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Powdery mildew hits Alberta

Pea growers in Alberta lose their innocence.

November 19, 2007  By Helen McMenamin

In 2003 and 2004, powdery mildew caused yield losses in Alberta pea crops,
not a common occurrence in previous years. Now, all pea growers across the prairies
recognize that they cannot afford to ignore powdery mildew.

The disease usually shows up in the last days before harvest and is of no consequence.
However, it can hit earlier, as happened in 2003 and 2004. The fungus can cover
the leaves, reduce photosynthesis and cause severe losses, according to Mark
Olson, Alberta Agriculture pulse extension specialist. "The fungus is everywhere,
so if conditions favour it, the disease will develop," he says. "Those
conditions are a susceptible crop and a few days of warm temperatures and cool
nights – conditions that raise humidity." Late seeding and high soil
nitrogen levels also increase susceptibility.

"If you seed peas late into high nitrogen soils, you're two-thirds of
the way to seeing powdery mildew infection. In that situation, you can almost
guarantee powdery mildew infection. It can strike any time after flowering starts.
Be ready to start scouting at flowering and be especially vigilant any time
you have cool temperatures overnight."


The temperature conditions do not have to be extreme, 20 to 25 degree C days,
with nights below 10 degrees C are enough. Once powdery mildew gains a toehold,
it spreads quickly, especially in leafed varieties.

At first, powdery mildew appears on the foliage, most often on the upper sides
of the lowest leaves, as fine white specks that are easily rubbed off. These
generally appear after the third cool morning. They quickly turn yellow, then
to purple or brown blotches with golden fruiting bodies, or cleistothecia, that
blacken as they mature. Spores released from these fruiting bodies are spread
by any breeze to other parts of the foliage, other plants or new fields.

What should growers do?
The fungicide Headline controls powdery mildew and gives residual protection
for 10 to 14 days. Olson has seen average yield increases of four or five bushels
from spraying, more than enough to cover the cost of fungicide and application.
The benefits include control of ascochyta. Just as important, the fungicide
improves the crop's standability, making harvest easier. Spraying must be prompt.

"About five days after infection, the fungus forms a waterproof coating
that fungicides can't penetrate," says Olson. "Desiccants are also
sealed out, so quality may suffer. Worse, it doesn't dry out and wet weather
turns infected foliage into a slimy mess that's almost impossible to combine."

Desiccants can make a quality difference in peas that is worth $1 or even $2
per bushel: more than enough to pay for crop monitoring and fungicide treatment.

Frequent light sprinkling has been suggested as a way for irrigation farmers
to control powdery mildew, but Olson does not believe it is effective. "The
theory is that the water washes the fungus off the foliage," he says. "In
my experience, it's not effective. I'd assume irrigated crops to be at the same
risk as any other pea crop."

The ideal answer is resistant varieties. Most of the new varieties are resistant
to powdery mildew, but seed has been scarce. Those resistant varieties may be
easier to find for 2005, although the 2004 harvest may have had an effect on
seed quality. Check provincial variety recommendations for disease resistant

Can be confused with downy mildew
Resistant varieties of peas are completely resistant. Reports of powdery mildew
on resistant varieties of peas are actually due to a different disease, downy
mildew. The two can occur together.

Downy mildew infection begins as mousey grey, felty-looking patches on the
undersides of leaflets. The leaflets turn yellow and die, and young stems become
distorted and covered with a white cottony growth. Infected pods have brown
blotches with green islands in them. Inside the pod, these blotches are felty
and nearby seeds abort or have brown sunken spots.

Downy mildew, a new problem on the prairies, is caused by a fungus called Peronospora
, while Erysiphe pisa causes powdery mildew. In 2004, Olson
saw fields with both powdery and downy mildew.

Alfalfa, lawn grass, willows, caragana, poplar and other species are susceptible
to powdery mildew, but the species that attack these plants are different from
peas. Similar weather favours all powdery mildew fungi, though, so disease on
these plants is a sign peas are at risk.

Powdery mildew fruiting bodies can overwinter, causing infection next year
in peas in the same or adjacent fields the following year. Olson's advice for
avoiding losses to powdery mildew is simple: "Seed early and avoid fields
with high residual nitrogen. Choose a resistant variety. If you seed a susceptible
variety of peas under these conditions, pencil the cost of fungicide into your
projections and monitor the crop closely. Use a four year rotation and don't
seed peas close to a field that was infected the previous year." -30-



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