First Report of Downy Leaf Spot of Walnuts caused by Microstroma juglandis in Turkey

In the spring of 2001, diseased trees of Juglans regia L. in Hatay, in the Mediterranean region of Turkey, showed typical symptoms of downy leaf spot, especially following moist and cool conditions. A survey was conducted in the affected regions, which included 12 orchards in Antakya and 5 orchards in Altinozu districts. Disease prevalence (% fields with plants showing foliar symptoms) in these locations was 83% and 60%, respectively. Disease incidence (% of plants with foliar symptoms) ranged from 69% to 87% (of between 15 and 36 samples per location). Initially, disease symptoms were characterised by a whitish efflorescence containing fungal structures and spores on the lower surface of the affected leaf, often concentrated along the veins, and a yellowish discoloration on the corresponding upper surface of the leaf. In the later stages of disease development, as the leaf spots enlarged, diseased tissues became necrotic. Sometimes, the leaves of infected walnut trees were abnormal and trees became defoliated. Examination of leaf infections using scanning electron microscopy revealed that whitish polygonal efflorescences were basidial structures forming oval basidiospores (6.2 to 7.8 x 2.9 to 3.4 µm).

The fungus was identified as Microstroma juglandis (Berenger) Sacc. (Hymenomycetes, order Exobasidiales), based on morphological and cultural characteristics of the basidium and basidial structures. Isolations on potato dextrose agar using small pieces of surface-sterilised leaf tissue yielded white or cream-colored, mucoid and yeast-like colonies (García-Jimënez et al., 1995). Colonies consisted of ellipsoidal to ovoid or clavate cells that reproduced by unipolar budding (Arx et al., 1982). Pathogenicity tests were performed on leaves of walnut seedlings by spraying with a spore suspension (approximately 10⁷ conidia per ml) and covering the plants with polyethylene bags for three days at 25±2°C and 75-80% relative humidity. Control plants were treated with sterile distilled water. Symptoms, identical to those observed in walnut orchards, became visible 21 to 24 days after inoculation. However, lesion development was mild because of the excessively warm conditions (30±2°C). In eastern Spain, downy leaf spot on walnuts was previously reported from the province of Valencia (García-Jimënez et al., 1995). This is the first report of downy leaf spot on walnuts caused by Microstroma juglandis from Turkey. The disease is generally not considered economically important. However, the impact of downy leaf spot in the eastern Mediterranean region of Turkey in 2001 was significant and caused serious concern to walnut growers. The survey indicates that downy leaf spot may be emerging as a new problem for the walnut growers in Hatay and other areas of Turkey.