IDENTIFICATION AND DAMAGE
Leaf curl first appears in spring as reddish areas on developing leaves. These areas become thickened and puckered, causing leaves to curl and severely distort. The thickened areas turn yellowish gray and velvety as spores are produced on the surface by the leaf curl fungus. Affected leaves later turn yellow or brown and either remain on the tree or fall off; they are replaced by a second set of leaves that develop more normally unless wet weather continues. The loss of leaves and the production of a second set decrease tree growth and fruit production. Defoliation in spring may expose branches to sunburn injury.
The leaf curl pathogen also infects twigs and shoots. Affected shoots become thickened, stunted, distorted, and often die. Only rarely do reddish, wrinkled areas develop on fruit surfaces; later in the season these infected areas become corky and tend to crack. If leaf curl infection builds up and is left uncontrolled for several years, the tree may decline and need to be removed.
LIFE CYCLE
It takes about 2 weeks after leaves emerge from buds before leaf symptoms appear. The fungus grows between leaf cells and stimulates them to divide and grow larger than normal, causing swelling and distortion of the leaf. Red plant pigments accumulate in the distorted cells. Cells of the fungus break through the surface of distorted leaves and produce elongate, saclike structures called asci.
Inside the asci are spores known as ascospores, which give the leaf a powdery or feltlike appearance. The ascospores are released into the air, carried to new tissues, and bud (divide) to form bud-conidia. The fungus survives the hot, dry summer as bud-conidia and ascospores on the tree's surfaces. When the weather turns cool and wet in fall, the ascospores germinate to produce bud-conidia. The new bud-conidia and the oversummering bud-conidia continue to increase in number by budding. Eventually a film of bud-conidia is formed on the tree's surface. In spring, the bud-conidia are moved by splashing water to newly developing leaves where infection occurs. Cool weather prolongs the period of disease development by slowing leaf growth. Development of leaf curl ceases when young tissue is no longer developing or when temperatures are above 79 to 87°F (26 to 30.5°C).
MANAGEMENT
To prevent peach leaf curl, treat peach and nectarine trees every year after leaves have fallen (late November). Copper-based fungicides including Bordeaux mixture (a slurry made of hydrated lime and copper sulfate) or synthetic fungicides can be used. However, to be effective, copper-containing compounds must have at least 50% copper; those containing less do not adequately control leaf curl despite advertising claims. If timed properly, a single fall/winter spray will normally prevent losses.
In areas of high spring rainfall or when spring rainfall is abundant, it may be advisable to apply a second copper spray or a lime sulfur treatment in spring, preferably before buds begin to swell, but definitely before budbreak (when green color is first visible).
Although symptoms of leaf curl are seen primarily in spring as new leaves develop, there is little you can do to control the disease at this time. Some people remove diseased leaves or prune infected shoots, but this does not improve control. (Pruning in fall, however, can reduce the spore inoculum overwintering on the tree.) Normally, diseased leaves fall off within a few weeks and are replaced by new healthy leaves, unless it is rainy. If leaf curl symptoms occurred on your trees in spring, be sure to treat the following fall, around late November, to prevent more serious losses the next year.