Seed & Chemical
Alternaria black spot control options expanded
By Top Crop Manager
Spraying for the disease is uncommon, but suppression can be achieved with sclerotinia control.
In the nice-to-know department, canola growers have an additional tool for
alternaria black spot control. While the disease is rarely targetted with fungicides,
a spin-off of sclerotinia control is that alternaria is often suppressed.
"I don't know of anyone spraying for alternaria," says Murray Hartman,
provincial oilseed specialist with Alberta Agriculture, Food and Rural Development
at Lacombe. "The problem is that there isn't any way to predict whether
there will be a response to the fungicide."
Alternaria is a more common problem in Polish type canola than Argentine types.
Still, research has shown that fungicide application can provide a yield response
in Argentine types as well. Those facts, though, do not change Hartman's observation
on how to predict when to spray since "there is no established protocol
to predict yield benefits like there is with sclerotinia."
Sclerotinia timing can provide suppression
While few if any growers spray specifically for alternaria black spot, suppression
can be achieved as a bonus if sclerotinia stem rot is targetted with fungicide
control. Three fungicides have both sclerotinia and alternaria on their label:
Quadris, Rovral Flo and the newer Lance, which received a label extension to
include alternaria control in 2004.
Lance provides another tool for growers to target sclerotinia as well as alternaria
and is valuable because it is a Group 7 fungicide. Since Lance is a different
fungicide than Quadris (Group 11) and Rovral Flo (Group 2), it gives canola
growers another rotational option when managing fungicide resistance.
Alternaria develops under similar environmental conditions as sclerotinia,
both like wet, humid conditions. However, alternaria research in Alberta found
that severe infections occurred in cool (10 to 15 degrees C), wet, windy weather
in July to early August, whereas sclerotinia prefers warm (greater than 15 degrees
C) and wet conditions. Both diseases are favoured when canola crops lodge in
late July to early August.
Application timing for fungicide control is different, though. Sclerotinia
applications generally occur during early flowering stages, while control of
alternaria is targetted at late flowering to early green pod stage.
Until researchers develop spray decision systems to predict when to spray for
alternaria, growers can at least be assured that if sclerotinia is sprayed,
suppression of alternaria is an added bonus. -30-
Timing of fungicide application
Fungicide, sclerotinia control, alternaria
Apply at 20 to 50 percent flowering
to control sclerotinia and suppress alternaria. Apply a second time seven to
14 days later up to full bloom if disease persists or weather is favourable
for disease development. Apply at late flowering to early green pod stage.
Apply at the early bloom stage prior
to 30 percent for sclerotinia control and suppression of alternaria. Apply at
early pod stage, which is approximately 90 percent petal drop.
Apply when the canola crop is in the 20 to 50 percent bloom stage. Best protection
is achieved when applied at 20 to 30 percent bloom stage, prior to petals beginning
to fall. A split application can be made up to the 50 percent bloom stage. A
single application at early greed pod stage or as a split application: the first
at full bloom followed by a second application at early green pod stage. -30-