Cereals
Aphids Found in Cereals: The aphid species that can become
pests of cereal crops generally do not overwinter in Manitoba, and
blow into Manitoba at some point in the spring or early summer.
Whether they become economical pests of cereals will depend on when
they arrive, in what numbers, and what the conditions are like after
they arrive. While scouting a field of oats near Fannystelle last
Thursday, we found two of the species of aphids that feed on cereals;
the bird cherry-oat aphid (Figure 1) and the English grain aphid
(Figure 2).
|
Figure 1. Bird Cherry-Oat Aphid |
Figure 2. English Grain Aphid |
Levels were quite low; we were using a sweep net to do
general insect assessments and only finding one or two aphids per 10
sweeps. But aphid levels are something we may want to start looking
at when scouting cereal crops. I find the easiest way to scout for
aphids, and insects in general, is to use a sweep net or shake
plants over a shallow bucket, tray, or some white surface. This is a
relatively easy and quick technique, and if aphids are not detected
consistently while doing this then they will not be a problem. If
aphids can be found consistently, then you need to assess how many
aphids per stem on average are present to determine if control would
be economical (the economic threshold is 12 to 15 per stem prior to the
soft dough stage). The bird cherry-oat aphid is the main vector of
barley yellow dwarf virus, so in fields where this aphid is found
pay attention for symptoms of barley yellow dwarf as well,
particularly in late seeded spring cereals. In the field, bird
cherry-oat aphid will look like dark specks on the plant. They are
darker then most other aphids in cereals, and you may have trouble
seeing the cornicles (the two tube-like structures at the back of
aphids).
| The rust that has been reported on spring
wheat in the Valley is also evident in the Southwest. This
picture was taken in a crop just north of Deloraine that was
only at the five-leaf stage. Another field was at full flag and
had up to a dozen pustules on mid-storey leaves. Both could
prove costly on susceptible varieties without intervention.
Remember that the most popular spring wheat cultivars do not
have good resistance to leaf rust. |
Figure 3. Leaf Rust
on Spring Wheat |
|
Figure 4. Close-Up Symptoms of Wheat Streak Mosaic |
Wheat Streak Mosaic has been seen on the spring crop now
from a few locations but does not appear to be widespread.
In the crop pictured below, in the Elgin area, there was a
significant infestation of foxtail barley. It may have
served as an overwintering host within the field for wheat
curl mites, the disease’s only vector. Thursday last week
started with a tailgate meeting to look at the intensity of
powdery mildew on a spring wheat crop. The
question-of-the-week has been, “Will powdery mildew move up
onto the upper leaves and how much is lost already to the
intense sporulation on stems and lower leaves?” The answer
is, in many cases, no – humidity in the upper part of the
canopy is limiting the movement to the very top of the
plants. Some exceptions exist where humidity has been
elevated by geography (e.g. the Interlake region) or by
localized microclimate effects (e.g. areas of fields
protected by shelterbelts, etc.). And while the disease down
in the canopy looks bad, it does not in itself harm yield
potential. The majority of yield comes from the food
factories of the uppermost two leaves, stem and glumes. The
remainder of Thursday was spent in a tour of the Southwest
region to look at a variety of crops and how disease
conditions differ from those in the Valley. |
Field Peas
|
Figure 5.
Mycosphaerella blight
on lowermost leaves
|
Phone reports on field peas were mostly about yellowing
of the new growth which often turned out to be a combination
of herbicide setback coupled with very rapid growing
conditions. Most crops we saw were recovering nicely and had
very little evidence of pathogenic disease. One field had
the onset of mycosphaerella blight, a manifestation
of the ascochyta complex. While not as debilitating as foot
rot, this leaf and stem disease has to be watched as it can
develop very quickly. We have had one diagnosis of foot rot
and none, so far, of anthracnose. |
Canola
|
 |
Crops across the south exhibit a wide range of growth
stages from early rosette to mid-bloom. Apothecia have been
observed in many locations although there are others where
the rainfall has been limited and surface soils are dry even
beneath crop canopies. Fungicide application where warranted
is first applicable at the 30% bloom stage (when petals
begin to fall) as pictured in the picture. |
Insect Monitoring Programs Update
Bertha Armyworm Monitoring:
Bertha armyworm counts have been low in most traps so
far, with the exception of a few traps in the Swan River valley.
Traps near Durban and Minitonas each had about 500 bertha armyworm
adult moths in them this week, and a trap near Birch River has 187
moths. A table for interpreting moth counts for bertha armyworm can
be found on the MAFRI website.
(click on the link that says Interpreting Bertha Armyworm Cumulative
Moth Counts). Weekly maps of trap counts in Manitoba and across the
Canadian prairies will be posted at this site starting this week.
The area with these high initial trap counts is the same area
that had some higher levels of bertha armyworm larvae last summer.
Although environmental factors and natural enemies can prevent a
potential problem from developing, we will have to pay attention to
what shows up in these traps over the next couple of weeks. If it
becomes evident that there will be greater than 1200 moths per trap
over the 6 week trapping period, crop scouts will need to make
looking for egg masses and larvae of bertha armyworm a priority
while scouting canola in July and August.
Insect Identification Quiz
Question: Some of these larvae were found on milkweed
plants at the edge of a sunflower field. Any idea what this larva
is?
Answer: This is a monarch butterfly larva. They are
commonly found on milkweed this time of year. They will not feed on
any crop plants near the milkweed.
Insects and Diseases in Vegetable Crops
Information on insects and diseases in vegetable crops in
Manitoba can be found at:
http://web2.gov.mb.ca/agriculture/mwvr/index.php
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