BRIEF HISTORICAL ACCOUNT OF OLIVE LEAF SCORCH ("BRUSCA") IN THE SALENTO PENINSULA OF ITALY AND STATE-OF-THE-ART OF THE OLIVE QUICK DECLINE SYNDROME
"Brusca" (leaf scorch or marginal leaf burn) is a disease of olive (Olea europaea) typically characterized by the desiccation and death of tissues at the tip and/or along the edge of the leaf blade, which can be followed by defoliation. Since the end of the 18th century this disorder has reappeared periodically, after long periods of quiescence, in the same olive-growing areas of the province of Lecce (Apulia, Salento peninsula, southern Italy). Over time, this disease has been the object of repeated investigations, first by a couple of local physician, who were the authors of its description, then, since the beginning of the 20th century, by professional plant pathologists. These studies have established that "brusca" may have multiple origins as determined by abiotic ("brusca non parassitaria" = non parasitic scorching) or biotic ("brusca parassitaria" = parasitic scorching) causes. "Brusca non parassitaria" can be induced by any physical cause that affects water supply to the leaf margin cells (e. g. insufficient moisture in the soil, water is lost too quickly from the leaves to be replaced adequately, damaged roots), or by hot dry winds, salty winds, nutrient deficiency/toxicity. By contrast, weak foliar pathogens (i.e. the discomycete fungus Stictis panizzei) or xylem-invading fungi or bacteria that plug the water conducting vessels can, in principle, be the agents of "brusca parassitaria". These agents may include Xylella fastidiosa, a xylem-limited bacterium, which has recently been found associated with an olive disease denoted "quick decline syndrome". This disease occurs in some areas of the Lecce province where severe cases of "brusca" have repeatedly been observed in the past. This coincidence, and the type of symptoms (extensive scorching and desiccation of the canopy) that somewhat recall those described in the early literature, may lead one to speculate that X. fastidiosa has been sitting in the area for about 250 years, rather than being a pathogen of recent introduction. This is not the case, as discussed in the present paper.