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Manitoba Insect and Disease Update
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May 27, 2003 |
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Compiled by: John
Gavloski, Extension Entomologist, Manitoba Agriculture and Food, Phone: (204)
745-5668; Fax: (204) 745-5690; and
David Kaminski, Plant Pathologist, Manitoba Agriculture and Food Phone: (204)
745-5656; Fax: (204) 745-5690
To report observations of insect and activity or control that may be of interest or
importance to others in Manitoba, please send messages to be above contact addresses.
To be placed on an E-mail list to be notified when new Manitoba Insect Updates are
posted, please contact John Gavloski at the address or numbers listed above.
Canola
Insects
Feeding on canola stems beneath the soil surface has been reported from quite a few
fields. Some plants were actually girdled by several "bites" and they did not
have the structural support to stand on their own. None of these plants though had any
noticeable decay or signs of root rot. Attempts at finding cutworms or wireworms around
these plants were not successful, and the damage is not what is typical of these insects.
Our best guess at this point is that this is flea beetle feeding. Most of these situations
are on canola that was planted early (April 24 to May 1st), and there is evidence of some
feeding to the cotyledons (although this was limited as in most cases seed treatments
containing insecticides had been used). Flea beetles will take shelter in cracks in the
soil when it is cool or less than favorable feeding conditions. Our best guess so far is
that the flea beetles are feeding on the stems belowground while taking shelter under
these unfavorable conditions.
There has also been a lot of concern from farmers over the past week about whether
canola seed treatments containing insecticides are controlling flea beetles. Remember that
the seed treatments in canola currently on the market will kill flea beetles once the flea
beetles have fed on the plant. The lindane-based seed treatments, which are no longer on
the market, had a repellent effect as well, but were much shorter lived than the current
treatments. Flea beetle numbers are high this year, so even with insecticide treated seed,
you have to expect some feeding damage, especially around the outside of the field. In
most cases this feeding is minor and won’t be of economical concern.
Foliar spraying of canola for flea beetle control is occurring in many fields. One
district in the southwest reported a total of about 7,500 acres of canola sprayed for flea
beetles, including some fields that had seed treatments containing insecticides. Some of
the sprayed fields had seed treatments with the high rate (extended protection) of the
insecticide. Some of these fields receiving foliar sprays may be the result of farmers
deciding to spray after seeing minor amounts of flea beetle damage (<25%) and thinking
that the seed treatment is not working. However, there are also fields where the seed
treatments will be near the end of their effective period. Seed treatments with a lower
rate of insecticide should provide 2 to 3 weeks of flea beetle protection, and those with
a higher rate of insecticide should provide 4 to 5 weeks of flea beetle protection. If the
canola can make it to the 3 to 4 leaf stages in these periods of time without significant
flea beetle damage, then foliar sprays should not be needed.
If trying to decide whether to spray canola for flea beetle control, the weather
forecast over the next few days and soil moisture are also factors to consider. Under hot
and dry conditions the canola seedlings have more trouble outgrowing the damage and flea
beetles feed more aggressively.
Diseases
Later in the week, we began to hear numerous reports of
canola that was suffering from pathogenic disease – wirestem. This
picture from Mark Sloane, Ag Rep at Pilot Mound, shows the characteristic brown collapse
of the juvenile tap root with healthy white tissue above and below the decay.
Symptoms appeared to be more pronounced the greater the depth of seeding – they were
on plants just now emerging after 3 weeks in the soil.
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| Over the weekend, the Diagnostic Centre was able to determine
that both Rhizoctonia and Fusarium were the fungi responsible
for the wirestem symptom. The length of time that seed treatments can provide
protection from soil-borne organisms is limited and, if at all possible, canola should NOT
have to emerge from more than 1" depth. A tight rotation with canola is also linked
to higher levels of soil-borne pathogens. Such cultural methods of disease
management are vital components of integrated pest management (IPM). |
Small Grains
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Nothing new since last report. Here is what a healthy wheat plant
looks like at the three-leaf stage, when crown (or secondary) roots begin to develop and
tillering becomes evident, too.This CWRS plant was seeded very shallow hence little or no
lengthening of the sub-crown internode. The length of that vital structure (a good
place to look for signs of root disease) is influenced by depth of planting but also by
type. For example CPS wheat should not be planted deeper than an inch because of the
limited extension of the scabbard-like coleoptilar sheath. |
General
A sunflower field near Carman is being treated with insecticides for cutworm control,
and a cornfield near Carman may need to have cutworms controlled as well. Cutworn
populations have also been reported to be high in the Northwest region. A variety of sizes
of cutworms are currently present. Redbacked cutworm is the main species, although some
army cutworms are present as well.
Wireworm damage was reported from a cornfield in the Somerset district.
Emergence of grasshoppers is underway. First and second instars of the migratory
grasshopper are now present in many ditches and field edges.
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