Leaf curl is a disease affecting the genus Prunus—most notably peach, nectarine, and almond—caused by the fungus Taphrina deformans. Infected trees exhibit leaves which are puckered and distorted, before ultimately curling downwards and becoming discolored. Larger trees may experience severe defoliation, while seedlings can die of the disease.
Leaf curl is most readily identified by the leaf deformation and discoloration evident in infected trees.
The first symptoms of leaf curl typically arise in spring, with affected leaves developing reddish patches. These areas thicken and pucker, eventually causing leaves to curl and distort. The early stages of the disease can look similar to the consequences of aphid infection. Thus, it is necessary to check for evidence of aphids before making a diagnose of leaf curl.
But as the disease progresses, symptoms become more obvious and distinctive. Leaves will experience chlorosis, while the initially puckered patches can become almost tumorous looking, with extremely severe infections turning red and purple. These growths take on a waxy, thick texture that has been described as feeling like a rubber Halloween mask. The fungal pathogen responsible for the disease produces spores on leaf surfaces, causing them to take on a dusty, velvety appearance as they develop. These spores can be blown onto the leaves of other branches or nearby trees, spreading the disease.
Eventually the infected leaves will turn yellow or brown and die, and may either drop off or continue to cling to branches. These leaves will typically be replaced by a second set of leaves which grow in and develop normally. However, the initial loss of leaves typically results in reduced overall growth, as well as a reduction in fruit development. In particularly sunny weather, branches exposed by defoliation can experience sunburn.
Unfortunately, the reach of the disease is not limited to tree leaves. Young shoots and twigs, when infected by the disease, will become distorted and stunted, and often die. Occasionally, even fruit will exhibit symptoms, producing thick warty, scabby, or corky growths on fruit surfaces. The impact of infection of the fruit is not merely visual, as these growths often crack, exposing the fruit to secondary infection by pests and opportunistic pathogens.
The most common consequence of leaf curl is a potentially severe reduction in yield. Seedlings infected by the disease may wither and die. Even fully-grown trees can eventually succumb to the disease if left untreated for too long.
Caused by the fungal pathogen Taphrina deformans, the life cycle of leaf curl usually plays out in the early spring, when conditions are wet.
The spores of T. deformans usually overwinter in sheltered areas, such as piles of infected leaves, as well as twigs, bud scales, and bark crevices on host trees. When conditions are wet for more than 12 hours, and temperatures warm to anywhere between 48° to 87° Fahrenheit (the disease’s ideal temperature is about 68°F), spores begin to germinate.
Spore germination tends to coincide with the first emergence of new leaves, which are particularly vulnerable to infection. The fungus easily penetrates into this new plant tissue, and disperses from there.
The fungus hijacks the ‘machinery’ of leaf cells, causing them to reproduce rapidly and uncontrollably, resulting in the characteristic swelling, puckering and curling of leaf tissue. The fungus reproduces within these growths and then cut through the leaf surface, creating structures called “asci,” which grow and release sexual spore. It’s the growth of these reproductive structures which gives infected leaves the gray, powdery look they develop in the late stages of the disease.
When temperatures become too warm or conditions are too dry, growth and reproduction are suspended. In the fall, as temperatures drop, T. deformans resumes its efforts.
In years where sustained warm temperatures occur early in the spring, the disease often fails to establish much of a foothold, and few if any symptoms of leaf curl are seen. But in years where there are long, uninterrupted periods of cool, wet weather, the disease can progress and spread quite rapidly.
As with many water molds, treatment of leaf curl largely relies upon proper sanitation, eliminating long periods of wet conditions, and the application of fungicides and proper nutrition.
Wet conditions are crucial to the growth and spread of T. deformans. It often spreads by water dislocating spores from leaf surfaces to adjacent trees. Thus, much of the treatment regimen for leaf curl depends upon implementing irrigation systems which deprive the fungus of the conditions it craves. Where possible, use ground-based types of irrigation—avoid the use of sprinklers or misters, as water drips on and through leaves, carrying with it the spores from infected leaves.
With more mild and limited infections, some growers prefer to trim off and destroy infected branches But there is little evidence of this being effective, and when infections are more entrenched, this is rarely a feasible approach. This is especially the case given that infected tissue often drops off and is replaced by healthy new shoots and twigs in a matter of weeks or months.
Usually, it’s advisable to hold off on treatment efforts until the end of the growing season. In the fall, after most leaves have dropped, apply a suitable protective fungicide. A variety of copper-based fungicides and solutions are popular choices. Chlorothalonil is also effective in protecting against leaf curl. Usually these fungicides are to be applied heavily, until tree surfaces are saturated and dripping. In the meantime, piles of dropped infected leaves and twigs should be removed and destroyed.
Because invasions of T. deformans are limited to leaves, shoots, and sometimes fruit, the disease can essentially be purged during the winter season when this tissue drops away (hence why the pathogen overwinters on plant surfaces and crevices).
Proper balanced nutrition is also key to successfully managing leaf curl. Excessive applications of nitrogen produce the rapid growth of leaves and shoots with weak structural integrity, due to supplies of calcium being overtaxed. Applications of calcium and boron, as well as products such as Fusion 360 Soil and Iota 0-0-1 which make soil nutrients more bioavailable. This ensures that tree growth occurs at a steady, sustainable rate, and that plant tissues can more readily mount a defense against T. deformans and other pathogens.