Technical Abstract: Penicillium solitum causes blue mold on apples during storage which results in economic losses. Information pertaining to growth and decay caused by this pathogen is important for developing disease control strategies. Therefore, we evaluated the effect of temperature on decay caused by P. solitum in apples, fungal growth in culture, and quantitatively and qualitatively assayed polygalacturonase (PG) activity. Decay was evident only on apple fruit incubated at 20°C following 21 days incubation. However, decay developed when apples were moved from low temperatures (0, 5 and 10) to 20°C, which shows that the inoculum was viable following cold exposure. P. solitum viability at low temperature was confirmed as decay developed on inoculated apples following incubation for 3 months at 0°C. Lower temperatures reduced conidial germination and mycelial growth on PDA, which may explain the delay in decay development in inoculated fruit which was stored at 0, 5, and 10°C. Crude extracts from P. solitum decayed apple fruit tissue possessed detectable PG activity using the qualitative plate assay at 20°C but not at 0, 5, or 10°C. However, a quantitative method (Gross, 1982) for determining in vitro PG activity using purified enzyme was observed at 0, 5, 10 and 20°C and decreased with temperature. Although P. solitum is able to decay apple fruit following prolonged cold storage, low temperature is beneficial as it delays lesion development by affecting various factors of pathogen virulence.