That's either late blight or gray mold, and the way to say for sure is to look at affected fruit. Late blight has a characteristic oily dark looking big roundish splotches on the fruit, usually on the underside which are not soft (when the fruit is green) and somewhat bumpy.
Prevention is usually the same. Better conditions if possible, if not spraying with copper octanoate as preventive will help some but not much (there are also other more aggressive alternatives).
Better conditions mean: 1. no rain on the plants (greenhouse or at least a transparent roof over them), makes the biggest difference.
2. better spacing. That jungle you have there is fungus heaven. In late blight territory it's single stem or bust.
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