I am growing green tomatillo (a mexican variety of tomato) but one of my plants is looking very sad and I need help to understand what could be the problem.
About the problem: Two days ago I noticed one of them developed a few light spots on two leaves on the top. Yesterday more light spots appeared, but the plant looked good. Today there are approx. the same number of spots, but one of the leaves has started curling its edges a little, and the other leaves are just hanging, they are not rigid anymore. I don't know what could be the problem. I have 10 plants in individual pots close together, and this is the only one that is having this issue.
About my setup: I'm an absolute newbie to gardening, so my setup is pretty rudimentary. I don't know anything about nutrients, ph, etc... My plants are 4 weeks old, I transplanted them from a propagator to individual pots at age 2 weeks, and I keep them in a conservatory. Temperature, humidity, etc. are not regulated. Temperature dropped below 10°C a few days ago, so that might be a factor (although the other plants are unaffected).
What I have discarded: I have checked for insects and eggs around and under the leaves, and there is nothing to be found. Also none of the neighbor plants are affected. I read it could be related to water "burning" the leaves, however I didn't water the plants yesterday, and yet more spots appeared. The compost I used I bought it from the store, as in a sealed bag, so I guess the soil is sterile.
I'm attaching a picture of how it looked today. Notice the leaves on the foreground and the background are affected, but not the one in the middle, nor the small ones down the stem.
Is this something I should worry about? Any help is greatly appreciated! Thanks!