In humid weather and in early mornings, a fuzzy mould can often be seen on the underside of the blotches or on the stem lesions. This fuzzy growth contains spores of the fungus. The spores spread in wind and water and cause new infections in wet weather. They can spread 20 kilometers or more. Rain-spread spores can cause infection even in a garden where tomatoes or potatoes have not been grown before.
Ripe fruit become soft and mushy if invaded by secondary bacteria.
Infected potato tubers usually have large brown sunken areas on the outside. When cut open, the browning can be seen to extend for a few centimeters beneath the skin into the tuber. Light brown streaks may also be seen throughout the flesh. Eventually, tubers may rot completely. Some of these strains are highly aggressive and can kill plants completely in a few days, if the weather is cool and wet.