Certified GKH Gardening Expert
This appears to be overwatering damage. Compensation for heat should be done with shade. Watering should, only, be done when the top 2 or 3 inches of soil are completely dry.
Soils that remain saturated without a period to dry out will suffocate plant roots. Be sure to allow for a dry period, and move the plants to shade during hotter portions of the day. You may need a fungicide to take care of any infections that can arise from wet soils.
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