Answer: It sounds like you have the same plants in the same pots year after year. Fungal diseases can "overwinter" in the soil and come back when weather condtions are favorable. I'd advise starting with fresh soil and plants, and sterilize the pots in a solution of 1 part bleach to 9 parts water and let them dry in the sun. Fungal diseases are common in your climate. Here are some general rules for helping control fungal diseases. Start by making sure that your plants are getting enough direct sunlight. You'll also want to make sure that there's enough room between plants for air to circulate freely. Overcrowding not only makes plants more susceptible to diseases, if leaves touch other plants, those diseases can easily be spread. The general advice to inhibit the spread of fungal diseases is to avoid wetting leaf surfaces, so water the soil, don't sprinkle foliage from above. You can also try a fungicide, but without knowing exactly which fungus it is, I can't recommend which one would be appropriate.
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