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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect and Disease Update

July 11, 2011                   

Compiled by: John Gavloski, Entomologist, Manitoba Agriculture, Food and Rural Initiatives
Phone: (204) 745-5668; Fax: (204) 745-5690
Holly Derksen, Plant Pathologist, Manitoba Agriculture, Food and Rural Initiatives
Phone: (204) 750-4248; Fax: (204) 745-5690

To report observations on insects or plant pathogens that may be of interest or importance to farmers and agronomists in Manitoba, please send messages to the above contact address.

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


 

Recent Insect and Plant Pathogen Activity

Cereals

Barley Yellow Dwarf Virus – There is a suspected case of Barley Yellow Dwarf Virus in a field of spring wheat near Portage la Prairie. The disease symptoms are showing up in patches throughout the field. The main symptom observed was chlorosis of the flag leaf starting from the tip and spreading down the leaf margins. English grain aphids were observed in the field and it is suspected that the oat- birdcherry aphid may also be present in lower numbers. Although English grain aphids can potentially be a vector for some strains of BYDV, the oat-birdcherry aphid is generally considered the stronger vector. Generally speaking, infection of BYDV has the biggest effect on yield when crops are infected in the early stages of development. However, when the flag leaf is showing disease symptoms there may also be an effect on yield. Growers may use insecticides to control the aphid population and therefore the spread of the disease.

Rust – Winter wheat plots at the University of Manitoba research farm at Carman are showing symptoms of both leaf rust (Puccinia triticina) and stripe rust (Puccinia striiformis). Many winter wheat crops in the area were sprayed with a foliar fungicide and therefore there have not been reports of rust in growers' fields. The observation of these rusts at the U of M farm indicates that the rust spores have arrived in Manitoba.

Leaf and Stripe Rust
Figure 1. Leaf and Stripe Rust

Armyworms: Some cereal fields in the Morden and Winkler areas have armyworms that are approaching economic levels. Armyworms likes to hide under debris on the soil during the day, so if seeing feeding on cereal leaves, look on the soil to see if armyworms may be the cause.

Stage of larvae is something to consider when determining the potential economic impact of an armyworm population. If most larvae are greater than about 2.5 cm, they will be finishing feeding and pupating soon, so control in these situations is generally not recommended. In most instances agronomists are noticing larvae and monitoring the situation but not seeing damage to plants or populations high enough to justify insecticide use. Highest populations seem to be in the Morden / Winkler area.

Figure 2 below shows armyworm larvae. Note in figure 2 the variation in colour, and the habit of curling up when the larvae are disturbed.

Armyworm larvae
Figure 2. Armyworm larvae

Canola

Blackleg: There have been a number of cases of blackleg in canola recently. While there are limited control options once the disease is detected in your crop it reiterates the importance of longer rotations between canola crops. It is recommended the canola growers use a 4-year rotation between canola crops. It is believed that new variants of the blackleg fungus (Leptosphaeria maculans) are present within Manitoba and many R-rated varieties are beginning to show higher blackleg incidence and severity ratings.

Sunflower

The sunflower agronomist for the National Sunflower Association has observed the presence of both Verticillium wilt and sunflower rust aecia.
Verticillium wilt - Early symptoms of Verticillium wilt in sunflowers include chlorotic lesions and interveinal chlorosis. These symptoms can advance to necrotic areas, vascular discolouration, and general wilting characteristics.
Sunflower rust - The image below shows an example of rust aecia. Note the cup-like appearance of the fruiting bodies and the characteristic grouping together to form a circle.

Sunflower rust aecia
Figure 3. Sunflower rust aecia
Photo credit: Claire Kincaid, NSA


Soybeans

Soybean aphid: Low numbers of soybean aphids were found in the Carman area this week. Those scouting soybean fields should look for these aphids. Note however that large numbers are necessary before economic damage occurs. The economic threshold is when there are on average at least 250 aphids per plant and the population is increasing, and the plants are in the R1 (beginning bloom) to R5 (beginning seed) growth stages. This threshold gives an approximate 7-day lead time before aphid populations are expected to exceed the economic injury level (670 aphids per plant), where cost of control is equal to yield loss. When soybean aphid populations are not actively increasing above 250 aphids per plant, natural enemies are keeping up with the aphid population. Insecticides should not be used in this case, as it will kill the natural enemies which may enable the aphid population to increase above the economic injury level.

Below is a picture of soybean aphids on a soybean leaf; taken in 2009 when populations were considerably higher than what we are currently seeing. The white specked are shed skins of the aphids.

Soybean aphids on soybeans
Figure 4. Soybean aphids on soybeans

General crop scouting

Hail damage – Thunderstorms moved through parts of Manitoba last week bringing hail to some areas. Hail damage to crops can create wounds for which pathogens to enter. In addition, the precipitation will have created moist environments conducive to many pathogens. For those areas that did not receive moisture this past week remember that heavy dews can often create enough moisture for pathogens to flourish.

 

Surveys and Forecasts

Diamondback moth monitoring: Remaining traps for monitoring adults of diamondback moth can now be removed. We now have the information that we need from these traps for estimating the number of generations and relative abundance in the province. Highest trap counts were in the Eastern and Northwest regions.

Table 1. Highest cumulative trap counts for diamondback moth in Manitoba as of July 11, 2011

Location

Cumulative
Trap count

Sewell

306

Beausejour

175

Gilbert Plains

107

Morris

80

The full data set for adult counts of diamondback moth can be viewed at: https://www.gov.mb.ca/agriculture/crops/insects/db/pdf/2011diamondbackdata.pdf

So far no high populations of larvae of diamondback moth have been found or reported, although low levels of larvae are being found in some fields. It is not a given that because trap counts detect a lot of adults have blown into an area that there will also be a lot of larvae to follow. We have had years before where high counts of adults did not result in significant numbers of larvae.

Factors such as weather and natural enemies can regulate the population. A parasitic wasp known as Diadegma is one the natural enemies can be quite effective at regulating diamondback moth numbers.

It is important that monitoring for diamondback moth larvae be an important part of canola crop scouting this summer, particularly in Eastern and Northwest Manitoba. But do not make assumptions on larval levels or plan management options based on the trap counts for adults. The best course of action for now is to focus on monitoring for the larvae. If any fields are found to contain high levels of diamondback moth larvae, please alert myself or a nearby Farm Production Advisor so we can provide alerts in future Manitoba Insect and Disease Updates of any regions where high larval counts are starting to appear.

Bertha armyworm monitoring: Counts from the bertha armyworm traps are starting to rise, but all areas of the province are still in the low risk category, which is a cumulative count of less than 300 moths. Highest counts so far are:

Table 2. Highest cumulative trap counts for bertha armyworm in Manitoba as of July 11, 2011

Location

Cumulative
Trap count

Glenboro

173

Neepawa

147

Durban

138

Roblin

120

Treherne

106

Mather

106

Data from the bertha armyworm monitoring program and a chart on how to interpret the data are available at: https://www.gov.mb.ca/agriculture/crops/insects/bertha/index.html

Crop Scouting Educational Opportunities

Crop Diagnostic School: A reminder that the Crop Diagnostic School continues this week (until July 14th) at the University of Manitoba Farm in Carman, MB.

Please remember to bring any samples of insects, plant pathogens or weeds you would like us to identify, and come with topics you would like to discuss.

More information on the Crop Diagnostic School can be found at:
https://www.gov.mb.ca/agriculture/crops/cdschool/