We've balanced the soils on many kiwifruit orchards using comprehensive soil tests and the right fertilisers. PSA only shows up sporadically in shady or exposed parts of orchards. It all starts in the soil. When the soil is balanced, the plant's immune system is fully functional. From there, the plant can self-protect and self-heal.
Look after the microbes. Add carbon. Build organic matter. Get the big four elements up to luxury levels. Your crops will be larger and earlier. They will not go off in cool storage. Insect attack will reduce to a minimum.