Effects of nutritional and environmental factors on Phyllosticta citricarpa, the causal agent of citrus black spot

Citrus black spot, caused by Phyllosticta citricarpa, has been reported in Tunisia since 2020 and has negatively impacted yield and marketability. The environmental conditions required for the growth of P. citricarpa remain poorly understood, limiting further biological and epidemiological studies. In the present study, the effects of different pH levels, culture media, temperatures, and citrus juices on the development of P. citricarpa were evaluated both in vitro and in vivo. The results showed significant differences in mycelial growth based on pH values, culture media, and their interaction. Mycelial growth ranged from 0 cm on OA medium at pH 5 to 7.26 cm on the same medium at pH 9, one month after incubation. Furthermore, P. citricarpa was able to grow within a temperature range of 15°C to 35°C, with optimal colony growth observed at 25°C. Among the citrus juices tested, juice from mature fruits of Citrus aurantium exhibited the strongest inhibitory effect on the mycelial growth of P. citricarpa in vitro and reduced symptom development in vivo assay. These findings enhance our understanding of the optimal conditions for P. citricarpa growth and highlight the potential of C. aurantium juice as a natural inhibitor for managing citrus black spot.