Probably the easiest disease of peach and nectarine to identify and one of the most puzzling is peach leaf curl. This disease is easy to identify even with your eyes closed because infected leaves and fruit have the thickened warty texture similar to a rubber Halloween mask. The brightly colored and distorted leaves on infected trees are strikingly obvious to neighbors driving by your farm, or in my case some years ago, my research plots next to a busy highway. At the same time, it is puzzling because a peach grower in the Midwest can go years without spraying for this disease and see little or no symptoms, and then suddenly get clobbered.
Peach leaf curl is caused by the fungus Taphrina deformans, which infects peach, nectarine, and almond trees. T. deformans was first introduced to America in the 1800s, and can be found in nearly every peach growing area of the world.
Leaf curl is distinctive and easily noticeable, and the severity of the signs depends on how early infection has occurred. Diseased leaves can usually be identified soon after they emerge from the bud, due to their red color and twisted shape. As the leaves develop, they become increasingly distorted, and ultimately thick and rubbery compared to normal leaves. Infected leaves can be white, red, and purple and to some eyes actually pretty, even though many leaves will eventually shrivel up and drop to the ground.
The Element of Surprise
The puzzling sporadic nature of this disease is due to a rather precise combination of temperature and moisture needed for infection. Peach leaf curl infections occur in the spring when wetting periods over 10 hours occur under cool conditions (46°F to 53°F), with rainfall greater than 0.5 inch. This is an effective infection temperature range of approximately seven degrees.
This is remarkable when you consider cherry leaf spot and apple scab can cause infections over a much wider temperature range — more than 30 degrees. These rather specific conditions may not occur every spring, which explains why some years peach leaf curl does not show up on unprotected trees. Leaf and fruit buds are vulnerable for several weeks after swelling, as are new leaves produced on terminal growth on into the summer. However, episodes of cool temperatures and extended wetting generally become uncommon as the spring progresses.
The occasional infections that show up on new terminal growth in midsummer are due to unusually cool wet weather. The time between infection and first symptoms ranges from nine days under warm conditions to nearly a month in cool springs.
Fungicides are Crucial
Peach resistance to leaf curl is determined by numerous genes and so varieties display a range of resistance. In general, varieties with large conspicuous showy bloom tend to be more susceptible to peach leaf curl than non-showy varieties, but there are plenty of exceptions. Genetics plays the primary role; however, showy bloom varieties generally develop sooner in the spring and presumably are exposed to more episodes of cool wet weather.
Climate plays a large role in disease severity. For example, a variety such as ‘Redhaven’ looks relatively tolerant in the Midwest but is very susceptible in the relatively cool and damp Pacific Northwest. Most commercial peach and nectarine varieties are susceptible to peach leaf curl.
A few peach varieties are available that have a relatively high resistance to leaf curl including ‘Frost,’ ‘Indian Free,’ ‘Muir,’ ‘Autumn Rose,’ ‘Avalon Pride,’ ‘Autumn Rose,’ ‘Charlotte,’ ‘Early Crawford,’ and ‘Oregon Curl Free,’ but these are generally not of sufficient quality for commercial production. Even varieties with relatively good curl resistance are somewhat susceptible during the first few years of growth.
In the summer months, Taphrina deformans exists as a white yeast-like growth on leaves and shoots, causing no apparent harm. However, this ensures that plenty of inocula is available for infections in the following season. Thus, unprotected peach orchards in their second leaf can show peach leaf curl even though no leaf symptoms were seen in the year of planting. Summer fungicide applications for brown rot control may provide some suppression of this yeast phase which explains why in low-crop years or in young orchards when brown rot sprays are reduced or omitted, peach leaf curl problems can be worse the following season.
Fall application of fungicide for leaf curl is preferred over spring since poor spraying conditions in the spring may prevent an application in time to prevent infection. In cool springs infections can continue after bud swell if weather conditions are in the conducive range. Under such conditions, a “better late than never” fungicide application after the start of bud swell can help to prevent additional infections. It is customary in cool wet peach growing climates to apply both a fall and spring application for leaf curl.