First report of Alternaria mali causing necrotic leaf spot of apples in Turkey

During surveys performed in apple orchards in Isparta province, small, circular, brown-bordered, purplish-brown spots were seen on apple leaves (Fig. 1). The enlarging spots coalesced and became darker on some apple cultivars (Fig. 2). One fungus was repeatedly isolated from leaf samples obtained from 6 different locations in the province. It formed dark olive, circular, velvety colonies on PDA. Mycelia were septate, pale brown and had an average diameter of 4.2 mm. Conidia, with an average size of 20.6 x 9.25 mm had both transverse and longitudinal septa and a short false beak and were formed in long chains. The pathogen was identified as Alternaria mali (Bulajic et al., 1996; Sawamura, 1990).

Pathogenicity of A. mali isolates was tested by inoculation with a conidial suspension (106 conidia per ml) on detached, wounded apple leaves and on apple seedlings in orchards. Symptoms were observed on detached leaves three days after incubation in a humidity chamber at 25°C and were similar to those developing on leaves of inoculated seedlings (Fig. 3). Reisolations yielded the same fungus. In addition, isolates were grown on Czapek-Dox medium for 6 days at same temperature, without shaking. At the end of this period culture filtrate was obtained and applied to the underside of wounded leaves as described in Johnson et al., 2000. Necrotic spots were observed on the leaves 24 hours after the inoculation (Fig. 4). These results supported the contention that A. mali causes disease by producing a specific toxin. This is the first report of Alternaria necrotic leaf spot of apples caused by A.mali in Turkey.