Different methods of evaluation of Monilinia laxa on apricot flowers and branches

Organic apricot production is currently not profitable. The main obstacle to sustainable profitability is the brown rot disease caused by the fungus Monilinia laxa. Differences exist in the apricot germplasm in the brown rot susceptibility. A good evaluation of Monilinia is essential for a precise diagnosis of the disease and for exhibiting differences between tolerant and susceptible cultivars and genotypes. For this purpose, different evaluation methods of disease susceptibility were developed and compared in a bi-parental population between 'Bakour' (tolerant to M. laxa) and 'Bergeron' (susceptible). The method consists in the observation of naturally infected trees on open field assessing the percentage of twigs showing symptoms of M. laxa infection. However, this method is weather dependent in relation with temperature, humidity and rain period during the flowering and needs to take into account the annual variation of Monilinia climatic contamination risk for each tree as a covariate. For this reason, new methods of evaluation of M. laxa were developed under the controlled conditions. In the first new method, a spore suspension was sprayed on flowers of each genotype, the second method consisted in putting exactly one drop of spores on the pistil, and finally for the third method a plug of mycelium was placed on the branch of each genotype, after removing the bark. Different levels of infection were observed within the bi-parental population for the three controlled phenotyping evaluations. No good correlation between the visual assessment in the field and the new controlled evaluation methods on branches or flowers was found. Surprisingly, symptoms in petals were not linked with the Monilinia infection according to our observations. However, a segregation was observed for the visual assessment and for the evaluation test on branches. Based onto the observed variability, a QTL approach can be applied for assessing the genetic components involved in M. laxa resistance.