July 27, 2004

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.

These apothecia have been active for > a week	Canola A quick recap on how Sclerotinia infections arise in canola . . . The resting bodies from previously infected crops germinate to produce apothecia when the soil has been wet (field capacity to saturated) for at least 7 days.  Ascospores that blast from the upper surface of the apothecia. Those that happen to land on canola flower parts (petals, sepals and anthers) can use the easily available sugars to produce mycelium.  When the flower parts fall, they land on leaves or in leaf axils.  Where free moisture stays for 24 – 48 hours, infection of the leaf or stem occurs.  Infected areas quickly collapse because of the enzymes produced by the fungus.  You can find crater-like spots on the back side of the leaves (red circled area) – early infection, and areas of spreading decay (red arrow) – later progression.  Inevitably these correspond to the infection that began from infected flower parts on the upper surface of the leaf. Once the infection reaches the stem, infection progresses up and down resulting in weakening, lodging and premature death of part or all of the plant.	(Click on images to expand)
	The big question is, “Are all of the late-seeded crops that have recently begun to flower at risk?”  Although there was some hot weather over the weekend, it has cooled off at night resulting in heavy dews in the morning.  In a lush canola canopy, this moisture remains on the lower leaves long enough for infections to initiate.  Wherever there’s been significant rainfall (i.e. > 10 mm in the last week), it is quite likely there are still numerous viable apothecia in your neighborhood.  Thus there is still a risk of Sclerotinia infection.  The “to spray or not to spray” decision should always be tempered by the potential value of the crop.  Remember that these later seeded crops could face lowered yield potential because of a higher probability of excessive heat during flowering.

Cereals

Wheat Midge – Although most wheat has already flowered and is no longer susceptible to wheat midge, some high populations of wheat midge have been reported over the last week in areas of the southwest. This could be of concern to those with later maturing wheat in these areas. A couple of wheat fields were sprayed to control wheat midge in the Rapid City area. High levels of wheat midge (as high as 2 to 3 midge adults per plant) was reported from the Waskada area, although I am not aware of control being needed in any fields near Waskada, since the fields these were reported from were past the susceptible stage. However, those with later seeded wheat in the area should monitor to determine if wheat midge is present at economic level.

Armyworm – Larvae of the true armyworm (Pseudaletia unipuncta) continue to be noticed in many fields, although currently I am not aware of any fields where control was necessary. Aside from assessing the average number of armyworms in a field or area of a field, the stage of the crop and stage of the armyworms should also be considered when making decisions regarding whether control with insecticides would be economical. If larvae are large (1-1/4 to 1-1/2 inches in length) most feeding has occurred and the larvae are no longer a threat to the crop. Data from the U.S. indicated that once wheat is in the dough stage armyworm leaf feeding does not do any additional economic damage. A field of winter wheat in the Winkler area was reported to have armyworm levels well above the threshold of 10 / m2 on average. However, the crop is quite mature, and the larvae have not been feeding on the heads. In situations like this, where the most critical yield forming period has past, the impact the remaining population would have on yield would not likely be as great as the cost of applying an insecticide, and the application of insecticides would not be advised.

Pulse Crops

Pea aphid

Aphids on peas – Spraying for pea aphids in peas has been reported from fields near Arborg, Neepawa, and Morris. Monitor fields of peas for aphids when peas begin to flower. Pea aphids mainly infest the growing tips of plants. You can either take 180 degree sweeps with a sweep net, or check plant tips (the upper 8 inches or 20 cm) for aphids at each stop. Try to get counts from at least 10 areas of the field and find the average to determine if insecticides are likely to be economical. If, at the beginning of flowering, there are on average 9 to 12 aphids per sweep or on average 2-3 aphids per plant tip, an insecticide application when 50% of plants have produced some young pods would be cost-effective.

Identification tips - The picture above shows a pea aphid inside the bag of a sweep net. Note that pea aphids are only about 3 mm long as adults, and smaller when young. The long “tubes” at the back of the body are called cornicles, and are a characteristic you can use to tell aphids from other insects. Also note the pear shape, and long legs (relative to body size).

First trifoliate with angular lesions of anthracnose	Anthracnose in Beans - We have begun to see and hear reports of anthracnose in edible beans.  The first symptoms are inconspicuous and might only be seen (or confirmed) by turning over leaves by hand.  In the picture below, we see the characteristic veinal collapse and discoloration by turning over one of the leaflets. The crater like lesions that soon form on the petioles and stems may show signs of spore production (light orange color).  Such lesions weaken flowering stems leading to breakage and loss of pods.  If fungicides are to be used successfully, they are best applied at early bloom or just after the first pin beans have formed.  If you happen to be scouting numerous bean fields, consider wearing disposable plastic booties (over your own footwear) and discarding them before entering another field, especially if you have walked in an infected field.  We do not want to be spreading pathogens to uninfected fields.

Canaryseed

Aphids -  A couple of fields of canaryseed in the Morris area have been treated with insecticides to control aphids. The economic threshold for aphids in canaryseed is when at least half of the stems have 10-20 aphids, and the crop is not yet at the soft dough stage. When monitoring for aphids in canaryseed, remember to look carefully in the head, as well as between the leaf sheath and the stem. Where aphids become clustered under the sheaths of canaryseed plants, the sheaths can become brown and swollen.