed seeding of the subsequent crop so that the mites are starved out. This takes from 7 to 10 days and achieves the removal of the “green bridge” that allows the mites to carry the disease to the new crop.

There is often a background level of the virus in native grasses along the edges of fields or in ditches. It is not safe to assume that volunteer cereals or over-wintering crop that was cut as green feed are free of the disease.

CAUTION: Yellowed plants are not necessarily infected with streak mosaic virus. We also observed plants yellowed from tan spot (fungal) infection, from frost, and from apparent nutrient stresses. Some of these causes can be addressed through intervention so a careful diagnosis may be warranted.

How serious can losses be? There are no hard and fast answers but it is generally those plants that were infected last fall that will suffer the most. Because over-wintering survival of winter wheat crops has been so favorable and early growth with heat in April has been vigorous, there is no question that an infected crop should be taken to harvest rather than destroyed and replanted with a spring cereal. By the time an adequate interval has passed, it would be getting towards early June and crop sown then has far less chance of reaching its potential.

Another disease that we have been watching since last fall on winter wheat is powdery mildew. This fungal disease has evidently over-wintered on some crops especially in areas of fields where airflow is reduced (often adjacent shelterbelts) and humidity remains relatively constant. Monitor such field for disease progress and consider a fungicide if powdery mildew continues to move upwards in the canopy. To date we have seen no leaf rust on crops on which the disease was already prevalent last fall.

Figure 2 (Powdery Mildew)
Click on Image to Enlarge

Cutworms in Fall Rye

Cutworms have caused considerable damage to a fall rye field near Wellwood. At least 80 of 140 acres of fall rye was heavily damaged, and in need of reseeding. The fall rye was seeded into recently broken ground (which was previously hayland).

Dealing with a cutworm problem can be complicated by the fact that there are many species of cutworms, and these can vary in their food preferences and even how effective insecticides would likely be in controlling them. The species of cutworm in this case was glassy cutworm, a species that prefers to feed on grassy plants. This is a species that does not often come aboveground to feed (like redbacked or army cutworm will), so insecticides that may work well against some of the species of cutworm that do come aboveground are not as effective against a species such as glassy cutworm where they are not coming aboveground to feed.

Figure 3 (Glassy Cutworm)
Click on Image to Enlarge

Glassy cutworms can be quite big early in the season, since they overwinter as partly grown larvae. Glassy cutworm is not a common crop pest in Manitoba, which makes this situation even more unusual, but there have been problems before when a cereal crop follows the breaking of pasture or hayland.

Knowing that it is glassy cutworm that is causing damage to the plants, there are a few things to consider in a situation like this where the field will be reseeded and the farmer wants to try to reduce the cutworm population as much as possible:

• Insecticides are often not very effective against glassy cutworms because most individuals remain beneath the soil surface, and don't emerge at night to feed. But control seems to be improved greatly when applications are made before or during a rainfall. Rainfall carries the insecticide down to the level of the larvae. If this is practical it might be something to consider.
• If insecticides are to be applied, using as high a water volume as is feasible (>200 L/ha)is recommended.
• Even if the control is poor, I suspect (although I haven't seen data to prove it) that canola is very low on the food preference list for glassy cutworm. Grassy crops are there preferred hosts.
• If the farmer is not in too much of a rush to apply insecticides to the field, waiting until after reseeding to apply the insecticide may provide the best control. The cutworm will have had their food source destroyed or greatly reduced, and this may force more of them to have to come to the surface and search for more food, exposing them to the insecticide.

Canola

Flea Beetles: Flea beetle activity and feeding is noticeable on volunteer canola in many areas of the province, although so far it has not been at levels that have been very high. Observations around Carman show that the striped flea beetle has been the dominant species, which is what we expect in late April and early May. The crucifer flea beetle, which is the more damaging of the flea beetles that eat canola, should start to emerge in larger numbers soon. The past couple of weeks have been quite cool and windy, which does reduce flea beetle feeding. When we get into some warm, calm weather we will start to get a better idea of what the flea beetle population is like.

General Insect Concerns

White Butterflies in your Crops?

Some have been calling regarding what all the white butterflies are that are quite numerous in some of the fields. These are cabbage butterflies. They are not pests of any field crops, although their larvae can be a problem in some cruciferous vegetable crops. The adult butterflies will often be present on almost any crops or plants in flower, often in big numbers, as they look for nectar. The larvae (referred to as imported cabbageworms) may occasionally be found on canola leaves, but do not cause enough damage to be considered economical pests of canola.