Ontogeny-Dependent Development of Diffuse Abaxial Scab Symptoms on Apple Leaves

Diffuse apple scab symptoms on the abaxial leaf surface are caused by the fungal pathogen Venturia inaequalis and occur even in well-managed orchards. The disease is challenging to manage, as the symptoms emerge on aging leaves later in the season and only develop on some leaves, suggesting the involvement of intrinsic defense mechanisms. We studied the development of diffuse abaxial scab symptoms on 'Jonagold' leaves in an orchard at three different shoot positions and at four different time points during the growing season to identify leaf-specific defense mechanisms. We identified infection moments by tracking the ascospore load and formation of new shoot leaves and determined the period required for proliferation of diffuse abaxial scab symptoms. Then, we correlated gene expression and phenolics profiles with the V. inaequalis DNA content to identify those inhibiting scab proliferation in asymptomatic leaves. Leaves in the middle of shoots showed higher symptom expression compared with shoot base and tip leaves. Disease inhibition in asymptomatic leaves was negatively correlated with several defense-related genes and phenolics. Altogether, our observations highlight that diffuse abaxial scab is one of the key challenges in apple growing and that the leaf ontogenic status during the infection process affects the development of these symptoms, with putative regulation by phenolic metabolism and ontogeny-related defense genes.