Phenotypization of citrus plants for tolerance towards citrus bot gummosis disease in Italy

Citrus bot gummosis is a widely distributed disease in Mediterranean environments. Symptoms consist of bark cracking, gummosis of main branches and internal necrosis. Among the involved fungi Botryosphaeriaceae species are predominant, especially in a new scenario where climate change is modifying the geographical distribution and the host range of many pathogens, revealing unexpected diseases, mostly in young orchards. Within Botryosphaeriaceae, Neofusicoccum parvum is one of the main pathogens. Marker-trait association studies and their application in molecular breeding are widely considered the most powerful strategies for the selection of new citrus cultivars. Since the main bottleneck of these kinds of studies is represented by the obtainment of a robust phenotypic data set, in this study a phenotyping assay for assessing resistance/susceptibility towards N. parvum was set up and performed on both a citrus germplasm collection, consisting of 40 genotypes belonging to sweet orange, mandarins, clementines, bergamots, citron, grapefruit and lemon groups, and on individuals (112) of a lemon segregating population obtained by crossing lemon ‘Interdonato’ × lemon ‘Femminello Siracusano 2Kr’. Phenotyping was performed by inoculating eight 20-cm detached twigs per genotype with a mycelial plug of N. parvum. Twigs were incubated in plastic boxes and maintained in a growth chamber at 25°C. Final assessment of disease development (lesion length) was conducted five days after inoculation. Preliminary results in citrus genotype assays revealed the wide variability in the trait of interest expressed by the considered citrus germplasm, even within the same genotype group, where the necrosis length ranged from 0.47 cm in the blood orange ‘Tarocco Gallo’ to 6.36 cm in the clementine ‘Comune CMC1’. In the lemon segregating population, necrosis length varied from 0.53 to 14.65 cm. The phenotyping characterization will enable a marker-trait association analysis identifying the genetic determinants of tolerance towards citrus bot gummosis disease.