Abstract
Yeast associates with many plant parts including the phyllosphere, where it is subject to harsh environmental conditions. Few studies have reported on biological control of foliar pathogens by yeast. Here, we newly isolated leaf-colonizing yeasts from leaves of field-grown pepper plants in a major pepper production area of South Korea. The yeast was isolated using semi-selective medium supplemented with rifampicin to inhibit bacterial growth and its disease control capacity against Xanthomonas axonopodis infection of pepper plants in the greenhouse was evaluated. Of 838 isolated yeasts, foliar spray of Pseudozyma churashimaensis strain RGJ1 at 108 cfu/mL conferred significant protection against X. axonopodis and unexpectedly against Cucumber mosaic virus, Pepper mottle virus, Pepper mild mottle virus, and Broad bean wilt virus under field conditions. Direct antagonism between strain RGJ1 and X. axonopodis was not detected from co-culture assays, suggesting that disease is suppressed via induced resistance. Additional molecular analysis of the induced resistance marker genes Capsicum annuum Pathogenesis-Related (CaPR) 4 and CaPR5 indicated that strain RGJ1 elicited plant defense priming. To our knowledge, this study is the first report of plant protection against bacterial and viral pathogens mediated by a leaf-colonizing yeast and has potential for effective disease management in the field.
Highlights
Among biological agents, plant growth-promoting rhizobacteria (PGPR) have been used to reduce disease symptoms and pathogen titers caused by bacteria and viruses, and developed into biological products[14,15,16]
A Bacillus pumilus INR7 endophyte was commercialized as Yield Shield; seed treatment with Yield Shield successfully managed Cucumber mosaic virus (CMV) on pepper (Capsicum annuum), angular leaf spot caused by Pseudomonas syringae pv. lachrymans on cucumber, and bacterial wilt caused by Erwinia tracheiphila on cucumber during field trials[17,18,19,20,21]
Community structure was determined by similarity between internal transcribed spacer (ITS) sequences of 832 yeast isolates, which were sampled during May to October in 2013, 2014, and 2015 because of the growing season of pepper plants under field conditions in Korea (Fig. 1)
Summary
Plant growth-promoting rhizobacteria (PGPR) have been used to reduce disease symptoms and pathogen titers caused by bacteria and viruses, and developed into biological products[14,15,16]. Pseudozyma flocculosa is a basidiomycetous fungal yeast that has been extensively characterized as an effective control agent for powdery mildew fungi, which are ubiquitous phyllosphere pathogens of numerous field and greenhouse crops. We isolated yeasts from leaves of field-grown pepper plants in South Korea using yeast semi-selective medium containing rifampicin antibiotic to inhibit bacterial growth. By screening 838 yeast isolates in a greenhouse, Pseudozyma churashimaensis strain RGJ1 was selected as a promising candidate for further field studies. Spray application of strain RGJ1 on pepper seedlings before and after transplanting into the field was very effective for disease suppression of bacterial spot caused by Xanthomonas axonopodis pv. Vesicatoria growth on agar medium by strain RGJ1, indicating that RGJ1 may induce systemic resistance in pepper plants to protect against bacterial spot disease. This study is the first report of plant protection against bacterial and viral pathogens mediated by the leaf-colonizing yeast P. churashimaensis
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