Phytoplasmas are obligate bacterial parasites of plant phloem tissue and of the insect vectors that are involved in their plant-to-plant transmission. Phytoplasmas were discovered in 1967 by Japanese scientists who termed them mycoplasma-like organisms. Since their discovery, phytoplasmas have resisted all attempts at in vitro culture in any cell-free medium; routine cultivation in an artificial medium thus remains a major challenge. Although phytoplasmas have recently been reported to be grown in a specific artificial medium, experimental repetition has yet to be reported. Phytoplasmas are characterized by the lack of a cell wall, a pleiomorphic or filamentous shape, a diameter normally less than 1 μm, and a very small genome.
Citrus greening disease is a disease of citrus caused by a vector-transmitted pathogen. The causative agents are motile bacteria, Candidatus Liberibacter spp. The disease is vectored and transmitted by the Asian citrus psyllid, Diaphorina citri, and the African citrus psyllid, Trioza erytreae, also known as the two-spotted citrus psyllid. It has also been shown to be graft-transmissible. Three different types of HLB are currently known: The heat-tolerant Asian form, and the heat-sensitive African and American forms. The disease was first described in 1929 and first reported in China in 1943. The African variation was first reported in 1947 in South Africa, where it is still widespread. Eventually, it affected the United States, reaching Florida in 2005. Within three years, it had spread to the majority of citrus farms. The rapid increase in this disease has threatened the citrus industry not only in Florida, but the entire US. As of 2009, 33 countries have reported HLB infection in their citrus crop.
Lethal yellowing (LY) is a phytoplasma disease that attacks many species of palms, including some commercially important species such as the coconut and date palm. In the Caribbean it is spread by the planthopper Haplaxius crudus which is native to Florida, parts of the Caribbean, parts of Australia and Central America. The only effective cure is prevention, i.e. planting resistant varieties of coconut palm and preventing a park-like or golf-course-like environment which attracts the planthopper. Some cultivars, such as the Jamaica Tall coconut cultivar, nearly died out by lethal yellowing. Heavy turf grasses and similar green ground cover will attract the planthopper to lay its eggs and the nymphs develop at the roots of these grasses. The planthoppers' eggs and nymphs may pose a great threat to coconut growing countries' economies, into which grass seeds for golf courses and lawns are imported from the Americas.
Phyllody is the abnormal development of floral parts into leafy structures. It is generally caused by phytoplasma or virus infections, though it may also be because of environmental factors that result in an imbalance in plant hormones. Phyllody causes the affected plant to become partially or entirely sterile, as it is unable to normally produce flowers.
Xylella fastidiosa is an aerobic, Gram-negative bacterium of the monotypic genus Xylella. It is a plant pathogen, and is transmitted exclusively by xylem fluid feeding sap insects. Many plant diseases are due to symptomatic infections of X. fastidiosa, including bacterial leaf scorch, oleander leaf scorch, coffee leaf scorch (CLS), alfalfa dwarf, phony peach disease, and the economically important Pierce's disease of grapes (PD) and citrus variegated chlorosis (CVC). In Europe it has attacked olive trees in the Salento area of Southern Italy causing the olive quick decline syndrome (OQDS). While distributions of X. fastidiosa–related diseases are mostly limited to the Americas, outbreaks have occurred in Taiwan, Slovakia, and other countries worldwide.
Aster yellows is a chronic, systemic plant disease caused by several bacterium-like organisms called phytoplasma. The aster yellows phytoplasma (AYP) affects 300 species in 38 families of broad-leaf herbaceous plants, primarily in the aster family, as well as important cereal crops such as wheat and barley. Symptoms are variable and can include phyllody, virescence, chlorosis, stunting, and sterility of flowers. The aster leafhopper vector, Macrosteles quadrilineatus, moves the aster yellows phytoplasma from plant to plant. Its economic burden is primarily felt in the carrot crop industry, as well as the nursery industry. No cure is known for plants infected with aster yellows. Infected plants should be removed immediately to limit the continued spread of the phytoplasma to other susceptible plants. However, in agricultural settings such as carrot fields, some application of chemical insecticides has proven to minimize the rate of infection by killing the vector.
Pestalotiopsis palmarum is the causative agent of a fungal disease of bananas, coconut and Date palms. The fungus causes leaf spots, petiole/rachis blights and sometimes bud rot of palms. Unlike other leaf spot and blight diseases, Pestalotiopsis palmarun attacks all parts of the leaf from the base to the tip. Whereas most diseases only infect the leaf blade or the leaf petiole.
Sabal palmetto, also known as cabbage-palm, palmetto, cabbage palmetto, blue palmetto, Carolina palmetto, common palmetto, swamp cabbage and sabal palm, is one of 15 species of palmetto palm. It is native to the Southern United States, as well as Cuba, the Turks and Caicos Islands, and the Bahamas.
Elm yellows is a plant disease of elm trees that is spread by leafhoppers or by root grafts. Elm yellows, also known as elm phloem necrosis, is very aggressive, with no known cure. Elm yellows occurs in the eastern United States, and southern Ontario in Canada. It is caused by phytoplasmas which infect the phloem of the tree. Similar phytoplasmas, also known confusingly as 'Elm yellows', also occur in Europe. Infection and death of the phloem effectively girdles the tree and stops the flow of water and nutrients. The disease affects both wild-growing and cultivated trees.
Grapevine yellows (GY) are diseases associated to phytoplasmas that occur in many grape growing areas worldwide and are of still increasing significance. Phytoplasmas are obligate cell wall-less bacterial pathogens, and rely on plants and homopterous phloem-sucking insects for biological dispersal. In plants, they are mainly restricted to the phloem tissue where they can move and multiply through the sieve tube elements. Almost identical symptoms of the GY syndrome are caused by different phytoplasmas and appear on leaves, shoots and clusters of grapevine. Typical symptoms include discoloration and necrosis of leaf veins and leaf blades, downward curling of leaves, lack or incomplete lignification of shoots, stunting and necrosis of shoots, abortion of inflorescences and shrivelling of berries. Those symptoms are related to callose deposition at the sieve plates and subsequent degeneration of the phloem. Although no resistant cultivars of Vitis vinifera or rootstocks are known so far, the various grape varieties differ considerably as far as symptom severity is concerned. It ranges from fast decline and death in highly susceptible cultivars to tolerant rootstocks as symptomless carriers of the pathogen. Currently, the only available control strategies include early eradication of infected crops, early eradication of infected source plants, and chemical control of vectors through regular insecticide treatments.
The red ring disease of coconuts and African oil palms is caused by the nematode Bursaphelenchus cocophilus. It is also identified in literature with an alternative scientific name Rhadinaphelenchus cocophilus. The common name, the red ring nematode, is derived from its distinguishing symptom.
Sugarcane grassy shoot disease (SCGS), caused by ‘Candidatus Phytoplasma sacchari’ which are small, pleomorphic, pathogenic bacteria that contributes to yield losses from 5% up to 20% in sugarcane. These losses are higher in the ratoon crop. A higher incidence of SCGS has been recorded in some parts of Southeast Asia and India, resulting in 100% loss in cane yield and sugar production.
Cadang-cadang is a disease caused by Coconut cadang-cadang viroid (CCCVd), a lethal viroid of coconut, African oil palm, anahaw, and buri palms. The name cadang-cadang comes from the word gadang-gadang that means dying in Bicol. It was originally reported on San Miguel Island in the Philippines in 1927/1928. "By 1962, all but 100 of 250,000 palms on this island had died from the disease," indicating an epidemic. Every year one million coconut palms are killed by CCCVd and over 30 million coconut palms have been killed since Cadang-cadang was discovered. CCCVd directly affects the production of copra, a raw material for coconut oil and animal feed. Total losses of about 30 million palms and annual yield losses of about 22,000 tons of copra have been attributed to Cadang-cadang disease in the Philippines.
The South American palm weevil, Rhynchophorus palmarum, is a species of snout beetle. The adults are relatively large black beetles of approximately one and a half inch in length, and the larvae may grow to two inches in length.
Citrus black spot is a fungal disease caused by Guignardia citricarpa. This Ascomycete fungus affects citrus plants throughout subtropical climates, causing a reduction in both fruit quantity and quality. Symptoms include both fruit and leaf lesions, the latter being critical to inter-tree dispersal. Strict regulation and management is necessary to control this disease since there are currently no citrus varieties that are resistant.
Cherry X disease also known as Cherry Buckskin disease is caused by a plant pathogenic phytoplasma. Phytoplasmas are obligate parasites of plants and insects. They are specialized bacteria, characterized by their lack of a cell wall, often transmitted through insects, and are responsible for large losses in crops, fruit trees, and ornamentals. The phytoplasma causing Cherry X disease has a fairly limited host range mostly of stone fruit trees. Hosts of the pathogen include sweet/sour cherries, choke cherry, peaches, nectarines, almonds, clover, and dandelion. Most commonly the pathogen is introduced into economical fruit orchards from wild choke cherry and herbaceous weed hosts. The pathogen is vectored by mountain and cherry leafhoppers. The mountain leafhopper vectors the pathogen from wild hosts to cherry orchards but does not feed on the other hosts. The cherry leafhopper which feeds on the infected cherry trees then becomes the next vector that transmits from cherry orchards to peach, nectarine, and other economic crops. Control of Cherry X disease is limited to controlling the spread, vectors, and weed hosts of the pathogen. Once the pathogen has infected a tree it is fatal and removal is necessary to stop it from becoming a reservoir for vectors.
CandidatusPhytoplasma fraxini is a species of phytoplasma, a specialized group of bacteria which lack a cell wall and attack the phloem of plants. This phytoplasma causes the diseases ash yellows and lilac witches' broom.
Thousands of plant diseases have been recorded throughout the world, many of these causing heavy crop losses. Early detection and accurate diagnosis is essential for the effective management of plant disease. Thus the first step in studying any disease is its timely detection of the diseased plant. Quick initial detection is largely based on the signs and symptoms of disease.
Little leaf of brinjal or eggplant is one of the most serious diseases of brinjal in the areas of its cultivation.
Papaya Bunchy Top Disease was first discovered in 1931 in Puerto Rico. Early on, the identity of the pathogen was highly contested due to the inability of isolating it; thus Koch’s postulates could not be fulfilled. Scientists have previously believed that Papaya Bunchy Top Disease was caused by a virus, a mycoplasma-like organism (MLO), or a phytoplasma, but these possible pathogens have since been disproven. Since the identity of the pathogen was unknown, all diagnoses were given solely based on a list of commonly associated symptoms. Through sequencing and microscopy, scientists identified the pathogen to be a part of the genus Rickettsia in 1996. The bacterium is described as being rod-shaped, small, gram-negative, and laticifer-inhibiting. Rickettsia causes diseases in animals, such as typhus and spotted fever, as well as in other plants, such as phony disease of peach and almond leaf scorch. Papaya Bunchy Top is found throughout the American tropics and has been economically important due to its major impact on fruit production. There is little information about the current economic impact.
