Leaf curl
Encyclopedia
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
disease
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...
caused by a fungus
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
(Taphrina deformans
Taphrina deformans
Taphrina deformans is a fungus and plant pathogen, one of the causal agents of peach leaf curl, the other being a virus.-Life Cycle:T.deformans infects species of the genus Prunus including P. amygdalus and P. persica . Infected leaves develop a whitish bloom as infection progresses...
, genus Taphrina
Taphrina
Taphrina is a fungal genus within the Ascomycota that causes leaf and catkin curl diseases and witch's brooms of certain flowering plants. One of the more commonly observed species causes peach leaf curl...
) or virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
(especially genus Begomovirus
Begomovirus
The genus Begomovirus contains more than 200 species and belongs to the taxonomic family Geminiviridae. They are plant viruses that as a group have a very wide host range, infecting dicotyledonous plants...
of the family Geminiviridae
Geminiviridae
Geminiviruses are plant viruses which have single-stranded circular DNA genomes encoding genes that diverge in both directions from a virion strand origin of replication . According to the Baltimore classification they are considered class II viruses...
) and characterized by curling of leaves. Although all leaf curl occurring in different plants (usually only citrus plants) is relatively the same, one of the most notable types is peach
Peach
The peach tree is a deciduous tree growing to tall and 6 in. in diameter, belonging to the subfamily Prunoideae of the family Rosaceae. It bears an edible juicy fruit called a peach...
leaf curl, caused by the fungus Taphrina deformans
Taphrina deformans
Taphrina deformans is a fungus and plant pathogen, one of the causal agents of peach leaf curl, the other being a virus.-Life Cycle:T.deformans infects species of the genus Prunus including P. amygdalus and P. persica . Infected leaves develop a whitish bloom as infection progresses...
. Taphrina deformans belongs to the subclass Protoascomycetes. Leaf curl is found in America
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
, Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, and New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
. It was first introduced in America
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
in 1852 and has now spread all over the country.
Problems with Leaf Curl
'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 StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
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 virusVirus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
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
Sweet potato
The sweet potato is a dicotyledonous plant that belongs to the family Convolvulaceae. Its large, starchy, sweet-tasting, tuberous roots are an important root vegetable. The young leaves and shoots are sometimes eaten as greens. Of the approximately 50 genera and more than 1,000 species of...
leaf curl, carried by the sweet potato whitefly
Whitefly
The whiteflies, comprising only the family Aleyrodidae, are small hemipterans. More than 1550 species have been described. Whiteflies typically feed on the underside of plant leaves.-Agricultural threat:...
. 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
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
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
Ascus
An ascus is the sexual spore-bearing cell produced in ascomycete fungi. On average, asci normally contain eight ascospores, produced by a meiotic cell division followed, in most species, by a mitotic cell division. However, asci in some genera or species can number one , two, four, or multiples...
that break through the cuticle
Cuticle
A cuticle , or cuticula, is a term used for any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...
of the leaf. One asci consists of eight ascospore
Ascospore
An ascospore is a spore contained in an ascus or that was produced inside an ascus. This kind of spore is specific to fungi classified as ascomycetes ....
s that create conidia
Conidium
Conidia, sometimes termed conidiospores, are asexual, non-motile spores of a fungus and are named after the greek word for dust, konia. They are also called mitospores due to the way they are generated through the cellular process of mitosis...
, 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.
Other common diseases caused by ascomycetes include: the Dutch Elm Disease, Chestnut Blight, Leaf Spot, and Ergot
Ergot
Ergot or ergot fungi refers to a group of fungi of the genus Claviceps. The most prominent member of this group is Claviceps purpurea. This fungus grows on rye and related plants, and produces alkaloids that can cause ergotism in humans and other mammals who consume grains contaminated with its...
.
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
Fungicide
Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...
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
Silverleaf whitefly
The silverleaf whitefly is one of several whiteflies that are currently important agricultural pests. The Silverleaf Whitefly is classified in the family Aleyrodidae, and is included in the large sub-order of insects, Heteroptera...
. 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
Lettuce
Lettuce is a temperate annual or biennial plant of the daisy family Asteraceae. It is most often grown as a leaf vegetable. It is eaten either raw, notably in salads, sandwiches, hamburgers, tacos, and many other dishes, or cooked, as in Chinese cuisine in which the stem becomes just as important...
chlorosis, processing problems in cotton
Cotton
Cotton is a soft, fluffy staple fiber that grows in a boll, or protective capsule, around the seeds of cotton plants of the genus Gossypium. The fiber is almost pure cellulose. The botanical purpose of cotton fiber is to aid in seed dispersal....
, 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 .