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Aloha zapatero, welcome.
I haven't seen this problem on my papaya, but I wonder if it could be broad mites? I have had a couple of brugmansia with broad mites which typically shows itself with small distorted leaves, usually slightly downward cupping, attacking the new leaves. Broad mites are microscopic and cannot be seen with the naked eye. Common insecticides do not seem to work on broad mites, at least I have not been able to find one...I remove all the infected leaves off and destroy them, and pray. Broad mites can be killed (so I am told) by a hot water bath (110 for 15 minutes) but of course that is impossible in your situation.
Sorry I cannot be of any real help, do you have a dept of agriculture in Thailand that would be able to identify the problem? I feel for you, especially as it is spreading. I don't think it is one of the common virus.
Perhaps others who are more knowledgeable will chime in.
The deformation leads me to ask you if you are fertilzing the plant with any particular thing?
I've not seen that sort of fungus on my papayas before, but I have seen deformation after using hasta grow, or seaweed based fertilzer. I would remove the leaf just in case.
Thank you all for your kind replies. I will try to respond to each:
Braveheart: Other plant people here have suggested the possibility of broad mites, however I have put the leaves under 20x magnification and have found nothing that I would recognize as a mite. But please see below.
rjudd: Good thinking! However, I've used no fertilizers. And the fungicides and insecticides that I have used have had no similar effect on the other plants...
fauna4: I followed your Agdia link and explored around in there. I found both your "mosaic" as well as "ringspot", however was unable to locate any pictures. But you set me onto the virus track, which I hadn't thought of. With a little searching of Google Images, I was able to duplicate the leaf pattern. Based upon those pictures, I now believe that it is PRSV potyvirus. Here is the link, if you are interested:
I spent some time poking around in the PaDIL (Plants and Diseases Image Library) site, and was pretty impressed. If I can contribute something back to the group, here is their Home link. (Pretty obvious from the above, I guess):
Hey Z. I should have posted a link to a site with pictures, but my brain is more on the diagnostic laboratory wavelength. I agree with destroying the plants. There are no cures for viruses, and the only real management practice is to destroy the plants to prevent spread to future plants. Sorry this happened to your plants.
I am no expert on papayas but is this the same mosaic disease that put a lot of papaya growers out of business for a while? That is the reason many are now growing the new genetically changed papayas that do no get the disease. Aloha