Infection and development of apple scab (Venturia inaequalis) on old leaves

Apple leaves were inoculated artificially and naturally with conidia of Venturia inaequalis under glasshouse and outside conditions. Fungal development on the inoculated leaves was examined microscopically. Results showed that conidia can infect the under surface of leaves of any ages, ranging from newly emerged leaves to ones that are up to 4 months old. On young leaves, mycelia grew at a faster rate and thus led to visual symptoms sooner than on old leaves. Mycelia on very old leaves grew so slowly that they did not lead to visual symptoms at the time of leaf-fall, even though some of these symptomless colonies had already produced conidia when viewed microscopically. A considerable amount of symptomless infections may therefore be expected on old leaves in autumn. As a consequence, these symptomless infections may have considerable impact on the amount of primary inoculum in the following season and thus on disease management.