Seed & Chemical
Scouting and counting vital to ascochyta control
By Helen McMenamin
Modified spray decision system guides fungicide application.
By Helen McMenamin
Fungi of the ascochyta complex can cut pea yields by 40 percent, but spraying
fungicides does not always pay. Scouting at least twice a week from early flowering
on, plus good record keeping and a close eye on weather forecasts can guide
fungicide applications. As a result, deciding whether and when to spray fungicide
against ascochyta are critical to a positive bottom line for the pea crop.
Pea growers in some parts of the world automatically protect flowering crops
with fungicides. According to Alberta Agriculture pulse research agronomist,
Ken Lopetinsky's work over three years and four sites in the Edmonton region,
that approach pays about half the time, with a break-even price of $4.50 a bushel.
Yields of untreated crops were 49 to 59 bushels and fungicide protection increased
yields by 11 to 20 percent.
Clearly, knowing when to spray is important. Even at the current low prices
for peas, fungicide applications that save 12 bushels (25 percent of a 50 bushel
crop) pays off in yield. Reducing fungal disease also improves seed quality,
standability and ease of harvesting.
Part of the challenge in deciding whether to apply fungicides is that they
offer protection from future disease, not a cure for symptoms. Even the new
systemic fungicides act mainly as protectants, though they do limit growth of
Bravo is a contact fungicide that prevents spores invading the plant, but does
not affect existing infections inside the plant. It lasts seven to 10 days.
Headline and Quadris act systemically, entering the plant to protect all surfaces
and limit growth of existing lesions. Quadris protects for 14 to 18 days, Headline
for 10 to 14 days and it protects against powdery mildew as well as ascochyta.
"Once you see symptoms, the disease has already damaged your crop,"
says Mark Kuchuran, technical specialist with BASF. "In a high-yielding
or high-value crop and humid conditions, ascochyta speckling on any more than
the very lowest leaves at first flowering will probably cause economic losses.
A fungicide pays off in those conditions. But, when disease risk is less clear-cut,
a scoring system helps in decision-making."
Ascochyta prediction system for
|Characteristic||Estimation risk scale||Score|
|Crop canopy||Thin: zero||Moderate: 10||Moderate to heavy: 15||Heavy: 30||–|
|Leaf wetness humidity, dew||Dry: zero||Low: 10||Moderate: 20||High/wet at 1:00pm: 40||–|
|Percent plants (crop) showing symptoms||None: zero||Low less than 20 percent: 15||Moderate 20 to 50 percent: 25||High greater than 50 percent: 40||–|
|Five day forecast||Dry: zero||Unsettled: 10||Showers: 15||Wet/rain: 20||–|
|If the estimated risk value is less than 65
points, no fungicide application is deemed necessary, but field inspections
should continue on a bi-weekly basis. Total score over 65 points, fungicide
application is advised.
|Source: AAFRD: Lopetinsky and Strydhorst.|
To help ensure a fungicide return on investment, Lopetinsky modified disease
prediction systems from Syngenta and the University of Saskatchewan to make
an easy-to-use version for farmers. It takes much of the guesswork out of the
decision to spray a fungicide on field peas. (It is not for chickpeas.) He uses
current and expected crop and weather conditions along with the crop's yield
potential, to gauge the need for fungicide application, and the likelihood of
a positive return. His modified scoring system is simple enough to be a practical
tool for farmers. "Ascochyta risk is different for every field," he
says. "You can't scout a few fields and predict the disease rate for all
pea crops in the area. You have to look at individual fields."
Take your dog scouting
"Scout at least twice a week, starting from first flowering, by late June,"
says Lopetinsky. "Go to the same spots in the field each time, so you'll
see whether the disease is becoming more severe."
To assess the extent of infection, examine 10 plants at each scouting spot
and note the number showing any sign of ascochyta. Use these figures to calculate
an average percent infection for the whole field.
To judge the risk of the disease spreading, assess plant wetness at time of
scouting, and a prediction over the next few days. These are based on the density
of the canopy, its wetness at present and the rainfall predicted for the next
In Lopetinsky's simplified scoring system, rate crop canopy as thin, moderate,
moderately heavy or heavy. For example, a heavy canopy is difficult to walk
through; a small dog beside you disappears in it. The crop canopy also indicates
yield potential with a heavy canopy likely yielding more than 50 bushels.
Plant wetness is also scored on a four point scale, from dry (no dew in the
morning) to low (dew drying by around 9:00 or 10:00 o'clock), moderate (canopy
wet until 11:00 o'clock) and wet (canopy wet enough to soak your pants at 1:00pm).
Note the score for the moisture predicted in the five day weather forecast.
Add the scores for all four risk factors. A total score of greater than 65 indicates
fungicide application is justified. Lower scores provide a baseline for monitoring
The choice of fungicide depends on your expectations of the weather. If humid
conditions persist after the chemical dissipates, another application may be
"Producers tend to spray too late," says Lopetinsky. "They lose
yield and quality, hoping dry weather is just around the corner and they won't
need to spray. But delay in spraying a fungicide can cost you yield and quality."