Signs of lack of water or excess heat?

[to be chafoin]

Where it is useful to know if it is "mildew" or its "cousins" is if you want to put your tomatoes in an open tunnel ...

Well precisely I do not recognize there mildew (I do not see the oily appearance on the leaves, nor the felting under the leaves and there are no stains on the stem ...) but maybe I am mistaken...

[izentrop]

It looks like one of these diseases http://ephytia.inra.fr/fr/C/5086/Tomate ... -infestans

[to be chafoin]

Frankly I'm not sure:

Wet, brown to black lesions, sometimes extending several centimeters, appear on the petioles and stems

At home, we went rather through yellow and if the current result comes close, I did not spot the classic start of mildew, which we can see in the photos of this link ...