Seed & Chemical
A multi-pronged fight against ascochyta
By Carolyn King
Alberta researchers are honing a variety of weapons in the fight against ascochyta blight in chickpeas.
Ascochyta blight is both very common and very devastating, making it one of
the most important factors in reducing yield, quality and production of chickpeas
on the prairies. So a team of researchers is looking for more effective ways
to control the disease under Alberta conditions.
"The disease is common not only in Canada but almost everywhere in the
world in every year and the impacts are severe. For example, one scientist found
that, in 2001, about 46,500 acres of chickpeas were seeded in Manitoba and North
Dakota, but only 30,000 acres were harvested, and the situation was similar
in Alberta and Saskatchewan," says Dr. Kan-Fa Chang. He is a plant pathologist
with Alberta Agriculture, Food and Rural Development and is leading the research
He adds, "It's not only yield loss. Once the crop is infected, the pathogen
can infect the seed inside the seed pod. It discolours the seed surface, which
reduces the seed quality and germination rate and sometimes the price. For instance,
the infected seeds may have a value of only about eight to 15 cents per pound
compared to a value of about 36 cents per pound for healthy large kabuli seeds."
Most chickpea growers are all too familiar with this disease, which is caused
by the fungal pathogen Ascochyta rabiei. It is carried on infected seeds
and crop residues. Under the warm (20 to 23 degrees C) and moist conditions
that favour this disease, the pathogen can grow rapidly and reproduce. The spores
are spread from plant to plant by rain splash but also can be carried by the
wind to nearby fields.
"Typically, growers spray with a fungicide three or four times in the
growing season to control ascochyta blight. Even then they may get a 25 percent
yield loss due to the disease. And when conditions favour the disease, the yield
loss may be even greater. If the disease is left with no control, it could wipe
out the whole crop," notes Chang.
Chang's team has been conducting a range of projects in a four year research
program that was nearing completion in the late fall of 2006. The program is
funded by the Alberta Agricultural Research Institute, the Alberta Crop Industry
Development Fund and the Agriculture and Food Council. The projects include:
comparing various seed row spacings and seeding rates, assessing seed treatments,
comparing foliar fungicides, screening chickpea varieties for resistance, and
identifying the pathogen's races (strains).
Seed spacing: The researchers evaluated three row spacings (20, 30 and 40 centimetres)
and three in-row seed densities (seeding rates of 20, 40 and 60 seeds in a three
metre row) to figure out the best options to manage the disease while maintaining
chickpea yields. Chang says, "We found that if you increase the in-row
density or reduce the row spacing, you will increase the ascochyta blight severity
on the crop. In other words, if the plants are farther apart, the disease will
be less severe. However, the disease incidence, which means the percent of infection,
may remain the same."
Seed treatments: They have compared various seed treatments, testing six fungicides
alone or in combination. Four of the fungicides significantly improved seedling
emergence. Two of those fungicides, Apron Maxx and Crown, are currently registered
for this purpose on chickpeas.
Foliar fungicides: The post-emergence foliar fungicide project compared various
fungicides sprayed at different frequencies during the growing season. The treatments
reduced the disease but at least four sprays were needed to achieve 50 percent
control. Treatments were more effective if the fungicide was applied while the
disease pressure was still low. Chang notes, "Systemic fungicides like
Headline or Lance are more effective but more expensive than Bravo or other
kinds of fungicides."
Screening for disease resistance: The researchers have screened 22 cultivars
and breeding lines for resistance under greenhouse and field conditions using
several different methods to induce the disease in the plants. They are also
working on a faster method for screening, which involves the difficult task
of growing and isolating the fungal toxin. The toxin can then be used in a screening
process that requires less space and less time. Chang adds, "but every
method has advantages and disadvantages. For instance, some of the other methods
are easier to do, but the disease impacts are harder to measure."
Chang outlines some of the progress that they have made. "We have confirmed
that the desi chickpea is more resistant than the kabuli chickpea. We've also
found that, in general, the fern-types of kabuli are more resistant to ascochyta
blight than the unifoliate kabuli types." Although they have not found
any new highly resistant chickpea lines, they have found some tolerant lines,
where the plants can become infected but the pathogen is less able to reduce
yields. In addition, the researchers have successfully isolated some fungal
toxins and are now using them to screen cultivars.
Fungal races: The researchers collected many samples of the fungus from chickpea
crops in Alberta. They are currently comparing the physiological characteristics
of the different fungal isolates and trying to group the isolates into different
races of the pathogen. Chang explains that knowing the degree of variation in
the pathogen will help efforts to breed resistant chickpea cultivars. He says,
"We have to figure out which races are most prevalent under natural conditions
at different locations or in different years. Based on that, we can use the
most prevalent races when screening the cultivars for resistance."
Chang notes, "We've found the pathogen Ascochyta rabiei has a big
variation in its pathogenicity or culture morphology. This is a very commonly
occurring situation under natural conditions where you have a big population
of a pathogen. And this very large fungal variation is what makes the disease
so difficult to control."
Another important reason for identifying the races is to see if any have resistance
to certain fungicides. Chang says, "Several foliar fungicides are available,
like Bravo, Quadris, Headline and Lance, for controlling this disease. But if
we repeatedly use the same kind of fungicide, that puts the pathogen under selection
pressure; genes and mutations for fungicide resistance are selectively passed
on to the next pathogen generation. We want to know if there is any fungal isolate
that already has resistance to a fungicide, because some growers who sprayed
with Quadris or Bravo four times, couldn't control the disease. That might indicate
the fungus has developed resistance to that fungicide."
As to this project's results, he says, "We found some fungal isolates
of the pathogen with resistance to Bravo and Manzate. But proportionately, we
found very, very few fungal isolates with resistance to Headline."
Chang emphasizes that future research on ascochyta blight in chickpeas will
depend on funding availability. He believes that continuation of the breeding
program to develop resistant chickpea lines is still the first priority to combat
the disease. "The variation of the fungal population is already very big,
plus when we apply fungicides, only resistant types and mutations survive, so
we are accelerating the development of fungicide resistance. As a result, new
races may emerge in different years or locations. So we have to keep an eye
on this aspect." Other important research avenues include the development
of new fungicides and regular field surveys to watch for emerging races of the
pathogen with resistance to existing fungicides.
Key control practices for growers
Based on the data the research team has analyzed so far and on previous research,
Chang recommends that chickpea growers use an integrated approach that combines
chemical and cultural methods:
- Use resistant chickpea cultivars.
- Use a seed treatment fungicide, either Apron Maxx or Crown.
- Closely monitor chickpea fields for ascochyta blight and other diseases,
starting right after crop emergence.
- If ascochyta blight appears, apply a foliar fungicide, such as Headline,
Bravo, Quadris or Lance, as soon as possible; the longer the wait to apply
a fungicide, the harder the disease will be to control.
- Rotate between the different groups of foliar fungicides, and be prepared
to spray three times in a season if necessary.
- Avoid growing chickpeas on the same field or an adjacent field year after
Chang stresses the importance of getting a good start for your crop with healthy,
disease-free seed. If a grower decides to save some seed for the next year's
crop, he recommends that they have the seed tested to determine the percentage
of infection with Ascochyta rabiei and all other pathogens, as well as
the seed's germination rate and vigour.
He says, "If you can get totally healthy, disease-free seed or certified
seed, you should use that. Saskatchewan recommends that, if you can't find completely
disease-free seed, the seed should at least have an infection rate of less than
0.3 percent (three infected seeds in 1000 seeds). However, diseased seed forms
an infection centre, and from that centre the disease can spread very quickly
if conditions are favourable. So I would recommend totally clean, pathogen-free
seed, even if you have to pay a somewhat higher price." -30-