The second most used molecule on wheat and sixth on barley, chlorothalonil is currently experiencing its last campaign following its non-renewal at European level. According to Jean-Yves Maufras d’Arvalis, “its disappearance could have fewer short-term consequences than one might fear. […] Provided, of course, to advance practices towards a greater integration of available levers, including genetics, ADOs and biocontrol.”

Active on septoria, the chlorothalonil “has for over 40 years, one of the best value / market price”, presents Jean-Yves Maufras engineer Arvalis Institute plant. Paradoxically, few consequences are to be expected at the time of the first treatment ( T1 ), even “if the main uses of this molecule are in T1”. One exception, however: “if the septoria leaf pressure is strong and early, the yield gain can be up to 10 q / ha,” adds Jean-Yves Maufras.

No more repercussions on the second treatment ( T2 ): “the intake is neither frequent nor decisive at this period”. On barley, chlorothalonil is much less used. “It is most often justified to control rhynchosporiosis, in association with other molecules at the beginning of the run (T1). “

The non-renewal of chlorothalonil also represents, and above all, the disappearance of a mode of action of great durability in the face of the problem of resistance.

The consequences could, however, be “much more serious in the long run . By removing an active mode of action on all strains including strains resistant to triazoles, QoI and SDHI, it deprives itself of a particularly useful protection tool and has demonstrated its ability not to generate resistant strains[…] The non-renewal of chlorothalonil also represents, and above all, the disappearance of a mode of action of great durability in the face of the problem of resistance. In more than 40 years of use, no strain resistant to this molecule has appeared on any culture and vis-à-vis any pathogen. At the same time, this could lead to “an increase in the use of other modes of action, and therefore more selection pressure on other chemical families”.

What alternatives are possible?

Faced with this disappearance, two types of alternatives are possible, according to the tests conducted by Arvalis. First option: replace chlorothalonil with ” another fungicidal molecule , equally effective against Septoria and with a preferential mode of action , preferably without increasing the cost of protection”. Among the candidate molecules:

– The folpel is “the molecule that has the most common points with chlorothalonil,” according to Arvalis trials. Recently marketed under the name of Sesto with approval for wheat leaf blotch, this active ingredient could be used as early as the spring of 2020. Partially controlling ramulariasis, an authorization on barley would “enrich the number of solutions available” from here 2021 or even 2022. Work is underway on the resistance risk management plan (Performance Network). “The first results indicate that while the MDR strains seem as well controlled as after the use of chlorothalonil, on the other hand the strains of type Tri HR are much less.”

– Mancozeb , another multisite available and already authorized on cereals. On the other hand, “its effectiveness (compared to folpel and chlorothalonil) seems a priori insufficient to justify (re) developing its use on cereals”. In addition, the molecule, under review, may well be reclassified CMR.

– Sulfur , multisite classified in the list of biocontrol products. “Its effectiveness can not compete with that of chlorothalonil,” notes Arvalis. It does, however, show good efficacy, “associated with a triazole or other biocontrol molecule (such as phosphonates) at the time of T1”. Work is also underway on resistance risk management. According to the results, “if the MDR strains seem to be less well controlled than after the use of chlorothalonil, the Tri HR (non-MDR) strains are just as well”. “On barley, the interest of sulfur has not been demonstrated”.

The second option is to treat less by mobilizing other levers of protection  : “use of resistant varieties and decision support tool , and if necessary use of alternative products of biocontrol”. The objective is to limit or even eliminate the use of the first treatment. “Removing T1 removes, at the same time, the question of replacing chlorothalonil. By decreasing the number of treatments, the selection pressure decreases in parallel, and should allow (at least as well as an application of chlorothalonil could do) to reduce the selection pressure and the progression of the resistance. “

As Arvalis concludes: “Chlorothalonil is not irreplaceable, even if no alternative solution is really equivalent to it today. No disaster is therefore to be feared in the short term “. To mitigate the effects of its disappearance, “the programs will have to be adapted by modulating the doses or mobilization of other modes of action, or more simply by mobilizing other levers of protection to suppress the T1”.