Infection of ascochyta in chickpea crops can happen much earlier than first thought. Ascospores released from infected chickpea stubble can spread and infect new plants early and in the absence of rain splash, according to Yantai Gan, a research scientist with Agriculture and Agri-Food Canada at Swift Current. Scouting and spraying early is your best defence.
Gan discovered the fungus survives the winter in crop debris then, in spring it releases spores that are more mobile than the spores produced in infected plants. These spores, once released from the crop residue, can be carried on air currents or wind, and if they land on a susceptible plant, can cause an infection if there are moist conditions for around 24 hours.
To find out when ascospores are released from crop residue, Gan grew disease-free chickpea plants in pots in a greenhouse and every few days, starting in early May, set some plants, called sentinel plants, out for a couple of days. He then grew the plants a little longer to see whether they had been infected with ascochyta. By the third week of May, some of the sentinel plants became infected during a short period in a field of ascochyta-infected crop residue. By the end of May, any plants exposed to the infected stubble were infected with ascochyta.
ROTATION PLUS FIELD SELECTION
The standard advice is to use rotation to minimize infection of the crop from infected crop residue by growing chickpeas no more than once in four years on a particular field. But Gan’s research extends that advice to growing chickpeas as far away as possible from last year’s chickpea stubble, preferably upwind.
Ascochyta spores produced in infected plants are spread only in rain or dew splash, so they can only infect nearby plants. That leads to the spreading patches of ascochyta blight in a crop. In infected stubble, the fungus reproduces sexually to produce ascospores, which are released as soon as spring conditions allow. These move on air currents.
“We don’t know how far these spores can travel,” says Gan. “With no wind the spores probably don’t go far, but a little breeze is ideal for them to float along. Stronger winds can get straw moving and carry infection that way. Just a tiny piece of straw can spread a lot of spores.
“Ascochyta can start from a single spore or a small particle of infected residue and once it’s in a chickpea crop, the disease can spread quickly. You start with a single infected plant, then there’s a patch and an acre of crop can be infected before you realize it.”
Gan advises planting chickpeas upwind from last year’s chickpea fields, or as far away as possible from last year’s stubble, and, sticking to at least a four-year rotation.
Clean seed is the other key to minimizing infection. Gan strongly recommends seed testing and only using seed with less than 0.3 per cent ascochyta infection.
If you or your neighbours grew chickpeas last year, he advises spraying fungicide a week or 10 days pre-flowering, at the seven-to nine-node stage. Infection at flowering has the greatest impact on yields as flowers may abort.
“You don’t want to spray too early, or plants will be too small to catch the fungicide; you need some crop biomass,” he says. “The ideal time is at the seven-or nine-node stage.”
Think of spraying all crops before flowering as a protective measure, much like seed treatment, Gan advises.
“The key thing is to budget for one application of fungicide to protect the crop early in the year,” he says. “Spray even if you don’t see any signs of disease. It’s almost like an insurance policy. You can leave it until a little later if you’re willing to accept more risk, but it’s a very beneficial investment.
“You may not see very small infections early in the year. But, with just a little moisture that infection can spread through the crop very quickly.”
Gan advises combining fungicides or rotating among products with different modes of action because some races of Ascochyta rabiei, the species that affects chickpeas, have been found to be resistant to the strobilurins — Headline, Quadris, etc. These products are still valuable because they form a protective coating on the plant and have some systemic activity to prevent spore formation and most races of fungus are controlled, but adding a second fungicide with a different mode of action, such as Headline Duo or a
tank mix gives more protection.
That first treatment gives about two weeks’ protection. After that, scout for early signs of ascochyta, especially if it’s wet, or there’s rain in the forecast.
VA RIETY SELECTION
“Don’t forget some varieties are more susceptible to ascochyta damage,” says Saskatchewan Ministry of Agriculture plant disease specialist, Faye Dokken-Bouchard. “No chickpeas have good resistance to ascochyta, but all desi varieties have fair resistance and so do Frontier and Luna kabulis.”
Some lentil varieties have good resistance to ascochyta (a different strain than what affects chickpea), but Dokken-Bouchard notes that good resistance does not mean complete immunity. She advises protecting them with a fungicide when disease pressure is high, say from poor rotations.
“Even good resistance can break down if the disease pressure is heavy,” she says. As long as the weather is dry during flowering, lentils don’t usually need more than one fungicide application. Also, ascochyta in lentils is caused by Ascochyta lentis, a different pathogen to Ascochyta rabiei, which affects chickpeas. But lentils are also susceptible to anthracnose, which can cause similar losses to ascochyta — up to 50 per cent, as well as botrytis and sclerotinia.
“Think of the disease triangle,” says Dokken-Bouchard. “We have the pathogen present as soon as spores are released from residue, or from plants from infected seed, we usually get some moisture in late May or June, providing the environment that favours the pathogen, and we have a susceptible host. So we’re likely to have disease.”
Dokken-Bouchard would choose to wait until there’s rain predicted to apply protective fungicide. Gan puts more faith in normal weather patterns than in weather forecasts. He’d spray when the crop is at the right stage rather than count on a forecast giving you enough warning to spray in time for the fungicide to be rainfast. They agree on the goals though.
“If we can make the host plant less susceptible by applying a fungicide, we can prevent or reduce infection during flowering, then the disease has the biggest effect on yield,” says Dokken-Bouchard. “If we can protect the crop during this most critical time, we can get the yield potential. Scouting every five to seven days, starting a couple of weeks after the first fungicide application, and keeping an eye on the weather can control ascochyta and other diseases with minimum fungicide applications.”
Helen McMenamin is a freelance writer from Lethbridge, Alta.