Major Locus for Tolerance to Botryosphaeria dothidea in Prunus

Botryosphaeria spp. is a fungal pathogen complex associated with wood infections in multiple perennial plant species worldwide. Isolates from peach orchards in eight different locations across Florida and Georgia were examined using morphological characteristics and nucleotide sequences of the internal transcribed spacer region (ITS). A preliminary survey identified Botryosphaeria dothidea (anamorph Fusicoccum aesculi), B. rhodina (anamorph Lasiodiplodia theobromae), and B. obtusa (anamorph Diplodia seriata) associated with wood canker in the genus Prunus in the Southeastern USA. All species were found to be pathogenic, when they were artificially inoculated on one year old peach seedlings. A survey of subtropical peach germplasm at the University of Florida did not detect significant levels of tolerance to Botryosphaeria canker. However, examination of interspecific crosses using P. dulcis (Mill.) D.A. Webb, and P. kansuensis Rehder detected a potential source of tolerance for B. dothidea pathogen in the almond cultivar ‘Tardy Nonpareil’ (TNP). Pathogenicity tests using conidial suspensions on wounded peach, and peach x almond seedlings confirmed the virulence of B. dothidea strains. A total of 865 F₂ and backcross progeny, generated by open pollinating and backcrossing tolerant peach x almond F₁s to peach, were evaluated for tolerance to Botryosphaeria disease using 3 different techniques: natural infection; infection trellis system for enhanced inoculation, and mycelial slurries of B. dothidea on wounded plants. The results from the three different screening techniques were highly correlated. Linkage analysis identified a region associated with disease tolerance in chimeric chromosome 6-8 in the UFSharp x (Flordaguard x TNP) backcross population. The red leaf phenotype in peach co-segregates with a reciprocal translocation between chromosomes 6 and 8. The Botryosphaeria tolerance locus maps to this chimeric chromosome. Clonal replicates of the backcross population, evaluated on susceptible ‘Flordaguard’ rootstocks at Byron, GA, indicated that susceptibility and tolerance of the scion are independent from rootstock susceptibility. Additionally, tree diameter or vigor was not correlated with disease tolerance. The F2 and BC1 populations confirmed the presence of a single dominant locus associated with tolerance. Mapping of additional SSR markers for tolerance to B. dothidea, and introgression of the tolerance region into commercial quality germplasm is ongoing.

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