Answer: It's so disappointing to have your nearly-ripe fruit rot on the vine! The problem may be caused by a fungal disease called anthracnose. It thrives in the conditions you describe. You have to look closely to detect the early stages of the infection: small sunken spots on the skin of the fruit, which gradually enlarge in concentric circles from the original sunken spot, causing rot as they grow.
It's rarely a problem when the weather cooperates, but to be safe, avoid overhead watering, supplying moisture at the base of the plant instead. Keep fruit off the ground by training plants to stakes or trellises, since fruit close to the soil is more succeptible. Pick fruit regularly as it ripens - if you leave it on the vine, it becomes more vulnerable to infection.
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