Papaya ringspot is a virus that announces itself on the leaves first and the fruit later, and once a tree has it, it has it for life. So the real work is spotting it early and stopping it from reaching the next plant. Aphids carry it between plants in brief feeding visits, and squash-family plants and weeds can harbor it.1PubMed Central. Papaya ringspot virus-P: characteristics, pathogenicity, sequence variability and control Even so, in places where it once threatened whole industries, a mix of vigilance and resistant trees has kept papaya growing.2AgBioForum. Transgenic Papaya in Hawaii and Beyond
Signs of Papaya Ringspot on Leaves, Stems and Fruit
The earliest sign is usually vein clearing and yellowing on the young leaves at the top of the plant.3Phyton-International Journal of Experimental Botany. Papaya Ring Spot Virus: An Understanding of a Severe Positive-Sense Single Stranded RNA Viral Disease and Its Management This turns into a mosaic, a patchy pattern of light and dark green, and it was the one sign found on infected plants of every age.4PubMed Central. Survey, Detection, Characterization of Papaya Ringspot Virus from Southern India and Management of Papaya Ringspot Disease
The leaf signs get worse as the tree ages. Young plants under about five months often show only the green mosaic.5PubMed Central. Survey, Detection, Characterization of Papaya Ringspot Virus from Southern India and Management of Papaya Ringspot Disease Older plants add yellow patches, leaf curling, puckering, blistering and stunting.6PubMed Central. Survey, Detection, Characterization of Papaya Ringspot Virus from Southern India and Management of Papaya Ringspot Disease Severe cases develop shoestring leaves, where the blade shrinks to narrow strands.7Plant Viruses. Occurrence of Papaya ringspot virus Infecting Papaya in Ivory Coast Leaves may also curl downward or upward and turn stiff.8Plant Viruses. Occurrence of Papaya ringspot virus Infecting Papaya in Ivory Coast The worst symptoms appear once the tree is fruiting.9PubMed Central. Survey, Detection, Characterization of Papaya Ringspot Virus from Southern India and Management of Papaya Ringspot Disease
Stems and fruit carry the ring-shaped marks. Water-soaked rings or oily streaks can appear on the stem and leaf stalks.10Plant Viruses. Occurrence of Papaya ringspot virus Infecting Papaya in Ivory Coast Green fruit shows darker green rings and arcs, and on ripe fruit these become yellowish blotches with darker green centers.11Plant Viruses. Occurrence of Papaya ringspot virus Infecting Papaya in Ivory Coast Infected fruit can also be misshapen.12PubMed Central. Survey, Detection, Characterization of Papaya Ringspot Virus from Southern India and Management of Papaya Ringspot Disease
To check a suspect tree, start with the youngest leaves at the crown rather than the old ones near the bottom. Hold a leaf up to the light and look for mottling and veins that have gone pale. Then run your eye over the leaf stalks and trunk for greasy-looking streaks or rings, and inspect any green fruit for rings, which strongly suggest ringspot.
Plenty of other problems look similar. Yellowing that starts on the old, lower leaves and moves upward usually points to a nutrient shortage, overwatering or drought, because ringspot shows first on the newest growth at the top. Spider mites leave fine speckling, bronzing and sometimes webbing on the underside of leaves, not a patchy mosaic or rings on fruit. Other viruses and herbicide drift can also cause mottling and distorted leaves, so mottled new growth alone is one clue among several. A lab test settles any remaining doubt.
How Ringspot Spreads From Plant to Plant
Aphids spread papaya ringspot.13PubMed Central. Papaya ringspot virus-P: characteristics, pathogenicity, sequence variability and control They pick the virus up during brief probing bites and pass it on within minutes. That is why killing aphids after they land rarely stops the spread: the virus has usually moved before an insecticide works.
The aphids responsible are often winged visitors blowing in from other plants, and most never settle or breed on papaya. They land, take a quick test bite to judge whether the plant suits them, and move on, which is all it takes to pass the virus along. You may never see a colony on your trees, yet they can still have been exposed.
Two strains of the virus share some hosts. The papaya strain infects papaya and also squash, melons and cucumbers.14PubMed Central. Papaya ringspot virus-P: characteristics, pathogenicity, sequence variability and control The watermelon strain infects squash-family crops but not papaya.15PubMed Central. Papaya ringspot virus-P: characteristics, pathogenicity, sequence variability and control The two are nearly identical genetically.16PubMed. The nucleotide sequences of the 3′-terminal regions of papaya ringspot virus strains W and P A nearby infected squash patch can therefore be a source for your papayas.
Weeds growing among papayas can act as reservoirs of the virus.17Revista mexicana de fitopatología. Presence of Papaya ringspot virus (PRSV) in weed associated with Carica papaya in Colima, Mexico Bitter melon vine, a widespread weed in the Caribbean, can carry the papaya strain,18PubMed. Momordica charantia is a Weed Host Reservoir for Papaya ringspot virus Type P in Jamaica and wild relatives of squash and melon can carry it as well.19PubMed. Natural Infection of Citrullus colocynthis by Papaya ringspot virus-W in Iran
Confirming the Diagnosis With a Test
Standard lab tests detect the virus about 2 to 3 weeks after infection.20PubMed. Ultra-sensitive detection of papaya ringspot virus using single-tube nested PCR Newer, more sensitive tests can find it after about a week, even in plants with no symptoms yet.21PubMed. Ultra-sensitive detection of papaya ringspot virus using single-tube nested PCR Some rapid tests are about ten times more sensitive than older ones.22PubMed. Reverse transcription loop-mediated isothermal amplification assay for rapid detection of Papaya ringspot virus
To get a sample tested, contact your local agricultural extension office or a plant diagnostic lab, which can often take samples from home gardeners. Ask whether they test specifically for papaya ringspot virus, since not every lab offers it. Send young leaves that show symptoms, sealed in a plastic bag and kept cool, and get them to the lab quickly. A note on what you see, and on where squash or melons grow nearby, helps the lab interpret the result. Growers buying nursery seedlings will find testing them before planting well worth the cost.
What to Do About an Infected Tree
No spray or treatment cures a virus-infected plant. The control is pulling out infected plants early, which removes the source aphids feed from. Steady surveying and removal of infected papaya helped lower ringspot in Hawaii’s Hilo area.23The Plant Health Instructor. Papaya Ringspot virus Combined with quarantine on moving seedlings and constant checking, it kept a whole district free for years.24The Plant Health Instructor. Papaya Ringspot virus
Do it carefully. Before pulling the tree, spray any aphids on it so they do not fly to neighboring plants as it is disturbed. Cut the tree up and bag it or bury it rather than leaving leaves and trunk lying in the garden, where they remain a source. After removal, check the remaining papayas weekly for new patchy green leaves so you catch further cases early. Do not move seedlings from a yard with infected plants to a clean one, since they may carry the virus without showing it. An infected tree’s fruit can still ripen, though quality suffers, but keeping the tree puts every other papaya and squash plant nearby at risk.
Keeping Aphids and the Virus Away
Reflective mulch and oil work better than insecticide alone. In squash, silver reflective mulch cut the infection rate to about a third of that in unprotected plants.25International Journal of Pest Management. Control of aphid‐borne papaya ringspot potyvirus in zucchini marrow(Cucurbita pepo)with reflective mulches and mineral oil‐insecticide sprays Mineral oil sprays combined with insecticide worked about as well, and using both cut infection to about a sixth.26International Journal of Pest Management. Control of aphid‐borne papaya ringspot potyvirus in zucchini marrow(Cucurbita pepo)with reflective mulches and mineral oil‐insecticide sprays Black mulch helped only partly, and blue or grey mulch did not help.27International Journal of Pest Management. Control of aphid‐borne papaya ringspot potyvirus in zucchini marrow(Cucurbita pepo)with reflective mulches and mineral oil‐insecticide sprays Insecticide alone gave only a small benefit.28International Journal of Pest Management. Control of aphid‐borne papaya ringspot potyvirus in zucchini marrow(Cucurbita pepo)with reflective mulches and mineral oil‐insecticide sprays
For mulch at home, lay silver plastic around young plants, since that is when protection matters most. The glare confuses aphids coming in to land, so fewer of them test-feed on the plant. The effect fades once the growing canopy shades the plastic.
For oil, use a horticultural mineral oil labeled for edible plants, and follow the label for dilution. Cover the new leaves, where aphids probe, and repeat after rain washes the oil off. Avoid spraying in strong midday heat, which can burn oiled leaves. Because oil works by interfering with virus transfer during probing, it protects best when applied before aphids arrive.
Removing the virus’s hiding places matters as well. Clearing weeds in and around papaya beds removes reservoirs.29Revista mexicana de fitopatología. Presence of Papaya ringspot virus (PRSV) in weed associated with Carica papaya in Colima, Mexico Planting papayas away from squash, melon and cucumber takes away a nearby source.
Timing Your Planting
In one tropical region, papaya planted August to December saw only 10 to 20% infection.30Pest Management in Horticultural Ecosystems. Influence of weather factors on aphid population and the incidence of Papaya Ring Spot Virus disease Crops planted in February and March were overrun within 60 days.31Pest Management in Horticultural Ecosystems. Influence of weather factors on aphid population and the incidence of Papaya Ring Spot Virus disease Aphid numbers rose with the hot temperatures of February to April.32Pest Management in Horticultural Ecosystems. Influence of weather factors on aphid population and the incidence of Papaya Ring Spot Virus disease So avoid planting just before your local aphid peak.
Peak aphid months differ by region. Watch for aphid flushes on squash, roses or other soft growth near you, since a sudden buildup there signals the season is starting. Ask your extension office when aphids peak locally, and plan to have young plants past their most vulnerable stage by then.
Resistant Papaya Varieties
Rainbow and SunUp were released in 1998.33AgBioForum. Transgenic Papaya in Hawaii and Beyond Rainbow is widely planted and helped save Hawaii’s papaya industry.34AgBioForum. Transgenic Papaya in Hawaii and Beyond Both carry a gene from a mild virus strain that blocks infection,35European Journal of Plant Pathology. Papaya ringspot virus resistance of transgenic Rainbow and SunUp is affected by gene dosage, plant development, and coat protein homology and in practice they control ringspot in Hawaii.36PubMed. Virus Coat Protein Transgenic Papaya Provides Practical Control of Papaya ringspot virus in Hawaii
Resistance depends on the local strain. An engineered papaya resistant in the USA was not protected against Australian and Thai strains.37PubMed Central. Gene Technology for Papaya Ringspot Virus Disease Management Rainbow can be infected by strains from Guam, Taiwan and Thailand, while SunUp tolerates more foreign strains.38Wikipedia. Papaya ringspot virus Outside Hawaii, then, these varieties may not protect you, and whether you can get them depends on local rules.
For a home grower, Rainbow seed and seedlings are available in Hawaii, and they are the practical choice there. Elsewhere, ask nurseries about locally tested tolerant varieties rather than assuming a Hawaiian variety will work. Even tolerant plants need aphid and weed control.
Other options are coming. Papaya itself has no natural resistance genes, so breeders cross it with wild relatives.39Acta Horticulturae. How to Identify and Control Papaya Ring Spot Virus Resistant crossbred plants have been produced after decades of failure.40Euphytica. Papaya ringspot virus resistance in Carica papaya via introgression from Vasconcellea quercifolia Gene editing of a single plant gene gave full resistance in trials.41PubMed Central. Enhancing papaya resistance to ringspot virus through CRISPR/Cas9-mediated gene editing of eIF4E Inoculating plants with a mild strain to shield them from a harmful one protected them only for a few months.42PubMed Central. Genetic characterization of a mild isolate of papaya ringspot virus type-P (PRSV-P) and assessment of its cross-protection potential under greenhouse and field conditions
Is Fruit From an Infected Tree Still Good
Plant viruses do not infect people, so the fruit is not harmful. It can, however, taste poor and have a tougher texture.43Business Queensland. Papaya ringspot disease It is also more likely to develop fungal rots and black spot.44Business Queensland. Papaya ringspot disease Eat it soon after picking rather than storing it, since it tends to spoil sooner.
Cut away any rotting spots. If the flavor is flat, use the fruit in cooking or smoothies, where sweetness and spices can make up for it.
Moving Papaya Plants Across Borders
Ringspot is listed as a harmful organism by Japan, New Zealand and several other countries.45California Department of Food and Agriculture. California Pest Rating Proposal for Papaya ringspot virus Governments block plant diseases that could cause crop losses,46Biosafety and Biosecurity. The Threats of Transboundary Plant Pathogens so buy seedlings from local certified nurseries rather than carrying them in.
A foreign strain of the virus can overcome local resistant varieties, so one imported plant can undo protection that works at home. Infected seedlings can look healthy, because symptoms take time to show, so a clean-looking plant is no guarantee.