Fungal isolates of M. laxa obtained from infected apple trees grown on PDA.
Brown rot fungus Monilinia laxa (Aderh. & Ruhl.) Honey is an important plant pathogen in stone and pome fruits in Europe. In 1997 severe and unusual dying of apple shoots in Slovenian orchards were observed (Celar and Valic, 1999). Isolated pathogen was identified as M. laxa, using classical plant pathology approach. Due to uncertain identification of causative agent induced inoculations using fungal isolates obtained from infected apples were performed in apples, apricots and sour cherries during the following year. As characteristic symptoms occurred only on infected apples predictions have therefore been made about a possible existence of host-specific biological specialization of the pathogen.
Two studies have been carried out to address existence of formae speciales within M.laxa isolates:
By amplified fragment length polymorphism (AFLP) marker system the genetic diversity and relationships among 67 Monilinia laxa isolates obtained from different host plants (apples, apricots, sweet cherries, peaches, plums) were assesed. Genetic diversity analysis showed significant differences between isolates from apple trees and isolates from other host plants. No further grouping according to any other host plant was observed. The results indicate host specialization of apple isolates and support the taxonomic grouping of apple isolates as M. laxa f. sp. mali Harrison.
A proteomic approach was applied in a study of M. laxa isolates from apples and appricots in order to analyse differentially expressed proteins in terms of host specifity and identification of candidate proteins for diagnostic marker development. Multivariate statistical analysis (PCA) discriminated isolates from two different hosts, providing new data that support the existence of a M. laxa specialized form f. sp. mali, which infects only apples. 41 differentially expressed proteins were identified and some proteins expressed only in apple isolates have been described as virulence factors in other fungi. Ten proteins identified only in apple isolates are potential candidates for the development of M. laxa host-specific diagnostic markers.