SiamJim, looks like either a leaf mold or a powdery mildew.

What are you seeing on the undersides of the leaves?

Here is a link that might be useful: tomato problem solver

Suze,

The underside of the leaves have the same white stuff. Some of the leaves, on the underside, are clear. It looks like it's suffocating the plant.

Do you know what I can use to kill mold or mildew. I'm having a hard time getting the right product at the local nursery here in Thailand.

Thanks,

James

James,
It's looking like leaf mold to me based on your picture, but I'm not 100% sure. Would suggest that you go to the problem solver link I provided in the previous response and compare the differences between Cladosporium fulvum (leaf mold) and powdery mildew. Go the pics of leaves and click to enlarge them and read further information.

For either condition, given your limited availabilty of products, treatment might include Cornell formula spray or a sulfur product. Understand though, that the spraying will not be a one time thing. You'll need to spray periodically to control the problem.

With both products, you will need to be very careful in usage. They can both make plants photosensitive and should not be sprayed when direct sun is on plants or temps exceed 80F. In other words, only spray in early morning or late evening.

In addition, sulfur sprays can't be done in conjunction with any sort of other spray program that involves oils (like Cornell formula). You'd need to wait a few weeks between doing one or the other, so pick one approach or the other.

Oh, and I'd carefully remove the worst of the leaves before you spray, disposing of them away from your garden beds.

Here is a link that might be useful: discussion on Cornell formula

Suze,

Thank you very much for the advice. I'll do some research and post a follow up for the archives.

All the best,

James