HIGH MAGNITUDE OF FIRE BLIGHT SYMPTOM DEVELOPMENT AND CANKER FORMATION FROM JULY ONWARDS ON TWO APPLE CULTIVARS UNDER SEVERE NATURAL INFECTIONS
S.G. Aćimović, J.S. Balaž, D.Đ. Aćimović, P.D. Reeb
Fire blight (Erwinia amylovora) causes blossom and shoot blight ultimately forming cankers on woody tissues. The magnitude of late season symptom development has not been reported on apple in natural infections. Quantitative and qualitative changes of fire blight symptoms were monitored from July 2007 to April 2008 on cultivars Idared and Golden Delicious in northern Serbia with Idared displaying the highest number of symptoms. In July, shoot blight dominated with 67-88% necrotic shoot tips and completely necrotic shoots. Early blight led to formation of 7-18% cankers of determinate type. Between mid-July and end of September, the number of cankers increased from 65 to 80% on both cultivars. New cankers were of indeterminate type, suitable for pathogen overwintering. Cankers originated from extensive late season pathogen stem invasion from previously diseased shoots. Necrosis size on both cultivars increased significantly between July and September. Increases in canker number and necrosis size indicate a high level of renewed pathogen activity late in the season and its ‘preparation’ for overwintering. Poor winter pruning in one orchard resulted in considerable number of symptom units which constituted potential infection sources. The high magnitude of late season symptom changes, detected under severe natural infections, shows that the pathogen is vigorously active within previously formed symptoms and that pathogenesis continues clandestinely. The results underline the importance of the often neglected part of late season fire blight development for pathogen survival and enforce the requirement for eradication of cankers as primary sources of inoculum for the next season.