Outbreak of a Leaf Disease Caused by Pseudocercospora ceratoniae on Carob in Sicily.

Abstract

Carob (Ceratonia siliqua L.), an evergreen tree typical of the Mediterranean flora, has been grown in Sicily (Italy) from time immemorial for its fruits, used mainly as food for cattle and horses as well as for industrial production of alcohol. Although the economic importance of carob as a commercial crop has declined over the last decades, carob trees still are a characteristic aspect of the landscape in southeastern Sicily. In early April 1998, a severe outbreak of a foliar disease was noted on carob trees in the Ragusa province. Symptoms initially consisted of small (2 to 3 mm wide) dark brown, vein-limited spots, visible on both sides of the leaf and, later in the season, surrounded by a pale halo. In a humid atmosphere, spots scattered over the leaf blade but usually were most numerous along the midrib, enlarged, and coalesced, forming large blotches. Severely affected leaflets dropped, leaving the petiole attached to the tree. As a result, the trees appeared defoliated. Severely defoliated trees did not produce fruits. The causal agent of this disease was identified as Pseudocercospora ceratoniae (Pat. & Trab.) Deighton (1), an hyphomycetous fungus reported previously as a pathogen of carob under the name Cercospora ceratoniae Pat. & Trab. (3). On carob leaflets, P. ceratoniae formed grayish caespituli, confined to the lower surface of the necrotic spots. Caespituli consisted of dense fascicles of conidiophores (up to more than 50 conidiophores per fascicle) emerging through the stomata. Conidiophores were simple, slightly ampulliform, and geniculate at the conidial scars, which were conspicuous and unthickened. Old scars often were situated laterally on a short denticle. Conidia, borne singly as terminal blastospores and varying considerably in length, were pluriseptate, filiform, substraight or slightly curved, frequently slightly obclavate, with an obtuse apex and a short constriction at the base toward the truncate, unthickened hilum. Conidia from pure cultures of the fungus grown on water agar under black light were suspended in water and sprayed onto pot-grown carob plants. Inoculated plants were kept in a moist chamber for 48 h and subsequently transferred to the greenhouse. After 12 to 14 days leaf spots similar to those observed on naturally infected trees developed on inoculated plants and the pathogen was reisolated. Control plants sprayed with distilled water remained symptomless. C. ceratoniae had been recorded previously on carob in various Mediterranean countries, including Italy (2), but since has attracted little attention, being regarded as a sporadically occurring pathogen. Both mild temperatures during the winter and exceptionally frequent and persistent rain during the spring may have favored the epidemic outbreak of the disease caused by this fungus.