Hot and dry conditions are irresistible to spider mites. For the winter, I usually buy one or two handspray bottles of pyrethrum insect killer for bug emergencies. Both products are synthetics based on natural insecticides and very safe. If powdery mildew creeps in, I try to solve the growing conditions rather than accelerate the spraying. This is important for three reasons.
High humidity can be a breeding ground for a number of diseases.
High temperatures stress the plants, making them susceptible to pests. A high oxygen content may be invigorating for us but not for the plants. A small squirrel fan or a box fan may be all that s necessary. I leave a squirrel fan running to inflate the roof, and it provides the air circulation. A box fan can be plugged into a thermostat control to turn it on when the temperature rises.