Journal of Plant Pathology (2014), 96 (1), 159-168 Aćimović et al. 159 HIGH MAGNITUDE OF FIRE BLIGHT SYMPTOM DEVELOPMENT AND CANKER FORMATION FROM JULY ONWARDS ON TWO APPLE CULTIVARS UNDER SEVERE NATURAL INFECTIONS S.G. Aćimović 1 , J.S. Balaž 1 , D.Ð. Aćimović 2 and P.D. Reeb 3,4 1 Plant Bacteriology and Fruit Crop Mycology Laboratory, Department of Phytomedicine and Environment Protection, University of Novi Sad, Faculty of Agriculture, Trg Dositeja Obradovića 8, 21000 Novi Sad, Serbia 2 Viticulture and Enology Laboratory, Department of Horticulture, 1066 Bogue St., A18 Plant and Soil Sciences Building, College of Agriculture & Natural Resources, Michigan State University, East Lansing, MI 48824, USA 3 Department of Fisheries and Wildlife, 480 Wilson Road, 180 Natural Resources Building, College of Agriculture & Natural Resources, Michigan State University, East Lansing, MI 48824, USA 4 Department of Statistics, College of Agricultural Sciences, Ruta 151, km 12.5, Comahue National University, Cinco Saltos 8303, Rio Negro, Argentina SUMMARY 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. Quantita- tive 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 dis- playing 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 indeter- minate type, suitable for pathogen overwintering. Cankers originated from extensive late season pathogen stem inva- sion from previously diseased shoots. Necrosis size on both cultivars increased significantly between July and Septem- ber. 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 symp- tom 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 un- derline 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 pri- mary sources of inoculum for the next season. Key words: fire blight, Erwinia amylovora, apple, late season symptom development, cankers, Idared, Golden Delicious. INTRODUCTION The fire blight agent Erwinia amylovora, the most de- structive bacterial disease of pome fruits, inflicts severe economic losses to apple (Bonn and Van der Zwet, 2000; Norelli et al. , 2003). Disease severity varies between years, depending on the abundance of overwintering inoculum, host and cultivar susceptibility, crop management system and environmental conditions (Van Der Zwet and Keil, 1979). Fire blight is characterized by specific symptoms. Early spring infections cause blossom and fruitlet blight followed by further spreading of infection that leads to the development of shoot blight. The rapid progression of infection in these succulent organs leads to the invasion of woody tissues of twigs and branches and to formation of dark cankers with depressed bark and raised cracked mar- gins (Crosse et al. , 1972). These cankers have well-defined (determinate) edges. They form through the defense activ- ity of cambium, which produces callus and a periderm bar- rier of lignified and suberized tissue around diseased parts (Schouten, 1989; Biggs, 1994; Rosenberger, 2007). Thus, the pathogen is limited in further advancement and rarely overwinters successfully in this type of canker (Beer and Norelli, 1977; Schouten, 1993; Steiner, 2000). Shoot infections established later in the spring and during summer are restrained in progress due to the on- coming hot and dry conditions which coincide with shoot maturation and growth cessation (Van Der Zwet and Keil, 1979). Only portions of the succulent top of infected shoots necrotize. However, necrosis can resume with the favorable moist and warm conditions of autumn (Billing, 2006). Depending on the size of the previously necrotized shoot portion, the renewed advancement of necrosis leads to formation of cankers with uncallused (indeterminate) edges. These newly formed cankers have dark undented bark, diffuse and rarely cracked margins. They develop mostly later in the season from blighted shoots and late infected flowers (Beer and Norelli, 1977; Steiner, 2000; Edizioni ETS Pisa, 2014 Corresponding author: S.G. Aćimović Fax: +1.517.353.5598 E-mail: acimovic@msu.edu; srdacix@gmail.com