New insights into resistance of Platanus to Ceratocystis platani, the agent of canker stain
Pilotti, M.; Di Lernia, G.; Brunetti, A.; Tizzani, L.; Lumia, V.
2nd International Symposium on Woody Ornamentals of the Temperate Zone July 1 – 4, 2012, Ghent, Belgium
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Canker stain is caused by the ascomycetous fungus Ceratocystis platani and is the most severe disease in Platanus spp. For several decades, canker stain has been destroying urban tree plantings and natural standings in southern Europe and in North America. As sanitary measures for disease control have proved to be ineffective, selection programmes to obtain resistance to canker stain have been carried out for several years. They have so far enabled: 1) the re-birth of a resistant clone of P. x acerifolia by crossing a resistant P. occidentalis tree and a susceptible P. orientalis tree (Vigouroux and Olivier 2004); and 2) the retrieval of resistance sources in natural P. x acerifolia (Pilotti et al. 2009). Additional work is needed to characterize resistance phenotypes quantitatively, to verify the phenotype repeatability in natural conditions, and to get a sufficient number of resistant genotypes which would prevent Platanus genetic erosion and any emergence of new virulent pathogen races.
This paper provides new insights into the canker stain-resistance-status of Platanus by investigating the resistance/susceptibility phenotypes of P. orientalis seedlings, a species which has not been taken into consideration previously in terms of resistance selection. Ninety-three seedlings were derived from a P. orientalis tree located in Athens (Greece). Four years after inoculation, nine individuals (9.7%) were surviving with an optimal vegetative condition and with an evident resistant reaction at the inoculation point. Three additional rounds of inoculation were performed on multiple ramets of one of these genotypes. All the trials clearly showed both the repeatability of the resistance response and its stability over time. The specific nature of the seedlings was clearly identified by describing morphological characters and by focusing molecular analyses on two distinct genomic regions (rDNA ITS region, and a portion of the gene LEAFY). The importance of finding resistance sources in P. orientalis is discussed. We give a possible explanation for why P. orientalis possesses some resistance to C. platani despite the lack of co-evolution between the host and the pathogen.
We also report for the first time the results of inoculation trials aimed at assessing any variations in infectivity of the pathogen in connection with the inoculum dose and the season at which the infection is carried out. The importance of this testing in terms of improving the protocols of resistance selection is discussed.