Leaf curl

Peach leaf curl is a plant disease characterized by distortion and coloration of leaves and is caused by the fungus Taphrina deformans,[1] which infects peach, nectarine, and almond trees. T. deformans is found in the United States, Europe, Asia, Africa, Australia, and New Zealand.[2] Peach leaf curl reduces the amount of leaves and fruit produced by peach and nectarine trees.[3]

This article is about fungal leaf curl in peach, nectarine and almond trees. For viral leaf curl in tomato and other Solanaceae, see Begomovirus. For viral leaf curl in beets and turnips, see Geminiviridae. For viral leaf roll in tomato and other Solanaceae, see Potato leafroll virus.

Leaf curl
Peach leaf curl
Causal agents	Taphrina fungus
Hosts	peach, almond
Distribution	America, Europe, Asia, Africa, Australia, New Zealand

Affected species

Peach leaf curl can mainly affect peach, nectarine, and to a lesser extent almond trees. In an isolated case in Hungary in 2011, peach leaf curl was also identified in apricot trees.[4]

Symptoms

Peach leaf curl is a distinctive and easily noticeable fungal disease, and the severity of the symptoms 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. The color of the leaves changes from the normal green to red and purple, until a whitish bloom covers each leaf. Finally, the dead leaf may dry and turn black before it is cast off. Changes in the bark are less noticeable, if at all. Fruit may fail to develop from diseased blossoms. Any fruit that does develop from a diseased tree is usually normal, but sometimes may also be affected, showing a reddish color.[3] Infected leaves fall early. The tree usually produces a second flush of leaves that is rarely diseased, except in an unseasonably cool and wet spring, because the fungus is not infectious at the normally higher temperatures in late spring and early summer.

Cause of peach leaf curl

The fungus T. deformans causes deformed young leaves, red blisters, and ultimately the whitish bloom that covers the leaf as the infection progresses. This white color is made of asci that break through the cuticle of the leaf. One ascus consists of eight ascospores that create conidia, which are ejected in early summer and spread by rain and wind. The fungus survives the winter on the surface of the host plant, such as on bark or buds.[2] In late winter or early spring, rainwater washes spores into the buds as they burst. Once this happens, no treatment is effective. In the spring, about two weeks after blossom, new leaves emerging from the infected buds are infected by the conidia. The disease may not occur every year due to variation in temperature and rainfall. Specifically, for successful infection, the fungus requires wet winters, where rain (not fog or dew) wets the tree for more than 12.5 hours at temperatures below 16 °C (61 °F).[5] The fungus cannot grow at temperatures below 9 °C (48 °F).[6]

Control of the disease

Various methods are applied.

• The most effective method[7] is to plant peach trees against a house wall under an overhanging roof, possibly covered by a mat during the winter, to keep winter rain from the buds before they burst (and incidentally to delay blossoming until spring frosts are over), until the temperature exceeds 16 °C (61 °F) in the spring, deactivating the fungus.
• Commercially, spraying the leaves with fungicides is the most common control method. The toxicity of these fungicides means they are not legally available to noncommercial growers in some countries. Spraying should be done in the winter well before budding. If trees are not sprayed early enough, treatment is ineffective. Copper-based mixtures (such as Bordeaux mixture) and lime sulphur are two fungicides commonly used.[2][8]
• Peach cultivars can be planted which show some resistance to peach leaf curl, or at least regenerate rapidly, such as the white-fleshed Peach 'Benedicte'.[9] For nectarines, the cultivar Kreibich is reported to have some resistance.[6]

If a tree has peach leaf curl in a particular year, the disease will inexorably take its course, but measures can be taken to sustain the tree or maximize crop yield: protecting the tree from further rain at temperatures below 16 °C (61 °F), providing nitrogen and excess water to minimize stress on the tree; applying greasebands around the trunk to protect from insect infestation; and thinning the fruit. It is unclear whether removal of infected leaves from the tree is beneficial.[6] Removing the infected leaves and fruit after they fall to the ground is sometimes also suggested but superfluous if, in the following winter, fungicides or rain protection are applied.

History

Peach leaf curl was first introduced in America in 1852 and has now spread all over the country. By 1947, the disease was costing the United States $2.5 to 3.0 million annually.[3]

References

1. Peach leaf curl, Taphrina deformans Archived 2007-08-29 at the Wayback Machine at West Virginia University
2. Gray, William D.. The Relation of Fungi to Human Affairs. New York: Henry Holt and Company, Inc., 1959. Print.
3. Chester, Kenneth Starr. Nature and Prevention of Plant Diseases. 2nd. Philadelphia: The Blakiston Company, 1947. Print.
4. Kovics, Gyorgi Janos. An unusual occurrence of Taphrina deformans on apricot trees in Hungary. University of Debrecen, Oct 2011.
5. Rossi, V. et al. (2006). Influence of Environmental Conditions on Infection of Peach Shoots by Taphrina deformans. Phytopathology 96:155-163. PMID 18943918
6. University of California (2012) Integrated Pest Management Program - Peach leaf Curl. <http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7426.html >
7. Royal Horticultural Society. Peach Leaf Curl.<http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=232 Archived 2013-01-11 at the Wayback Machine>
8. Pscheidt, Jay W. (Jan 1, 2009). "Peach--Leaf Curl". An Online Guide to Plant Disease Control. Oregon State University. Archived from the original on June 12, 2010..
9. LWG [Bavarian State Institute for Viticulture and Horticulture]. Field trials on peach and nectarine trees regarding peach leaf curl.<"Archived copy". Archived from the original on 2013-06-20. Retrieved 2013-01-07.CS1 maint: archived copy as title (link)>

External links

• Index Fungorum
• USDA ARS Fungal Database
• Royal Horticultural Society - Peach Leaf Curl
• Treatment of peach leaf curl