First Report of Alternaria Leaf Spot on Camellia in Italy.

Abstract

Camellia cultivation has a long history in the Lake Maggiore area of northern Italy where a wide selection of varieties is present. Camellias are appreciated for their large, colorful flowers that bloom from late fall through early spring. In July 2005, a previously unknown foliar disease was observed on a collection of 2- to 12-month-old camellia cultivars (Camellia japonica) grown in several nurseries located in the Verbania Province (northern Italy). The disease was observed on plants grown in pots (10 to 24 cm in diameter) that were maintained either in the open or in a greenhouse and was present for the entire growing season. However, symptoms were more severe during the summer with temperatures ranging between 25 and 30°C with high relative humidity values. During the months of June and July of 2005, severe attacks involving as much as 70% of plants were observed on C. japonica cvs. Mrs. Tingley, Burnside, Hagoromo (synonym Magnoliaeflora), and Giuseppe Traverso. The disease was again observed in 2006. On the upper side of the younger leaves, small necrotic spots (3 to 8 mm in diameter) initially developed mainly at the margin of the leaves and near the petioles. Necrotic areas were surrounded by a chlorotic halo that turned progressively black. The necrotic areas often coalesced, generating larger spots with a diameter ranging from 15 to 30 mm. Severely affected plants were defoliated. Infected plants sometimes died. The presence of lesions on mature plants decreased aesthetic quality and market value. Leaf spots contained dark brown, multicellular, pyriform conidia. Conidia, generally in short chains, were 20.5 to 34.8 μm (average 29.3 μm) long, 6.9 to 12.2 μm (average 9.9 μm) wide, with 3 to 4 longitudinal cross walls, and an average of 5.7 single cells. From 15 samples of infected leaves, several isolates of a fungus identified on the basis of its morphological characteristics as belonging to the Alternaria alternata complex (2) were consistently isolated on potato dextrose agar containing 25 mg/l of streptomycin sulfate. Pathogenicity tests were performed by spraying leaves of healthy 6-month-old potted C. japonica cv. Burnside plants with a spore and mycelial suspension (1 × 105 CFU/ml) prepared by using a mixture of three isolates obtained in 2005 grown on PDA for 30 days at 23 ± 2°C in a growth chamber (12 h of light per day). Plants without inoculation served as a control. Five plants were used for each treatment. Plants were covered with plastic bags for 3 days after inoculation and maintained at 25°C in growth chambers. The first lesions developed on leaves 3 days after inoculation, while control plants remained healthy. Sixty days after artificial inoculation, 25% of the inoculated plants were dead, while the control plants remained healthy. From lesions of infected plants, a fungus belonging to the A. alternata complex was consistently reisolated. The pathogenicity test was carried out twice. The presence of A. alternata on C. sinensis, the commercial tea plant, was reported in India (1). Previously, a flower blight caused by A. tenuis was reported in the United States (3). This is, to our knowledge, the first report of A. alternata on C. japonica in Italy and probably in the world. The disease was present in 2005 and 2006 in several commercial nurseries affecting 50% of plants of susceptible cultivars.