This started on a few of my fig trees 2-3 weeks ago. Until now I've just cut the discoloured leaves off. 

 

The trees are growing in well-draining, elevated rows and it hasn't rained in approx. 2 weeks, so I'm ruling out root rot. 

 

Any ideas what else it could be? Should I do something about it? If so, what?

 

Should I be concerned? 

 

? Possibly https://www.gardeningknowhow.com/edible/fruits/figs/treating-fig-anthracnose.htm

My figs get rust and lose nearly all their leaves when it gets dry, which doesn't seem to cause permanent damage.

I don't know specific ID, but possible fungal leaf spots, like Cooked suggests an anthracnose, or an entomosporium leaf spot, which often appear in wet weather.  The good thing is that, from the photo, it appears to be only on older leaves.  The newer leaves, since dryer weather, appear to be unaffected.  That gives you a chance to consider that treatment may not be needed. Some pests and diseases come and go with the seasons, without doing major damage. 

 

 My opinion is that you have two options, or use both.

1. remove affected leaves to reduce inoculum (spore dispersal).  Wait it out and monitor new growth frequently, especially if it rains again.  If it doesn't rain, the disease may not spread to newer growth, and you may not need to do anything except improve soil fertility and water management. Avoid over-head watering. 

2. begin a preventive spray schedule. horticultural oil (delayed dormant 1%, as opposed to a stronger 2% dormant concentration that could burn foliage) will prevent superficial infections and also suppress mite infestations that potentially spread fig mosaic virus. 70% neem oil every two weeks is what I would prefer, but I have not seen this formulation in Thailand, only azadirachtin concentrate neem products, which is a great botanical pest repellent, but does not address fungal disease like the oil. 

A biological fungicide solution could also be considered, on two week intervals for prevention, one week apart to suppress active infections. Trichoderma h. is available. There are other fungicide options, chemi and home brews.

 

Drought dormancy as mentioned is common, but not necessarily preferred. It is extremely stressful for the plant and allowing it would be contrary to optimal growing conditions.  Best if you have an irrigation system and budget for water and fertility improvements throughout the growing season.