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Ascochyta pathogen insensitive to strobilurin fungicides in chickpeas

In 2007, Ascochyta rabiei, the fungal pathogen that causes ascochyta blight, was found to have widespread insensitivity (resistance) to the strobilurin class of fungicides, and since then researchers and companies have been working on developing better solutions and management practices. “Insensitivity developed very rapidly for the strobilurins because they have a single site mode of action and are used intensely in chickpeas. Insensitivity is now common in Alberta, Saskatchewan and the United States,” says research scientist Dr. Bruce Gossen with Agriculture and Agri-Food Canada’s Saskatoon Research Centre. He says the fungicide industry co-operated with government right from the start in monitoring and developing solutions for Ascochyta control.

Strobilurins are in the Group 11 fungicide class and are known as quinone outside inhibitors (QoIs). This class is prone to resistance because they act on a single site for control, and the pathogen can easily overcome the resistance if the selection pressure is high enough. The fungicides Headline EC from BASF and Quadris from Syngenta were the two strobilurin fungicides on the marketplace, although Headline EC is no longer registered in chickpeas. 

BASF manager of Research and Development Wayne Barton says his company has been working with government and farmers to monitor the situation in chickpeas after the launch of Headline in 2003. Early on, BASF realized the risk in chickpeas, and when the first insensitivity was found, the company was able to bring Headline Duo on to the market. That product was first introduced in 2007 to address Ascochyta insensitivity in chickpeas. Headline Duo contains a pre-mix of two chemical groups, a strobilurin and a carboxamide. With the inclusion of carboxamide, Headline Duo is still delivering good control of Ascochyta rabiei. When Headline Duo was registered, BASF removed Headline EC from the chickpea label to help growers manage Ascochyta insensitivity.

“Since 2007, BASF has been working with provincial and federal officials, including the Pest Management Regulatory Agency and industry stakeholders, to ensure chickpea growers have access to a fungicide solution that will control all strains of Ascochyta,” says Barton.

Understanding fungicide insensitivity
Gossen explains that fungicides historically contained multi-site modes of action, often including persistent elements such as heavy metals in their formulation. With the phasing out of the heavy metals and a shift to reduced-risk active ingredients, the fungicides became more susceptible to insensitivity, mostly because they were non-persistent and contained a single-site mode of action. Reduced sensitivity to fungicides usually showed up on crops with highly intensive use, such as orchards, horticultural crops and intensive field crops such as chickpea.

Several factors influence how quickly fungicide insensitivity can develop. Qualitative insensitivity develops when a change in a single gene results in complete loss in sensitivity. The risk of qualitative insensitivity developing is high. Quantitative insensitivity occurs when many genes, each with a small impact, affect sensitivity. When qualitative insensitivity develops, the result is total loss of control. Quantitative insensitivity results in a decrease in control rather than a total loss of control. The application rate is important for quantitative insensitivity, where a high rate can control intermediate levels of insensitivity. 

Frequency of fungicide use is also an important factor. Repeated application, as in the case of chickpea where three applications per season are common, puts more selection pressure on the pathogen. 

The many combinations of factors result in different levels of risk in different host/pathogen systems.

Gossen says the risk of developing insensitivity to strobilurin in Ascochyta rabiei was high because the pathogen is genetically diverse, its sexual spores are air-borne, fungicides are applied several times per season and insensitivity has been reported in related fungi. 

Research confirms widespread insensitivity
Initially, strobilurin fungicides provided effective reduction of ascochyta blight, but the extensive use of these fungicides starting in 2003 led to a breakdown in effectiveness. To assess how widespread the insensitivity was, Gossen ran a study from 2004 through 2008. He collected A. rabiei from commercial chickpea fields and assessed their sensitivity to strobilurins. He observed a rapid decline in the effectiveness of the two strobilurin fungicides Headline EC and Quadris. 

In 2004 and 2005, virtually all of the isolates were sensitive to the strobilurins. Samples collected in 2006 found insensitivity in more than one-half of the populations collected. By 2008, virtually all of the samples collected were insensitive to strobilurins. Quadris and Headline showed similar loss of control. “We had pretty clear proof that the insensitive isolates were causing lack of control with the strobilurins,” says Gossen. “In other studies we saw across Alberta and Saskatchewan and in the United States, there was a loss of control with strobilurins.”

Gossen also looked at the fitness of the isolates. He did not see any difference in fitness between the sensitive and insensitive isolates. “So if there is no difference in fitness, even if we withdraw the fungicide, the insensitive populations will likely persist for a long time.”

New fungicides provide options
In addition to Headline Duo, Bayer CropScience and Syngenta also have fungicide options to replace single active strobilurins. Syngenta’s Ed Thiessen says that Bravo 500 is still a good choice for Ascochyta control because it is a Group M product that has multi-site activity. As a multi-site product, the chance of insensitivity developing is rated low. As well, Syngenta is testing a new non-strobilurin fungicide for this market. “When choosing fungicides to manage resistance, it is important to go with a product with multi-site activity or a product with dual modes of action,” says Thiessen. 

Thiessen says he likes the newer chemistries because they have some systemic control, helping to protect new plant growth from the pathogen. Contact products like Bravo only control the pathogen on existing plant growth, but they remain key rotational fungicides for resistance management.

Bayer CropScience has Proline 480 fungicide registered for Ascochyta control in chickpea. Proline is also a Group 3 triazole product with prothioconazole active ingredient. Proline is rated as having a medium risk of insensitivity development.

Gossen has tested several of the new fungicide products with dual or alternate modes of action. He found that the new modes of action are working well, but some will require monitoring for insensitivity.

Of the products Gossen looked at, no insensitivity was found with the Group 7 products, including Lance (boscalid) and a new research compound from DuPont called Vertisan (penthiopyrad). A small percentage, 15 percent, had less sensitivity than the baseline, and monitoring for developing insensitivity should be initiated within the next few years. 

Chlorothalonil (Bravo) had a few isolates with low sensitivity, and Gossen says that is likely not a problem at this point, but is a niggling concern that should be monitored. Gossen did not have any research data on Proline (prothioconazole, a DMI or demethylation inhibitor).

Thiessen says that although industry and government have worked together to raise awareness of fungicide insensitivity, more needs to be done to help prevent further problems from developing with the new chemistries. “There still seems to be a lack of awareness of what to do about it. We all need to promote good management practices with the use of fungicides, especially in chickpeas where fungicide use is fairly intensive,” says Thiessen.