Screening of Blackberry and Raspberry Cultivars for Susceptibility to Fire Blight Disease in Turkey

Abstract

Fire blight is the most destructive bacterial disease of rosaceous plants containing berry fruits. The present study was conducted to determine host resistance among the extensively grown raspberry cultivars (Willamette, Rubin ve Aksu Red, Heritage, and Royalty) and blackberry cultivars (Bursa-1, Bursa-2, Bursa-3, Chester, Loch Ness, and Jumbo) to Erwinia amylovora, which is the causal agent of the fire blight disease. In the greenhouse and field experiments, the plants were inoculated with two virulent strains of E. amylovora to evaluate whether cultivar–strain interactions exist. Disease index (%) and disease severity (%) were determined regarding diseased plant symptoms and classified into four susceptibility groups (HR, MR, S, and HS). Raspberry cultivar Willamette and blackberry cultivar Bursa-1 appeared to be highly susceptible with average 50% disease severity ratings. One of the raspberry cv., Royalty, was moderately resistant with an average 25% disease severity rating. This is the first study determining resistance reactions of native and common blackberry and raspberry varieties against fire blight in Turkey. As the cultivation of berries expands to new regions and larger acreages, a serious outbreak of fire blight may be potentially much more destructive in the future. Therefore, phytosanitary measures are needed to prevent any further spread of the bacterium to new blackberryand raspberry-growing areas. The study will serve as an initial guide for growers and breeders for their appropriate raspberry and blackberry selections in Turkey. Blackberry (Rubus fruticosus) and raspberry (R. idaeus), belonging to genus Rubus, family Rosaceae, are widespread perennial shrubs. They are known to contain naturally occurring polyphenol antioxidants that can regulate certain beneficial metabolic processes in mammals. Also, they have a high abundance of healthy antioxidants and nutrients such as anthocyanins, salicylic acid, ellagic acid, pectin, B and C vitamins, and fiber. In herbal medicine, they are reputed to be effective in regulating many health problems (Ahn et al., 1996; Carvalho et al., 2013; Heinonen, 2007; Sellappan et al., 2002). The berries have been grown in many parts of world including the United States and Europe. Turkey is one of the leading countries in berry cultivation and especially blackberry has gained considerable importance during recent years. Consumption of wild berries used to be popular, especially in the Black Sea region; however, in recent years, cultivated berries are grown in large areas as a result of breeding studies. The Marmara and Black Sea regions are the most environmentally desirable areas for raspberry production (Onur, 1996). Adaptation performance of blackberry cultivars imported from the United States has shown variable performance in 16 regions of Turkey for 9 years (Agaoglu, 2003). Ipek et al. (2009) investigated genetic diversity of some blackberry cultivars (Arapaho, Black Satin, Bursa-1, Bursa-2, Bursa-3, Chester, Dirckson Thornless, Jumbo, Navaho and Loch Ness, and Boysenberry) using amplified fragment length polymorphic markers. In addition, 12 different raspberry cultivars: Summit, Heritage, Williamette, Meeker, Tulameen, Cola II, Newburgh, Canby, Rubin, Aksu Red, Bursa Dwarf, and Yalova-I, were tested for plant adaptation studies (Barut, 2000). Fire blight, caused by the bacterium Erwinia amylovora (Burr.) Winslow et al., is a disease that affects many Rosaceous species with apple (Malus sp.) and pear (Pyrus sp.) being of the greatest economic significance (Bonn and van der Zwet, 2000). The most striking symptom is blackened cane tips, which bend over and die, resulting in a ‘‘shepherd’s crook’’ appearance. As the disease progresses down the cane, the veins of the leaf and portions of the leaf surrounding the midvein turn black. Entire leaves may wither and die. Typically, discoloration and dieback are limited to succulent young growth. Infected peduncles turn black and the young developing berries become brown, dry, and very hard. Entire fruit clusters may be infected, but generally a few berries in each cluster remain healthy. Losses result from berry necrosis and from tip dieback of primocanes (Schilder, 2007). The disease was first observed in the late 1800s in raspberry (Ries and Otterbacher, 1977). Fire blight outbreaks are less common in Rubus species but can reach economically damaging levels under certain conditions (Braun et al., 1999). Lehman demonstrated that bacteria isolated from raspberry with fire blight symptoms infected wound-inoculated Rubus spp. canes, but not apple shoots, and that E. amylovora isolated from apple twigs did not infect raspberry canes (Lehman, 1933). Management strategies in apple and pear have focused on timely applications of copper, antibiotics such as streptomycin, and removal of inoculum sources by pruning and the use of resistant cultivars (Beer, 1990). Of these options, cultivar resistance is the most desirable for its cost-effectiveness and longterm stability. Development of resistant cultivars requires advanced planning in breeding programs to incorporate and maintain a diverse range of resistance genes in parental lines, but little is known about raspberry and blackberry resistance to fire blight. In Turkey, fire blight was observed the first time on blackberry (Rubus fruticosus cv. Chester) during 2008 and 2009 (Bastas and Sahin, 2012). Because of the sporadic nature of fire blight in Rubus, genetic resistance may be a better option for control than preventive sprays or predictive models. The purpose of this study was to identify and quantify resistance in currently available commercial raspberry and blackberry cultivars to assist growers in making appropriate selections for their disease situation and to aid breeders in the selection of parental plant material for breeding purposes. Materials and Methods Plant materials and growing conditions. The studies were carried out on a total of 11 blackberry cultivars (Chester Thornless, Jumbo, Loch Ness, Bursa-1, Bursa-2, and Bursa-3) and raspberry cultivars (Heritage, Williamette, Rubin, Aksu Red, and Royalty) grown widely in Turkey. The test plants were propagated by digging suckers and were selected among 3-year-old saplings showing uniform growth. Twenty saplings from each cultivar (10 plants for each Ea strain) were transplanted into plastic pots of 20 cm diameter filled with 4 kg of soil, and they were grown for 20 d at 25 ± 2 C, 60% to 75% relative humidity (RH), and under 12,000 to 14,000 Lux from tungsten-filament lamps for a 16-h photoperiod. After transplantation, the plants were fertilized once a week (each pot) Received for publication 8 Aug. 2014. Accepted for publication 17 Oct. 2014. This study was funded by Selcuk University Scientific Research Project’s coordinators. Assistant Professor, Research Leader, and Lecturer. Professor and Research Leader. To whom reprint requests should be addressed; e-mail kbastas@selcuk.edu.tr. 1492 HORTSCIENCE VOL. 49(12) DECEMBER 2014 with 15 g ammonium sulfate, 15 g diammonium phosphate, 15 g potassium sulfate, and 30 mL of a liquid fertilizer having 0.05% manganese, copper, zinc, boron, and molybdenum (Kacar and Katkat, 1999). The soil used in the experiments is characterized by 1.9% total soil organic matter, 0.08% total salts, 63% soil saturation percentage, and 8.1% lime (CaCO3) with soil pH of 7.8 in distilled water (1.5 v/v). The plots were trickle-irrigated as needed. In addition, sulfur dust (Thiovit, 80% microlized sulfur; Syngenta) was applied once (4 g·L water) for powdery mildew and mite control. Twenty-five days later after the transplantation, seven of 14 potted plants from each cultivar were placed on the ground 1 m apart at the experiment field of Selcuk University, Konya province, and were watered regularly throughout the growing season. Erwinia amylovora strains and media used. A strain of E. amylovora (Eab14) isolated from blackberry and a virulent strain EapK from apple (Selcuk University Faculty of Agriculture Department of Plant Protection Bacterial Culture Collection) were used in the experiments. The strains were obtained from the preselected strains causing 80% to 90% disease severity on the basis of the virulence test reported by Norelli et al. (1984). The virulence was tested on cv. Williams pear seedlings. They were verified as E. amylovora by streaking on Crosse and Goodman (CG) media (Crosse and Goodman, 1973), where it displayed typical colony morphology. The bacterium was grown in a liquid culture of 523 medium (Kado and Heskett, 1970) and stored at –20 C in solution with 30% glycerol. Two days before inoculations, frozen stock was streaked on plates of CG medium and incubated at 28 C. Plant inoculation. All plants were kept at 85% to 90% RH supported by misting, before the inoculations. Bacterial suspensions were prepared from growing colonies on CG at 28 C and were diluted in sterile distilled water (SDW) to give an absorbance of 0.15 at 660 nm. This represented 10 colony-forming units (cfu)/mL based on viable plate counts. Inoculum was maintained on ice during inoculation of the plants. Inoculation of the youngest two to three leaflets at the actively growing shoot tips was attempted by cutting through them with a scissor that had been dipped into 10 cfu/mL EapK and Eab14 strains of E. amylovora. The treated shoots were labeled with flagging tape for evaluation purposes (Norelli et al., 1984; Zeller and Meyer, 1975). Additionally, five plants each of blackberry and raspberry cultivars were inoculated with sterile water as controls. All seedlings received an additional misting immediately after inoculation and they were covered with a polyethylene bag for 24 h to maintain high humidity. To enhance disease development, the greenhouse was maintained at 24 ± 2 C and kept shaded using shadecloth. Also, the inoculated plants were misted with water four times at equal intervals to maintain