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First Report of Pestalotiopsis maculans Causing Necrotic Leaf Spots in Nursery Plants of Arbutus unedo and Ceratonia siliqua in Spain

Forestation of agricultural lands has led to a great increase in the production of plants in forest nurseries in southern Spain. During a disease survey of several nurseries, performed in 1998 and 1999, a necrotic leaf spot was found causing defoliation in seedlings of two mediterranean forest species, the ericaceous shrub Arbutus unedo and the leguminous tree Ceratonia siliqua. The affected plants were 9 to 24 months old and growing in fertilized peat in containers. Symptoms on both species consisted of large necrotic lesions (up to 20 mm in diameter) that were located mainly, but not exclusively, along the leaf margin. On A. unedo, necrotic spots were bordered by a red halo, while on C. siliqua the halo was dark. The fungus consistently isolated from both hosts was identified as Pestalotiopsis maculans (Corda) Nag Raj (=Pestalotiopsis guepinii), based on morphological characters (1). Acervular conidiomata (up to 200 μm in diameter) developed on the necrotic lesions of leaves incubated at 100% relative humidity and 20 to 24°C, and in cultures on potato dextrose agar (PDA) at 20 to 24°C over the course of 7 days. All isolates had 5-celled smooth conidia. Apical and basal cells were hyaline, while the three median cells were brown; the upper two were darker than the lower one. Conidia were 22 to 30 μm (mean length) and 5 to 9 μm (mean width). There were typically three (range 1 to 4) apical appendages averaging 17 μm long. The average basal appendage was 6 μm long. One-year-old seedlings and detached leaves from healthy field trees of A. unedo and C. siliqua were sprayed to runoff with an aqueous conidial suspension (10⁶ conidia ml^-1) of two isolates of the fungus. All inoculated and control plants sprayed with water only were incubated in a growth chamber at 100% relative humidity and 20 to 24°C for 48 h and then in the same growth chamber at 50 to 80% relative humidity or in the greenhouse (10 to 30°C and 40 to 80% relative humidity) until symptoms developed. Detached leaves were incubated to prevent desiccation in a humid chamber at 100% relative humidity and 20 to 24°C for 2 months. After a period of 2 to 4 months, lesions developed on all inoculated leaves but not on noninoculated controls. Lesion morphology on both hosts was similar to that observed in naturally infected plants in the nurseries. P. maculans was reisolated from lesions of all infected leaves but not from control leaves. Although other species of Pestalotiopsis have been reported infecting leaves of C. siliqua (1), to our knowledge, this is the first report of a Pestalotiopsis sp. on A. unedo and of P. maculans on C. siliqua.

Reference: (1) T. R. Nag Raj. Coelomycetous Anamorphs with Appendage-Bearing Conidia. Mycologue Publications, Waterloo, Ontario, Canada, 1993.