April 26, 2014
IF YOU ARE growing peaches, nectarines or almonds in Marin County, chances are you will become familiar with their nemesis, Taphrina deformans. This fungi causes peach leaf curl.
The name comes from the symptoms seen on the young spring leaves whose cells have been stimulated by the fungi to divide, enlarge and accumulate red pigments until they are swollen and puckered.
This disease favors cool, wet weather. If the fungi are around and conditions are favorable, it can substantially reduce fruit yield. Not only are leaves affected, but green twigs and shoots also can succumb to the fungi and become stunted, deformed and often die.
When warm weather hits, somewhere at or above 79 to 87 degrees, the disease stops. That doesn't mean the fungi went away, though.
Marin's maritime influence and abundance of untreated and infected home fruit trees create the perfect disease triangle of pathogen, susceptible host and conducive environment, especially if we have had some late cooler wet rains.
The best management strategy is to plant resistant varieties such as "Frost," "Muir," "Q-18" and "Indian Free" and varieties derived from the "Redhaven" peach. "Kreibich" is a resistant nectarine. Even with resistant varieties, spraying may be necessary when the trees are young.
Removing infected growth before ascospores — microscopic spores that develop during the winter on infected dead, fallen leaves — emerge helps.
For most home gardeners, store-bought copper fungicide products are the easiest to use. In Marin's climate they require at least three applications from Thanksgiving to Valentine's Day to be effective. Add a little horticultural oil or a sticker adjuvant to increase efficacy.
The right time to start a copper fungicide treatment program is in the late fall or winter. Prune peaches hard and follow with good sanitation and a fungicide application to the point of runoff. Keep in mind that copper can kill important soil fungi. Soak the tree, but try not to soak the ground.
A spring application is advised at bud swell, about two weeks before bloom, but is not effective after bloom. Never spray when trees are in bloom or when bees are present.