Severe Outbreak of Leaf Spot and Blight Caused by Botrytis cinerea on Majesty Palm in Southern Italy.

Sicily is the most important region of Italy for ornamental palm cultivation. Majesty palm (Ravenea rivularis Jum. & H. Perrier) is one of the most stately palms for cultivation in the tropics and subtropics, and has been recently cultivated in containers for indoor and outdoor use in eastern Sicily. R. rivularis, which grows on river banks, is native to Madagascar, and appears to behave as a rheophyte in the seedling stage. This palm is not frost tolerant and will grow in full sun but tends to grow best in partly shaded areas or under greenhouses conditions. Between December and March in 1999, 2000, and 2001, a severe leaf spot and blight was observed on young (6-month- to 3-year-old) plants of majesty palm growing in plastic-covered houses and in open fields in nurseries in Sicily. Affected plants had brown necrotic spots and gray mold on the necrotic leaf tissues. No symptoms were detected in mature (4- to 5-year-old) plants grown in the same nurseries. To isolate the casual agent of the disease, 160 small pieces of tissue cut from leaf spots collected in four nurseries were surface sterilized (20 s in HgCl₂ at 1 g/liter), washed with sterile water, and plated on potato dextrose agar (PDA). In addition, conidia and conidiophores were scraped from the leaf surface, suspended in sterile water, and streaked on the agar surface. After 2 days, single hyphal tips were transferred to PDA. Botrytis cinerea Pers.:Fr. was consistently isolated from affected leaf tissues. Colonies of B. cinerea on PDA were at first colorless and became gray to brown with the development of conidia, which ranged from 5.5 to 10 × 7 to 12 μm (average 7.5 × 9). Sclerotia were black, irregular in size and shape, and from 1.4 to 4.5 × 1.5 to 2.7 mm. Inoculating 8-month-old seedlings of R. rivularis tested pathogenicity of six isolates obtained from different nurseries. Wounded (with a needle) and nonwounded leaves of 10 plants (9 wounds per plant) were sprayed with 20 ml of a conidial suspension (10⁵ conidia/ml) of each isolate. An equal number of noninoculated plants were used as controls. All plants where incubated in a greenhouse at ambient temperature (21 ± 2°C) and 72 h of continuous leaf wetness. Five days after inoculation, leaf spots appeared on most of the wounded (approximately 80%) and the nonwounded (about 10%) leaves. No symptoms were observed on control plants. Koch's postulates were satisfied by reisolation of the fungus on PDA. On the basis of 3 years of observations in eastern Sicily, majesty palms were more readily infected by B. cinerea after rainfall, and freezing temperatures injured young plants. Leaf blight caused by B. cinerea was previously reported in Liguria (northern Italy) on Phoenix canariensis (1). The fungus does not appear to be a major disease problem in cultivated ornamental palms other than R. rivularis in Sicily or southern Italy. However, B. cinerea could be a limiting factor in the cultivation of majesty palm in eastern Sicily, and protective fungicides, especially in winter, are necessary for limiting losses. To my knowledge, this is the first report of B. cinerea leaf spot and blight on R. rivularis. Reference: (1) A. Garibaldi et al. Malattie delle piante ornamentali. Calderini Edagricole, Bologna, 2000.