Digital picture analysis for precision screening of sour cherry progeny for necrotic leaf tissue caused by Blumeriella jaapii

Sour cherry generally holds limited resistance to attack by cherry leaf spot fungus (Blumeriella jaapii) but differences in field performance are found between cultivars and in breeding progeny. Traditionally the degree of attack is estimated visually together with the type of reaction on the leaves. A more precise and less subjective method of evaluation is of interest for screening germplasm. A method of drying leaves, scanning dried leaves and a digital picture analysis protocol was developed to provide exact data on the percentage area of attacked necrotic tissue per leaf and with options of analysing size distribution of single necrotic spots to reveal detailed patterns of infections. The new method was used to characterize leaf attack in autumn on 20 individual leaves from single trees from a sour cherry breeding genepool of over 100 genotypes in a non-sprayed situation with high inoculum present. The percentage of necrotic leaf surface varied highly within the 20 leaves from one genotype, being dependent on the level of inoculum dispersal and air humidity/precipitation over time and space. The mean percentage of attacked leaf surface varied from 0 to almost 35% of total leaf area and the maximum from 0 to 60% necrotic leaf area, indicating the potential large impact on photosynthetic capacity in attacked trees. The distribution of and absolute size of single necrotic spots demonstrated differences in how individual genotypes react to infections, from small spots in tolerant genotypes to large collapsing necrotic spots in very susceptible genotypes. A comparison of attack data with the percentage of remaining leaves on a tree in autumn revealed that some genotypes retain leaves very late even if attacked by leaf spot disease. These trees may maintain a fairly high photosynthetic capacity even if some percentages of the leaf have become necrotic.