Integration of biological and conventional treatments in control of pepper bacterial spot
Introduction
Bacterial spot is one of the widespread and economically most important pepper diseases worldwide. The disease may be caused by Xanthomonas euvesicatoria, Xanthomonas vesicatoria and Xanthomonas gardneri species that belong to spot-causing xanthomonads (Jones et al., 2000, 2004; Obradovic et al., 2004). However, X. euvesicatoria strains are identified as the most widespread in pepper fields (EPPO, 2018). Xanthomonas perforans, a related species causing bacterial spot of tomato, has not been reported as a pepper pathogen.
Bacterial spot, caused by X. euvesicatoria has been a major limiting factor of pepper production in Serbia, due to endemic nature of the pathogen, favourable climatic conditions, questionable seed quality and limited control practices (Obradovic et al., 1999, 2001; Obradović et al., 2000). Based on differential reactions of 11 pepper genotypes, four physiological races of the pathogen (P1, P3, P7, P8) have been identified so far, with P8 being most widespread (Ignjatov et al., 2012). Currently, there are no commercially available pepper cultivars resistant to the pathogen races present in Serbia (Obradovic et al., 2004; Ignjatov et al., 2012).
Pepper bacterial spot management practices include preventive and curative strategies. Cultural practices, such as disinfection of soil and substrates in seedlings production, planting of healthy certified seeds and transplants, maintenance of optimum temperature and water regime in protected areas, removal of plant residues, implementation of appropriate agro-technical measures and cultivation of less sensitive varieties, are important for disease prevention. Unfortunately, they are often omitted or fail to provide satisfactory control, especially when weather conditions favour spread of the pathogen, resulting in severe epidemics. New races of the X. euvesicatoria, antibiotics and copper resistance development, make the disease control even more difficult (Marco and Stall, 1983; Adaskaveg and Hine, 1985; Ritchie and Dittapongpitch, 1991).
The most common disease control is still based on preventative application of copper bactericides, alone or in combination with ethylene-bis-dithiocarbamate (EBDC) fungicides and antibiotics (Marco and Stall, 1983; Sherf and MacNab, 1986; Vallad et al., 2010).
Roberts et al. (2008) and Fayette et al. (2012) reported suppression of bacterial spot on tomato plants with the use of various combinations of famoxadone, famoxadone plus cymoxanil, mancozeb and copper. However, the overuse of copper compounds led to appearance of copper resistance in X. euvesicatoria populations (Marco and Stall, 1983; Adaskaveg and Hine, 1985; Ritchie and Dittapongpitch, 1991; Mirik et al., 2007; Ignjatov et al., 2010). There have been studies showing toxicological problems associated with EBDC use and cancerogenic properties of their metabolites (Janjić, 2005). Moreover, residues of these pesticides have been reported on treated vegetables (Gullino et al., 2010). Therefore, after development of new active substances, the use of EBDC in plant protection might be reduced or forbidden in the future (Gullino et al., 2010; Janjić, 2005).
Antibiotics, especially streptomycin, have been successfully used for many years in control of tomato and pepper bacterial spot, until streptomycin-resistant bacterial populations emerged and became widely distributed (Stall and Thayer, 1962). Development of resistance to kasugamycin in Xanthomonas spp. is also possible due to similar mode of action with streptomycin (Woodcock et al., 1991). Although the use of antibiotics in plant protection is restricted in most EU countries, as well as in Serbia, variation in bacterial population sensitivity to kasugamycin (50 μg ml−1) has been observed among X. euvesicatoria strains isolated from pepper in Serbia (Obradović and Ivanović, 2007; Ignjatov et al., 2010). Limited efficacy of chemical treatments, as well as adverse negative environmental effects, stimulated plant pathologists to search for more suitable disease management solutions (Stall et al., 1986; Ritchie and Dittapongpitch, 1991; Obradovic et al., 2004a).
There were several attempts of using biological agents in control of pepper and tomato bacterial spot (Jones and Stall, 1998; Ji et al., 2006; Mirik et al., 2008; Abbasi and Weselowski, 2015). Bacteriophages, viruses that infect bacteria, have been recently studied as a promising natural antimicrobial agents in different pathosystems, including pepper and tomato spot-causing xanthomonads (Jones et al., 2007; Buttimer et al., 2017). Xanthomonas euvesicatoria specific bacteriophage KФ1, isolated from rhizosphere of pepper plants in Serbia (Gašić et al., 2011), showed significant efficacy in control of pepper bacterial spot in greenhouse conditions (Gašić et al., 2018). Moreover, combination of X. vesicatoria specific bacteriophages and acibenzolar-S-methyl (ASM), that activates systemic acquired resistance (SAR) in plants, was presented as a new alternative approach in control of tomato bacterial spot (Obradovic et al., 2004a, 2005; Jones et al., 2007). Treatments with ASM in combination with bacteriophages, or bacteriophages and harpin protein, significantly reduced bacterial spot of tomato ( Obradovic et al., 2004a ). Although ASM showed high potential in control of bacterial spot of tomato and pepper, some studies indicated it can negatively affect yield. Low yield is a limiting factor for using ASM to control the disease (Louws et al., 2001; Romero et al., 2001; Abbasi et al., 2002a, b). In order to achieve disease control without affecting yield, it is necessary to determine the concentration, time of application and number of treatments with ASM.
In this work, we explored the benefits of different strategies that could be considered as part of an integrated management of pepper bacterial spot in Serbia. Under field conditions we studied the efficacy of bactericides that are traditionally used in practice, as well as substances not registered for pepper bacterial spot control in Serbia, several biocontrol agents, and the integration of different biological agents and resistance inducers. Incorporation of novel alternate methods into the existing crop protection programme may provide more effective, durable and sustainable disease control.
Section snippets
Pepper plant development in response to different concentrations of ASM
Experiment 1. This experiment was conducted in a growth chamber at the Institute of Vegetable Crops, Smederevska Palanka, Serbia. Pepper plants cv. Early California Wonder grown in 10-cm (510 ml) pots containing soilless medium (Klasmann Substrate TS2; Klasmann-Deilmann GmbH), at 3–4 leaf stage, were used in the experiment. To evaluate the response of pepper plants to ASM, drench and foliar treatments were applied using three active ingredient concentrations: 0.0015, 0.0025 and 0.0035%. For
Pepper plants development in response to different concentrations of ASM
Experiment 1. When used in indicated concentrations, ASM did not produce any negative effect, such as chlorosis, spotting or necrosis, on pepper leaves. However, all three concentrations of ASM significantly reduced the plant growth. The height of the plants and the total weight of the fresh plant tissue were significantly affected as compared to the untreated control (Table 2). Treated plants showed a height declining trend along the time after the treatments. In the second measurement, 17
Discussion
Bacterial spot has been limiting pepper production in Serbia, especially when weather conditions favour the disease development. The disease management is a challenge due to limited efficacy of commonly used control strategies relying mostly on copper bactericides. Reduced copper sensitivity among X. euvesicatoria strains, as well as concerns about the environmental impact of copper residues, contributed to the increased interests in developing more effective control strategies that will
Acknowledgments
This research was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, project III46008, and COST Action CA16107 EuroXanth. The authors thank to Olgica Janković for her technical assistance.
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