Papaya Bunchy Top Disease was first discovered in 1931 in Puerto Rico.Davis, M. J. 1996

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 virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...

, a mycoplasma-like organism (MLO), or a

phytoplasma Phytoplasmas are obligate intracellular 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-lik ...

, 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 ''Rickettsia'' is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The term "ricke ...

'' in 1996. The

bacterium Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were amon ...

is described as being rod-shaped, small,

gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...

, 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.Agrios, George N. 1969. There is little information about the current economic impact.

Hosts and symptoms

Papaya is the only known host of the disease.Davis, M. J. 1993

Symptoms typically appear around 30 to 45 days after infection. Early symptoms are

chlorosis In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...

and stunting of young leaves, accompanied by internode and petiole elongation. As the disease progresses petioles and leaves become rigid and thickened. Chlorosis spreads to older leaves followed by necrosis and water-soaked spots appear on the petioles and stems. In diseased plants,

flowers A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechanism ...

and

fruit In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in particu ...

are rarely produced and during advanced stages of the disease, the papaya plant becomes denuded except for a few stunted leaves that remain at the apex. If fruit do set during infection, they are bitter tasting and unmarketable. Dieback can occur in some varieties of papaya when infected. A lack of

latex Latex is an emulsion (stable dispersion) of polymer microparticles in water. Latexes are found in nature, but synthetic latexes are common as well. In nature, latex is found as a milky fluid found in 10% of all flowering plants (angiosperms ...

flow from the leaves, stems, petioles, and fruit.Ploetz, R. C. 1994. was historically a very important symptom of diagnosis but has since found to be unreliable; making it important to look at many different symptoms before making a final diagnosis.Webb, R. R. 198

Development rate and severity of symptoms are highly variable due to host, genotype, and region.

Environment

Papaya Bunchy Top Disease is found only in tropical areas within the Americas. It is vectored by two species of leafhopper, '' Empoasca papayae'' Oman (Sein) and '' Empoasca stevensi'' Young. The species that vector the disease is dependent on the geographical location. ''E. papayae'' is most commonly associated with vectoring the disease because of its wide distribution throughout the American tropics. ''E. stevensi'' has a smaller distribution and is mostly attributed to cases in Trinidad.

Management

Tolerant papaya cultivars to Bunchy Top Disease have been found and bred to help stop the spread of disease. This is the most practical method of disease management but production of these

cultivar A cultivar is a type of cultivated plant that people have selected for desired traits and when propagated retain those traits. Methods used to propagate cultivars include: division, root and stem cuttings, offsets, grafting, tissue culture ...

s is limited to areas with a low disease pressure. Use of clean

propagation Propagation can refer to: * Chain propagation in a chemical reaction mechanism *Crack propagation, the growth of a crack during the fracture of materials * Propaganda, non-objective information used to further an agenda * Reproduction, and other fo ...

material and quick removal of diseased trees are also important to prevent spread. Use of Insecticides has also been proven affective to limit leafhopper vectors. The application of

antibiotics An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention o ...

, such as

chlortetracycline Chlortetracycline (trade name Aureomycin, Lederle Laboratories) is a tetracycline antibiotic, the first tetracycline to be identified. It was discovered in 1945 at Lederle Laboratories under the supervision of scientist Yellapragada Subbarow and ...

or tetracycline hydrochloride, into the soil around infected plants has shown some success in treating Papaya Bunchy Top Disease but has yet to become commercially available.

Importance

Infected papaya plants rarely flower or set fruit. Fruit production can therefore be significantly decreased. This can cause major agriculture losses within the American tropics, leading to economic impacts. Also if plants do produce fruit, it tends to be bitter and thus unmarketable. However, there is little current information on how this disease economically impacts the American tropics.

Pathogenesis

'' is a

, rod-shaped, laticifer-inhibiting

. It is typically 0.8 to 1.6 µm long and 0.25 to 0.35 µm wide. The bacteria are found within the sap of the plant. The bacteria infect the plant's cells, destroying the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...

along with its organelles, causing the cells to eventually collapse.

References

{{reflist * * * * Bacterial plant pathogens and diseases Papaya tree diseases