Papaya ringspot virus (PRSV) is a pathogenic plant virus in the genus Potyvirus and the virus family Potyviridae which primarily infects the papaya tree. The virus is a non-enveloped, flexuous rod-shaped particle that is between 760–800 nm long and 12 nm in diameter. It is transmitted between plants by mechanical activities like pruning and by numerous aphid species such as Myzus persicae. No seed transmission has been detected. There are two major types of this virus, but both are serologically indistinguishable and are so closely genetically related that they are now considered the same virus species. The type that gave the virus its name are the Type P isolates (PRSV-P). This type infects papaya and several members of the melon family (Cucurbitaceae). The other type, Type W isolates (PRSV-W), does not infect papaya. Isolates of PRSV-W do infect cucurbits such as watermelon, cucumber, and squash and were originally known as Watermelon mosaic virus 1.
History Hawaiian papaya production has been severely affected twice by PRSV. The virus was introduced to Oahu as early as 1937. The disease was mild for a number of years until it either mutated or a more aggressive strain was introduced around 1950. Within 12 years, the amount of land under papaya production dropped 94%. Production was then moved from Oahu to the Puna region of Hawaii island (the "Big Island") under strict quarantine. In 1971 PRSV was found in home gardens but efforts were taken to prevent its spread. The virus emerged in commercial farms in 1992 and by 1995 production in Puna was impossible. Commercial growers again relocated to the Hamakua coast but with only limited success. Hawaiian papaya production was halved by the end of the decade. Transgenic papaya varieties that are resistant to PRSV entered production in 1998 and resuscitated the industry.
Distribution and origin Both pathotypes are distributed worldwide. PRSV-P, for example, is known to be present in the Middle East, Africa, South and Central American. It has also been found in China, France, Germany, India, Italy, Mexico, Taiwan, and the United States. PRSV-W isolates have been found in the United States, the Caribbean, Mexico, Italy, Germany, Australia, France, India, the Middle East, and South America. Using genetic phylogeny studies, researchers suspect the virus originated in Asia, likely India, about 2,250 years ago. From there it slowly spread through the continent reaching China about 600 years ago. It was also introduced directly from India to Australia and the Americas within the last 300 years. Papayas were introduced to India only 500 years ago, at which point the virus made the jump from cucurbits. However, the virus has switched back and forth between pathotypes many times in its evolution.
PRSV-P Symptoms are typical of viral diseases. Papaya exhibits yellowing, leaf distortion, and severe mosaic. Oily or water-soaked spots and streaks appear on the trunk and petioles. The fruit will exhibit bumps and the classic "ringspot". A severe isolate of PRSV has also been shown to cause tissue necrosis. Cucurbit symptoms tend to be similar to papaya symptoms including blisters, mosaic, yellowing, and leaf distortions. This virus produces two types of inclusion bodies visible under a light microscope with proper staining of epidermal strips. One inclusion is the typical cylindrical inclusion (CI) which is considered diagnostic for the potyvirus group, and the other is called the amorphous inclusion (AI). The presence of both inclusions can be diagnostic for this virus.
PRSV-W
In squash, watermelon and other cucurbits, PRSV-W causes mottling and distortion of leaves and fruit. PRSV-W is considered to be one of the limiting factors in the growing of cucurbits in Florida. PRSV-W should not be confused with Watermelon mosaic virus 2, another potyvirus that infects cucurbits around the world, including Florida, and which is now known simply as Watermelon mosaic virus (WMV). PRSV has a different host range, different serological properties, and no nucleotide sequence homology with WMV. WMV also has different cytoplasmic inclusion bodies that can differentiate it from PRSV-W. Polyclonal and monoclonal antibodies have been made for both of the CI and AI proteins as well as for the capsid protein of PRSV-W.
Vectors Aphids are the predominant means by which PRSV is transmitted. PRSV is a non-persistent virus, meaning it does not enter beyond the feeding mouthparts of the aphid, and does not circulate or multiply within its insect host. Non-persistent viruses are transmitted quickly and easily between plants. Many species of aphid can transmit PRSV, particularly the Peach Aphid and Melon Aphid. Seed transmission has been observed at rates of 2 infected second generation plants out of 1355. This method of transmission is marginal compared to transmission through aphid vectors. However, if infected seed survives, it could then function as a source on inoculum, which then could be spread by insect vectors. Disease transmission can also occur by planting infected seedlings in fields where the virus is not present. Using "clean", virus-free seedlings for planting is of very high importance.
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