There are several fungicides labeled for control of pecan scab disease. A list of recommended fungicides, all of which are effective, can be found in the LSU AgCenter Fungicide Application Recommendations for Pecan Disease Control. Because scab disease can occur any time during the growing season it is necessary to make several preventative fungicide applications from spring through summer. Fungicide applications should be made before infections take place in order to prevent disease buildup. Applications made after infections have occurred will help reduce the occurrence of further infections but will not eliminate disease that is already present.
Fungicides are grouped by their mode of activity against fungi. To try to reduce the development of strains of the scab pathogen that are resistance to a particular fungicide group, it is recommended that a single fungicide not be used as the only fungicide in a seasonal application program. Once a pathogen becomes resistant to a specific fungicide it will be resistant to all of the products in that fungicide group. Similarly using different products from the same fungicide group will not reduce the chances of developing resistance to the entire group. Although there is no guarantee that any method of use will delay resistance development, it is recommended that each fungicide application be made with a product from a different group than was used in the previous application. Hypothetically, the chances of resistance selection are reduced when the number of fungicide groups used in a group rotation schedule increases. Another approach is to use products that contain a combination of two groups of fungicides.
Certain fungicide groups have a greater tendency for pathogens to develop resistance to them. It is probably best not to use these groups when there is a lot of scab disease already present on the trees, because higher pathogen populations increase the odds for resistance selection. The fungicide groups with the highest tendency for resistance selection should be used in the first half of the growing season which is when scab disease intensity is usually lowest. The fungicide groups with recommended products labeled for pecan that have the highest tendency for resistance include Group 3 (DMIs) and Group 11 (Strobilurins). The group with the lowest tendency appears to be Group 30, with Group M in the middle. It should be noted that resistance to all of these groups has been reported with pathogens on other crops. Thus, use a variety of fungicide groups for pecan scab disease control and make applications in a preventative manner before disease builds up to levels that will harm the crop and make control difficult.
For the list of the currently recommended fungicides for scab control, please see the above mentioned Fungicide Application Recommendations for Pecan Disease Control on the Plant Pathology page of the Pecan Station website.
Question answered by Dr. Randy Sanderlin, Pecan Research-Extension Station plant pathologist.