Make sure to comepletly dry your horses face off. I live in Florida so so it's very humid and if I don't dry my horses face off completely he tends to get a slight case of rain rot
I don't bathe often, even in summer; However, I rinse the horses down daily! I live in Arizona so it's very hot (it was 122° F yesterday!) I *always* rinse down after a ride. Sometimes before (I ride at 5am to beat the heat so it usually isn't necessary to rinse before but sometimes I do (even if it's just their legs and chest)) I normally rinse down all the horses several times throughout the day (the heat dries them up in minutes!) I *always* rinse under their tail, between their hind legs and under their mane (after rides especially). Many people don't but with the heat I find it necessary. I also get a sponge and rinse their face and behind their ears.
Hope this helped,
Only bathe your horse with shampoo and conditioner like once a month or before a show, no more than that cause it will take away the natural oils of the horse. I just use mane and tail on my horse and it works great.
But you can hose them off with water every day. But just do a lot of brushing.