European Journal of Plant Pathology 107: 787–794, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. Effect of bactericides, phosphonates and nutrient amendments on blast of dormant flower buds of pear: a field evaluation for disease control Emilio Montesinos and Pere Vilardell Institute of Food and Agricultural Technology-CeRTA, University of Girona, Avda. Lluis Santal´ o s/n, 17071 Girona, Spain, and Mas Badia Agricultural Experiment Station, La Tallada, Girona, Spain (Fax: +34972418399; E-mail: emonte@intea.udg.es) Accepted 15 July 2001 Key words: disease control, kasugamycin, fosetyl–Al, ethephon, nutrient amendments, Pseudomonas syringae, pear Abstract Blast of dormant flower buds (BDFB) of pear is a disease of economic importance in the major pear production areas of Europe. To obtain information concerning control measures and disease origin, chemical control trials were performed which included bactericides (kasugamycin and copper), phosphonates (fosetyl–Al and ethephon), and nutrient amendments (boron, calcium, and microelements). Although Cu levels in bactericidal treatments and microelements in nutrient amendments increased significantly in trees, there was no significant effect on disease control. However, incidence of disease was reduced significantly with phosphonate derivative compounds, and effects were observed only the year after the treatment was performed. Additional field trials were done to determine optimum dose and application timing, and a schedule consisting of three spray applications of fosetyl–Al (240 g a.i. hl -1 ) during May and June was the most effective treatment. This schedule was evaluated in 31 field trials performed in commercial orchard plots from 1989 through 1998. In the year after the treatment, average disease incidence decreased in 30 of 31 trials. The decrease of disease incidence was significant in 71% of the trials (average decrease of 46%). Neither the presence nor the population levels of P. syringae were consistently related to disease levels nor to the fosetyl–Al treatment effects on blast incidence of dormant flower buds. Introduction Blast of dormant flower buds (BDFB) of pear is a disease of complex origin characterized by a par- tial or complete necrosis of flower buds during tree dormancy or budbreak (Montesinos and Vilardell, 1991a,b). Necrosis may affect internal primordial flow- ers, leaves and scales, and usually starts at the tip part of the bud and progresses to the base. Depend- ing on disease severity, flowers per cluster may be reduced, buds may produce abnormal flowers, or buds may be killed completely. Later, axilary buds appear at the base of necrotic flower buds or bud scars. When disease incidence is high, vegetative growth of trees is delayed. Then, trees become alternate bear- ers annually. These symptoms are similar to those described for blossom blast of pear and stone fruits (English et al., 1980). The disease caused severe pro- duction losses in Europe in some years and has been reported in Spain (Montesinos and Vilardell, 1987; 1988), France (Giraud M, CTiFL, pers. comm.) and Italy (Miglio G, pers. comm.). Certain cultivars, espe- cially young trees of Conference, Doyenne du Comice, Abate F´ etel, and General Leclerc are very susceptible. Others, such as Williams (sin. Bartlett), Passe Crassane, and Alexandrine are less susceptible. The aetiology of the disease is complex. Pseudomonas syringae pv. syringae and associated ice nucleation active bacteria have been related to symp- tom development in cold years, and Koch’s postulates have been completed for P. syringae (Montesinos and Vilardell, 1987; 1991b). It has been suggested that abiotic stresses are also associated with the disease. The nutritional state of trees during dormancy and