Brown rot (caused by Monilinia species) is a well-known disease of peaches around the world. It tends to be more problematic in areas with high spring/summer precipitation and high humidity. Wind, rain, and insects help to distribute spores to cause flower blossom or fruit infections during the growing season. Spores may also form latent fruit infections that can cause postharvest rots. If infected fruits are left on the tree and not harvested, they may remain there and become mummified. In some cases, twig cankers may occur and wound gumming is produced.
Perhaps you had a dry spring/summer and experienced minimal losses due to this disease. Or, maybe you were faced with more rain than usual, the difficulty of making timely fungicide applications, reduced efficacy of materials you have historically used, and the disease simply “got away on you.” I know of cases in South Carolina where either a spring freeze or early season hailstorm resulted in crop damage that caused some growers to “walk away” so-to speak from problem blocks. In the latter case, we recommended a minimal spray program to help ensure that brown rot epidemics would not result but not everyone elected to do this because of cost.
If you had a brown rot problem last year, you have an opportunity this winter to take proactive steps to reduce the potential for a problem. According to my friend and colleague, Dr. Guido Schnabel (Extension fruit pathologist, Clemson University), orchard sanitation efforts this winter will go a long way to reduce inoculum and disease pressure this coming season. The first step is to deal with mummies. Because they are a source of inoculum for spring infections, these should be carried out of the orchard and disposed of.
At locations where there is a sod row middle and the orchard floor is vegetated, mummies that fall to the orchard floor may have sufficient spring moisture to become active spore producers. In locations where the orchard floor is kept bare by herbicides, mummies may dry out sufficiently so as not to be a problem. The second step is to deal with limbs that have cankers. The best practice is to remove them from the orchard and burn them. However, putting the prunings in the row middle and driving over them with a flail mower (to remove bark) can facilitate a more rapid decomposition and reduce inoculum level considerably. This is a useful practice for limbs that may also be infected by constriction canker (Phomopsis) or fungal gummosis (Botryosphaeria).
Research by Schnabel and colleagues has demonstrated resistance of brown rot fungi to various fungicide classes. Selection for resistance can occur when fungicides with a single site of action are used repeatedly for years. Whatever the reason, the number of tools available and the efficacy of those tools ultimately depend on their wise use by the grower. That is why thorough sanitation (now) is a wise step to take. Careful choice of fungicides during the growing season and rotation of products with different modes of action can help limit resistance development. However, if you did have product failure last year, you may want to visit Dr. Schnabel’s website (www.peachdoc.com) to learn more, especially about his brown rot resistance management program.
This column by Dr. Desmond R. Layne, “Now's Your Chance!” appeared in the January 2010 issue of The American Fruit Grower magazine on pages 52-53.