The pathogen invades host plant cells causing plant death. The pathogen spreads by movement of asexual spores: sporangia (airborne spores) and zoospores (water-swimming spores). In some countries of Europe and South America, the pathogen can produce a soil-persistent spore known as the oospore, but this spore has not been identified in the United States. In the united states, this fungus-like organism overwinters primarily in plant debris as mycelia, a filamentous thread-like growth of the pathogen. Phytophthora infestans is the oomycete, or water mold pathogen, responsible for tomato late blight. White, fuzzy pathogen growth can also be found in association with the fruit lesions. Symptomatic tomato fruits appear mottled, often with golden to dark brown, firm, sunken surfaces. Discoloration may also occur on the flowers, causing them to drop. Stem and petiole lesions are brown and are typically not well defined in shape. White, fuzzy growth may be found on the undersides of leaves or on lower stems. When the weather is very humid and wet, late blight infections can appear water-soaked or dark brown in color, and are often described as appearing greasy.

Infected leaves typically have green to brown patches of dead tissue surrounded by a pale green or gray border. Symptoms of late blight may be found on any above-ground part of the tomato plant. However, it is most infamously known for its destruction of potato, S. Late blight is a plant disease that affects a variety of Solanum spp., including eggplant, pepper, nightshade weeds, and petunia.

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