Brown Rot Resistance to Indar

Earlier this year, researchers in New York State reported their first cases of brown rot resistance to Indar (fenbuconazole). There have not been any reports of Indar failures in Ontario yet, but the New York experience is important - we could end up having the same problems if proper resistance management is not followed.

Indar is registered in Ontario for brown rot on stone fruit as well as black knot on sour cherries and plums. Indar is one of several fungicides in the same chemical group (triazoles) registered for brown rot. The members of this fungicide group are often referred to by their mode of action - usually as "SI" (sterol inhibitor) or alternatively as "DMI" (demethylation inhibitor). Other members of this group used against brown rot include Nova (myclobutanil) and Topas or Mission (both are propiconazole). Resistance to one member of a group usually means resistance to all members of the group and this was indeed the case in the Indar resistant strains of brown rot in New York.

In one New York case of resistance to Indar, it is theorized that repeated use of Indar exclusively for years against blossom blight and brown rot in one nectarine orchard led to the selection of a resistant strain of brown rot. Of course, not everyone sprays in that fashion and other cases may have arisen from many uses of Indar over the last decade or so, even if Indar was not used exclusively.

Repetitive use of one product should be an easy mistake to avoid. It is repeated ad naseum in articles, talks, and on some product labels to not use the same product repeatedly. For stone fruit, there is a fairly wide range of materials registered for brown rot in Ontario (including a new one this year - Pristine), so using one product continuously is definitely not necessary. Everyone has heard the refrain of rotating product chemistries to avoid resistance, as well as using adequate water volumes to obtain good coverage, and all the other means to get the most out of your pesticide use. So why would there be any confusion about repeated use of a single product?

One possible reason is that we always mention rotating between chemical families or groups, not just between products. In the past, it might have been difficult to keep track of which family a particular product belonged to as well as the families of previously used products. There are now charts at the end of each commodity calendar clearly showing the family for all products listed in OMAFRA Publication 360 -Fruit Production Recommendations 2006-2007. Referring to those charts when planning your fungicide strategy will help avoid overuse of any particular chemical group.

A second reason that confusion may arise about the reasonable number of times a product can be used in a row or in a season comes from product labels themselves. Many product labels carry conflicting statements about the acceptable number of uses. For instance, some labels suggest weekly applications are necessary for most of the season at one point on the label while stating that no more than two (or even one) applications should be applied in a row! This confusing information persists even though most new labels carry valuable advice in a resistance management section.

To lessen the chance of developing resistance to any pesticides, use the information on chemical families in Publication 360 when choosing appropriate products and critically evaluate any information that appears to imply that repeated use of one product or products with the same mode of action is acceptable. All products have a "group" number on the front of the label; always rotate products with different group numbers. There are very few insects or diseases that are unlikely to develop resistance if exposed exclusively and repeatedly to one control product. There are a few exceptions to this rule, but assuming resistance is a potential problem is the best way to be aware of it and avoid strategies that may promote it. For the most part, the best strategy is to follow known resistance management strategies to keep as many products and families of chemicals efficacious and in our marketplace for as long as possible.

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