Symptoms
Peach leaf curl, a disease caused by the fungus Taphrina deformans, first appears
as reddish areas on developing leaves in the early spring. These reddish areas
soon become thickened and puckered, causing the leaf to curl. The thickened areas
turn pale yellow then grayish white, signaling spore production on the distorted
leaf tissues.
Affected leaves turn brown or yellow and can either stay on the tree or fall off,
causing decreased fruit production and tree growth. Due to this defoliation, fruits
may become exposed, and therefore, prone to sunburn injury.
The fungus will also infect young, green twigs and shoots. In the case of severe
and recurrent instances, shoots quickly become stunted, thickened, distorted or
die, which severely reduces the vigor of the tree. The appearance of reddish, wrinkled
or distorted areas on the fruit surface, though rare, can turn fruit corky and
cracked.
Life Cycle
The spores overwinter in bark crevices and around the buds on tree twigs. The disease
cycle starts when the overwintering yeast cells wash onto the swelled buds and
emerging leaves in the early spring. The saprophytic yeast phase of the fungus
switches to the parasitic mycelial phase by producing a short hypha that penetrates
the protective layer. The fungus continues to invade the host tissue between the
outer cells until it reaches the parenchyma cells below.
Long periods of cool (50°F to 70°F), wet (>95% humidity) weather facilitate
infection. If warm temperatures follow bud swell and
leaf development is rapid, infections are rarely established.
Leaf symptoms begin to show about two weeks after leaves emerge from the buds. The
fungus grows around leaf cells and stimulates them to divide and grow larger than
usual, causing swelling and puckering. Distorted cells accumulate red plant pigments.
Fungus cells break through the surface of the leaves and produce sexual fruiting
bodies, which in turn produce sexual spores called ascospores. These ascospores
give the leaf a powdery, grayish white appearance.
The yeast phase begins with budding ascospores discharged from curled leaves onto
peach twigs and bud surfaces. The fungus survives on the tree’s surface as ascospores
and bud conidia during the hot, dry summer. When the weather turns wet and cooler
in the fall, ascospores germinate once again to produce more bud conidia.
Management
Resistant Variety
The best option for backyard growers is to use resistant peach and nectarine cultivars.
Peach cultivars with tolerance to leaf curl are ‘Frost,’ ‘Avalon’ and ‘Mary Jane.’
However, these cultivars can still be susceptible during the first few years after
planting and should be treated with preventative products.
Cultural Control
If you are observing leaf curl, wait until the end of the season to manage the disease.
Even though some people prune infected areas, this has not been proven to be effective.
Pruning in the fall prior to fungicide application can reduce the number of spores
overwintering on the tree and reduce the amount of fungicide needed.
If leaf curl is severe during a specific season, maintain tree vigor by thinning
more fruit than normal, irrigating to reduce drought stress and applying extra
nitrogen fertilizer.
Fungicides
With high disease pressure and notably wet winters, an application of fungicide in
the fall is appropriate when 90 percent of the leaves have fallen despite the early
spring application.
Dormant fungicide application should always be timed with temperature and bud swelling.
During this time, apply chlorothalonil, Ziram, liquid lime sulfur or copper.
During 2017, the Mid-Atlantic region experienced an unusually warm February that
influenced bud swelling and infection earlier than normal. Growers who applied
fungicide at that time were able to control the disease. This highlights the need
for tracking developmental stages for disease management.