Cereal Crops
Many pathogens present in winter wheat
Last week we visited more winter wheat fields, this time on the eastern side of
the province. There is a lot of winter wheat out there this year. Best
estimates put the amount at about 330,000 acres – as much as we’ve seen anytime
in the last decade. Some of it was seeded into fields that had no crop in 2005
(because of excess moisture) either to stabilize the soil or because the fall of
2005 was the first opportunity to work the soil. In any case, there is a
significant proportion of the winter wheat that was not seeded into the
preferred situation, that is canola stubble. While the over-wintering survival
of the crop has generally been good to excellent, there are also significant
pest concerns which we have been mentioning in the past three reports. These
include wheat streak mosaic (now documented from at least six separate locations)
and its vector, the wheat curl mite; powdery mildew; leaf rust; barley thrips;
and most recently barley yellow dwarf – another viral disease. Many crops also
have some tan spot, a residue-borne disease. To top it all off, some of the
most advanced crop has just headed and will have flowered by the time you are
reading this. Recent heat and peaks of nighttime humidity have us considering
the risk of Fusarium head blight (FHB).
Fusarium risk?
We intend to resume mapping FHB risk in 2006 now that the Manitoba Ag-Weather
Program has grown to the stage where we feel the model can give us a
representative map. Based on other years, we planned to have it up and running
by mid-June in order to cover the susceptible period (early flowering) in both
winter and spring wheat crops. However, the winter crop is ahead of schedule. As of this week’s Crop Report (June 5th), most stations are reporting
that the accumulation of heat since April 15th is above
normal. Rainfall, on the other hand, has been “all over the map,”
ranging from Steinbach, which has received only half its long-term average, to
Roblin, which has received more than three times the norm. A key to actual risk
of infection is significant rainfall within the seven days prior to the day risk
is assessed.To spray or not to spray for plant
pathogens?
Is a fungicide warranted? This question must
be answered on a field-by-field basis. There are just too many
variables to make general recommendations over a large geographic
area.
Following two difficult years in Manitoba,
growers will certainly be scrutinizing carefully the cost-benefit of
each input and, to justify a fungicide application, there should be
a reasonable expectation of return on investment. If FHB is
determined to be the disease with the greatest potential impact, Folicur (tebuconazole), recently granted full registration on wheat,
is the product-of-choice. You can find the label on the
PMRA website or you can find additional details on page 7 of the
addendum to the 2006 Guide to Crop Protection.
Add to the list of [over-wintering] viral diseases – barley yellow dwarfAlso, last week we received confirmation that a sample we collected from a winter
wheat field south of Holland was infected with Barley Yellow Dwarf virus. The
crop had been seeded before the end of August into barley stubble. There had
been significant regrowth of the barley following harvest. The disease
apparently infected the crop last fall when aphids were plentiful (they were not
detectable in the crop two weeks ago) and the distribution in the field was very
even. This is yet another example of a problem, apparent as yellowing from a
distance, that bears further investigation to be certain of cause. The impact
in wheat should be less than it would be in barley – all cereals are host and
can suffer from stunting and reduced yield from lack of chlorophyll in affected
leaves.
Figure 1.
Click on image to enlarge.
Thrips in cereals - the good and the bad
In many cereal fields this
year, white streaks are noticeable on the leaves of many plants. As mentioned in
last weeks update, thrips are a possible cause of this type of injury to the
plants, but seeing and counting thrips is often difficult because they are quite
small insects (less than 2 mm long). One technique I find useful for verifying
the presence of thrips and many other insects in cereal fields is to tap the
plants over a bowl, shallow pail or tray with a white surface. Insects such as
thrips and aphids are often more noticeable when doing this then when just
examining individual plants. Thrips will look like a small yellow or black speck
moving on the bottom of your tray or pail. The barley thrip is one of the more common thrips that feeds directly on cereal
crops. Adults are black, and juveniles are pale or yellowish. Heavy feeding can
result in significant yield losses and be economical to control, but in most
fields this feeding will be noticeable but not economical. The economic
threshold commonly used for thrips in cereals is: Threshold (thrips/stem)=(Cost
of Control /expected $ value per bushel)/0.4. For wheat at $4.00 per bushel and
control costs of $15/acre (insecticide and application) this works out to about
9 thrips per stem on average. For oats at $2.00 per bushel and control costs of
$15/acre this works out to about 19 thrips per stem. Insecticides to manage
thrips are only economical when applied before heading is complete.
 |
 |
| Figure 2. Young thrips |
Figure 3. Adult thrips |
Click on images to enlarge.
In most fields that we have examined this year some of the black thrips have had
white bands running across their bodies (Figure 4 shows a magnified picture of
this type of thrip). These are called banded thrips, and are actually beneficial
thrips because they feed on the species of thrips that are potential pests on
the crops. In some fields the population of these banded thrips is quite high.
This is an extra consideration for anyone using counts of thrips per stem to
determine if the population would be economical to control, since you would not
count the banded thrips.
Figure 4. Banded thrips
Click on image to enlarge.
Alfalfa
Alfafla weevilAlfalfa weevil has become a
lot more prevalent on alfalfa in the eastern part of the province in
the last couple of years, and is starting to become quite
noticeable again this year. Although not detected at economical
levels yet, this is something to be aware of when monitoring alfalfa
fields. We have been noticing a mixture of both adults and the
youngest larval stages when monitoring alfalfa in the Carman area
over the past week.
Figure 5. Alfalfa weevil adult
Click on image to enlarge.
Insect Monitoring Programs Update
Traps baited with pheromone that attracts diamondback moths have
been set up across Manitoba again this year to monitor the arrival
and abundance of diamondback moth. So far counts have been very low
in the traps; our highest count in any trap so far is 13. So at
present our risk of diamondback moth being at economical levels is
low. The lack of larvae being found in fields we have scouted also
verifies this lower risk so far for 2006.
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
HomeContact
UsAbout
ManitobaDepartmentsLinksPrivacy 
