The Backyard as Laboratory and Workshop Series 7: Papaya Ring Spot Virus (PRSV) - Scourge of Papaya Worldwide.

There is no stopping the viral scourge of papaya (papaw, pawpaw), wiping out plantation after plantation worldwide - even with attempts through genetic engineering.  It's because the virus is not only complex (it infects cucurbits like melon and squash - and other plants), but it persistently mutates into resistant types that invade quarantined areas and overcome transgenic defense. There is one hope every backyard can look up to -  the return of the native papaya varieties preserved in their indigenous state.

Dr Abe V Rotor

Living with Nature School on Blog
Paaralang Bayan sa Himpapawid with Ms Melly C Tenorio
738 DZRB AM Band, 8 to 9 evening class, Monday to Friday

Photo of a sick papaya at home in QC. The disease is systemic, that is, the virus resides in the whole system of the plant and infects all tissues from roots to fruits.  The leaves are the first to show the symptoms which may be mistaken for iron (Fe) deficiency being chlorotic (lacking in chlorophyll), mosaic, deformed and wrinkled like being scorched  by heat and sunlight. Even stunting may be thought of as deficiency symptom of other elements. The plant doesn't die, but remains stunted and exhibits rosette leaf arrangement. Some gardeners even think it is a fancy variety, and by keeping it as ornamental would only exacerbate the spread of the disease by mere physical contact and through biological transmission, particularly by aphids (Myzus persicae) as well as, in my observation though not mentioned in the books, the white fly of the genus Bemesia.       

The virulence of PRSV cannot be underestimated, from early infection during seed germination, to later infection at any stage of the plant. Which means that the virus is incipient in the embryo and openly infectious even in the senile age of the tree. As the infected tree faces slow death it becomes a source of viral inoculant in the open field within the range of the vectors, including man. Note the ring spots on the fruit from which the virus got its name. The spots predispose the fruit to secondary infection, leading to bacterial rot and fungal attack. Thus, not only production is gravely affected but the quality of the fruit as well to the point of becoming unfit for human consumption.     

My son Marlo (above) harvests a ripening fruit of native papaya we planted in our residence in QC. Note the lanky stand but  healthy condition of the tree. The photo at the right (from the Internet) shows a variety apparently immune to the disease like our own. Indigenous varieties have been reported to be resistant if not immune to PRSV, whether it be the P or W biotype - and possibly against the mutants arising from both pathotypes, and other viruses which we may not know. Commercially, the native papaya is of lesser importance, but it can supply the needs of the family and immediate community for ripe papaya (for the table and puree), and green papaya (for tinola and pickles). 

The commercialization of the Hawaiian papaya owing to its heavy and early fruiting, and feasibility in large scale  production was a boom but at the same time exacerbated the spread of the disease on global proportion.  Today, virtually no place is secure and safe from PRSV in spite of strict quarantine laws and regulations. 

When I was a farmhand, my dad grew native papaya in our backyard in Ilocos. We harvested only the fruits as they ripen in succession at few days interval. Fruits are sweeter when they are picked ripe. Fruits in the market are ripened with carburo or ethylene gas. They taste flat, the color is dull, and the texture gummy. When we needed green papaya, we would simply thin out the small ones or bansot, and leave the large, healthy fruits to reach full maturity. 

By the way, papaya is dioecious. Only the female papaya is cultivated, but a few male trees are spared for pollination. A third gender now and then would arise.  The tree bears small fruits hanging on elongated peduncles. The fruits are generally not edible, green or ripe.

Would you guess the productive life span of a native papaya? Five years to ten years of continuous fruiting. And it reaches a height of ten to twenty feet so that harvesting requires a pole with a basket (salukang Ilk). Well, the native papaya is not laden but the fruits though small, are luscious and sweet. As the tree gets older it branches out into two, three or four and the main branches are productive. Branching papaya is more resistant to wind and also to long dry season and pests, which includes the fruit bat (if you don't harvest the ripe fruits ahead of this nocturnal feeder). 

Papaya is the only species under the genus Carica, and the only edible species of importance among its five relatives. No wonder when it was orphaned from its non-edible kin, and transported for widespread cultivation, the virus became concentrated in the species, and through repeated and expanded cultivation in other countries, the virus mutated and evolved into more virulent types.  

I was in high school in the mid fifties when an agriculturist who trained in Hawaii promoted the Hawaiian papaya to be planted in the Philippines.  I bought a packet of seeds which dad and I planted.  Indeed our Hawaiian papaya became an "apple to the eye" in our locality. In a short time many backyards had growing Hawaiian papaya trees. And the native papaya was almost forgotten. . 

The sixties and seventies brought agricultural technology which revolutionized agriculture. It was a short live green revolution. It was the start of "globalization in farming" where the frontiers of agriculture were not only expanded but diversified in order to meet the expanding market. Agriculture took the helm of development with little consideration on the welfare of the environment.  It did not give much importance to environmental degradation, much less rehabilitation. Let me cite these cases to prove my point.   

• Plantations of Hawaiian and Peruvian ipil-ipil were wiped out by Psylla plant lice, while our native ipil-ipil remained unaffected.
• Hawaiian pineapple, so with other foreign varieties, failed to adapt, while our native pineapple called Formosa continued to thrive. 
• Varieties of rice developed by IRRI ultimately disappeared from farmers' fields, as our native rice varieties returned.
• Many corn varieties failed, while our native corn varieties persisted - even if production is low (only around one ton per hectare).
• Bangkok santol introduced leaf galls caused by mites that brought its own demise. The pest still persists in its progeny (cross of Bangkok and native santol). 
• The large Anglo-Nubian goats did not adapt to local conditions, while our local goats live on.  So with St Gertrude bullock that gave way to our native stock.  
• Several trials were made to plant soybean, white bean (for pork and beans), and potato.  We failed for the same reason. We cannot tailor the land to the crop.  
• Foreign varieties of plants and breeds of animals failed top get acclimatized under Philippine condition. All these  - and many other introduced crops and animals - failed. 

The Native Gene is Our Hope 

Let's go back to the native genes, 
the unspoiled, pristine gene pool 
that developed through thousands, 
if not millions of years of evolution.  

Genetic engineering is not the answer, 
but to harness the old faithful genes - 
genes that enhanced our survival 
as Homo sapiens, 
genes that co-evolved 
with those of other organisms. 

Co-evolution is the key 
to our success as a species. 
Technology is not, and never will. 
Tinkering with nature, and life itself
ushers the decline and ultimate 
demise of mankind. ~      
--------------------------------------------------
“The potyvirus Papaya ringspot virus (PRSV) is found throughout the tropics and subtropics. Its P biotype is a devastating pathogen of papaya crops and its W biotype of cucurbits. PRSV-P is thought to arise by mutation from PRSV-W … PRSV may have originated in Asia, particularly in the Indian subcontinent, as PRSV populations there are most diverse and hence have probably been present longest. Our analyses show that mutation, together with local and long-distance movement, contributes to population variation, and also confirms an earlier conclusion that populations of the PRSV-P biotype have evolved on several occasions from PRSV-W populations.”

On the evolution and molecular epidemiology of the potyvirus Papaya ringspot virus

Bateson MF, Lines RE et al, JGV Journal of Virology

--------------------------------------------------

Developments
· Vaccination. Like vaccination technique in humans, the host plant receives a mild strain of PRSV. Resistance is gauged by the delay in the onset of symptoms to reduction in the severity of symptoms. But inoculation of the mild strain also causes pathogenesis, which means that the plant did not gain true resistance.

· Transgenic papaya (Rainbow and SunUp) are claimed to have differential resistance to the Hawaiian strains of PRSV, but such resistance can be eroded by other viral strains found in other countries.

· Pathogen Derived Resistance (PDR) is a technique of inserting a gene fragment from the pathogen into the transgenic crop, leading to two transgenic lines claimed to be resistant to PRSV, but like the “vaccinated” and transgenic "varieties," are sooner or later overtaken by increasing virulence and mutation of PRSV into new strains.

· Deregulation aims at breaking out from world’s objection against GMO. Some countries like US and Japan, import Hawaiian papaya on a very limited scale. Backlash against GMO includes surreptitious destruction of experiments and plantations.

Objection to any type of GMO research rages in most parts of the world as people are “going for natural” food, medicine, clothing, homes, life style, etc. And they look at GMO as a Frankenfood (from the novel Frankenstein, by Mary Shelly).

· Durability of Resistance: Exposure to foreign strains of the virus is a serious risk, as the transgenic Rainbow papayas have been shown to be susceptible to PRSV from Guam, Taiwan and Thailand.

Acknowledgement: Wikipedia