Preliminary investigations on Italian strains of Alternaria causing a new apple disease

A new apple tree disease, probably caused by a new pathotype of Alternaria alternata, was observed for the first time in Alto Adige (North-East Italy), in the year 2000. In the following years the disease spread southwards in Trentino, Veneto, Piedmont and, perhaps, Emilia-Romagna. The disease usually appears on the leaves in late spring as little, irregular to round, brown-reddish or black spots, sometimes surrounded by a reddish halo. Spots increase in number during the season and coalesce to form wide necrotic areas. Affected leaves can fall prematurely. Brown-blackish spots, 0.5 to 2 mm in diameter, appear also on the fruits. Incidence of affected fruits varies over the year and generally increases in the pre-harvest period. Preliminary investigations showed that the Italian strains of Alternaria from both leaves and fruits are probably able to cause the disease by producing host specific toxins, but these strains do not belong to the pathotype mali of A. alternata present in other apple-growing areas of the world. A bioassay was developed for detecting the Alternaria strains pathogenic to apple. Different inoculation methods were tested on wounded or unwounded leaves and fruits. A ring-test was carried out using five fungal isolates to evaluate repeatability and reproducibility of the bioassay. Necrotic spots consistently appeared only on wounded tissues of inoculated organs after 24-48 h of incubation. Availability of a robust bioassay is the first step for studying population dynamics of the apple-pathogenic strains, characterizing strains at the morphological, ecological, biochemical, and molecular levels, and investigating host–pathogen interactions.