Polyphasic classification of Alternaria isolated from hazelnut and walnut fruit in Europe.

Brown apical necrosis of English walnut and grey necrosis of hazelnut are destructive fruit diseases caused by a complex of opportunistic fungi including several small-spored catenulate Alternaria taxa. Thirty Alternaria isolates recovered from walnut and hazelnut fruit that were pathogenic on their respective host were compared along with type or representative isolates of A. alternata, A. tenuissima, A. arborescens, and A. infectoria using morphological and molecular criteria. Morphological examination using standardized procedures separated the walnut and hazelnut isolates into three morphological groups: the A. alternata group, the A. tenuissima group, and the A. arborescens group based upon common characteristics of the conidium and the sporulation apparatus. To evaluate genetic relationships among these groups, AFLP markers, inter simple sequence repeat (ISSR) markers, and histone gene sequence data were compared. Based upon AFLP data, the A. alternata and A. tenuissima groups comprised a single lineage, and the A. arborescens group comprised a separate lineage. ISSR data supported the grouping by AFLP data except for three isolates of the A. alternata group that clustered with the A. arborescens group. Base substitution of the H4 gene supported the discrimination of the A. arborescens group from the A. alternata and A. tenuissima groups. Tests of hypotheses based upon groupings derived from the various data sets supported the discrimination of the A. arborescens group but did not support the discrimination of the A. alternata group from the A. tenuissima group.