White rust outbreaks on chrysanthemum caused by Puccinia horiana in Turkey

Chrysanthemum white rust is a destructive disease of many chrysanthemum (Dendranthema X grandiflorum) and related species (Dickens, 1990). The infection is caused by a fungus called Puccinia horiana Henn. This pathogen is an EPPO A2 quarantine pest, and a significant quarantine pest that is not known in Turkey.

Chrysanthemum cut flowers are only grown in Izmir province of Turkey. In 2006, the area under chrysanthemum production was about 5 ha. The major areas where it is grown commercially are Seferihisar, Narlıdere, Torbalı and Urla counties. During February and March of 2007, a devastating outbreak of white rust disease severely damaged the chrysanthemum crop in 12 different glasshouses, resulting in yield losses of 80%. Disease symptoms were observed on various parts of the plant, particularly in the leaves (Fig. 1). The symptoms began as pale green to yellow spots up to 5 mm in diameter on the upper surface of infected leaves. Eventually the spots turned brown and became necrotic. Spore-forming pustules were observed on the lower surface of the leaves and were buff to pink colour. As these pustules matured, they became white in colour. Microscopic examination of the pustules revealed the presence of teliospores on pedicels up to 52 µm long; pale-yellow, oblong to oblong-clavate, slightly constricted , 30-52 x 11-18 µm, with thin walls, 1-2 µm thick, and thicker at the apex, 4-9 µm (Fig. 2). Based on these morphological characteristics, the fungus was identified as P. horiana, as described by Kapooria & Zadocks (1973).

Pathogenicity tests were conducted on 3-month-old chrysanthemum plants, by spraying unwounded leaves with a suspension of basidiospores (approximately 5 x 10⁴ per ml). Control plants were sprayed with sterile water. Inoculated and control plants were enclosed in transparent plastic bags and incubated at 20±1°C for 4 days with a 16-h photoperiod. The bag was then removed. Two weeks after spraying leaf lesions developed on inoculated plants. No disease symptoms were observed on control plants. This is believed to be the first record and outbreak of white rust disease caused by P. horiana on chrysanthemum cultivars grown in Turkey.