Leaf curl is a plant disease caused by a fungus (Taphrina deformans,[1] genus Taphrina) or virus (especially genus Begomovirus of the family Geminiviridae) and characterized by curling of leaves. Although all leaf curl occurring in different plants (usually only citrus plants) is relatively the same,[clarification needed] one of the most notable types is peach leaf curl, caused by the fungus Taphrina deformans. Taphrina deformans belongs to the subclass Protoascomycetes. Leaf curl is found in America, Europe, Asia, Africa, Australia, and New Zealand.[2] It was first introduced in America in 1852 and has now spread all over the country.
'Leaf curl has caused a lot of damage over the years. It makes the amount of leaves and fruit produced by trees significantly lower. It has caused many trees to die. The disease also costs the United States 2.5 to 3 million dollars annually. However, it is believed that the effects of the disease are preventable. If the correct sprays (usually lime sulfur) are used correctly, a 98% yield of healthy fruit is obtained. If trees are not sprayed with specific chemicals early enough, and if the tree is treated after the host tissue is infected, treatment is ineffective.
Symptoms
Leaf Curl has characteristics that are very differentiating and easily noticeable. Diseased leaves can usually be picked out early after leaving the bud due to their reddish color and arched shape. As the leaves develop, they appear distorted and fold their tips backwards. Diseased leaves are usually thicker and softer than the normal, unharmed leaves. The colors of the leaves are also unique. Instead of the normal green spring leaves, the colors turn yellow, followed by purple, until finally a whitish bloom covers each leaf. Twigs may show signs of sickness, such as being black in color and swollen. Fruit can be affected, showing a reddish color. Infected leaves fall early. The tree may produce a second crop of leaves that is never diseased, because the fungus cannot survive at the higher temperatures in late spring and early summer. The fungus prefers the high humidity in the early spring because it permits spores to germinate. If the tree is infected for consecutive years death may occur.
Causes of Leaf Curl
When a virus is the cause of leaf curl in a plant, usually an insect will carry the virus to the plant. This is such the case in sweet potato leaf curl, carried by the sweet potato whitefly . The whitefly also has been found to transport the leaf curl virus to tomatoes. This has been seen since 1997 in Florida, and has since spread to other southern states. The virus was recently seen in South Carolina, and with the abundance of whiteflies in the state, has the potential to spread out of the southeastern United States. The Geminiviridae virus has also been seen to spread by the transport of nursery soils. The virus appears to stay in soil, and when a new crop is planted, affects the new host.
When a whitefly (and some other carriers of viruses) eats leaves on an infected plant, the virus enters its saliva and is spread when the fly eats at a healthy tree. This explains the rapid amount of trees infected in a given area.
When a fungus causes the sickness, a different process occurs. The fungus (Taphrina deformans) causes the whitish bloom that covers each leaf as infection progresses. This color is made of asci that break through the cuticle of the leaf. One asci consists of eight ascospores that create conidia, which are ejected in early summer and moved by methods of rain and wind. It is believed that this fungus survives the winter by staying on the surface of the new host plant, such as on bark or buds. In the spring, new buds are affected by the conidia as the leaves come out of the buds. The fungus produces the ascospores on the surface of the already infected leaves. Often the disease does not occur every year due to the variances in temperature and weather from year to year. The fungus has higher infection rates following cooler winters, has optimal temperatures for infection, and requires rain.
Control of the Disease
Although other methods are under investigation, spraying the leaves with fungicides is the most common and efficient control of the disease. It is important for spraying to occur well before budding. Different areas in climates depict the type of fungicide used and how often, since experiments have shown that the fungi are temperature dependent. Some fungicides commonly used include, among other things, copper based mixtures and lime sulfurs.
If a plant appears to have symptoms of leaf curl, precautions can be taken to maximize the crops for that spring. The trees are often treated with nitrogen and excess water to minimize stress on the tree. So the tree can focus on producing good peaches, thinning the tree out might also help. It would be advantageous to take away the infected leaves and fruit after they fall to the ground so that next years tree might not get infected. Fungicide might also be used before winter on the tree .
Future of the Disease
Scientists are on a mission to stop leaf curl infections. There have been observations that some crops have certain characteristics that make them not susceptible to the virus or fungus, even ones of the same species that are infected in other parts of the world. Experiments are being conducted and hopefully less susceptible crops will be able to be produced and stop the infection.
Due to millions of dollars of debt in the U.S. Agriculture Department in 1992, universities, state departments, and the USDA adopted a national plan of research and action against the silverleaf whitefly. Since then, crop damage has been somewhat reduced, but the researchers are still hard at work and the degree of debt is steadily increasing. The harm to the agriculture does not rest solely on the leaf curl disease when regarding the whitefly. It even carries the viruses to cause immature ripening in tomatoes, lettuce chlorosis, processing problems in cotton, and blotching in squash plants. In fact, every tomato field in Florida has been infected with a geminivirus.
The researchers are just trying new things and hoping that they move in the right direction. For example, breeding the lines of crops that are less affected by whiteflies would be more beneficial, but the issue of time is a problem. Another option is the introduction of wasps into environments. Researchers found that female exotic wasps deposit eggs under whitefly larva, which emerge from the eggs and destroy the larva. This is still being explored because introducing exotic species into new environments can have severe implications to the new environment and its members .
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