Abstract
Hairy root disease (HRD) caused by rhizogenic Agrobacterium biovar 1 strains affect tomato, cucumber, eggplant, and bell pepper grown in hydroponic greenhouses and can cause considerable yield losses worldwide. Recently, Paenibacillus xylanexedens strains (ST15.15/027 and AD117) with antagonistic activity against rhizogenic agrobacteria were identified. In this study, we present results of greenhouse trials of two consecutive growing seasons (2019 and 2020) to examine the potential of these two biocontrol organisms (BCOs) under practical conditions. BCO-treatment at a 107 colony forming units (CFU)/mL density resulted in a considerable reduction of the HRD infestation rate, confirming the biocontrol potential of the two P. xylanexedens strains. Results revealed that a single BCO strain (ST15.15/027) performed equally well as the mixed inoculum of both strains. The same level of biocontrol activity was even achieved when the BCO inoculum density was reduced to 105 CFU/mL. qPCR analysis further showed that Paenibacillus was still present in rockwool substrate near the end of both trials, indicating that they persist well in a rockwool environment and that application at the start of the trial is sufficient to protect tomato plants until the end of the trial. Altogether, these results are highly valuable for further optimization and exploitation of P. xylanexedens as a biocontrol product for the control of HRD in hydroponic greenhouses.
Highlights
Application of the biological control organisms (BCOs) mixture of the two P. xylanexedens strains ST15.15/027 and AD117 resulted in a considerable decrease of the hairy root disease (HRD) infestation rate, as compared to the untreated control by the end of the trials (Figure 1)
In 2019, the infestation rate decreased significantly from 42% in the untreated plants to 6% for plants treated with the BCO mixture (χ2 = 14.63, p-value < 0.001), while in 2020, a decrease in HRD infestation from 27% for untreated plants to 4% for plants treated with the BCO-mixture was observed (χ2 = 4.57, p-value = 0.03; Table S1)
When examining the effect of a single strain application (ST15.15/027), the results revealed a similar reduction of 4–6% in the HRD infestation rate, as compared to the mixed BCO inoculum (4%; χ2 = 0.00, p-value = 1), indicating that the same level of HRD control could be achieved by applying only one BCO strain
Summary
Rhizogenic Agrobacterium biovar 1 is the causal agent of hairy root disease (HRD), which affects many hydroponically-grown crops, including tomato, cucumber, eggplant, and bell pepper [1,2,3]. Application of hydrogen peroxide is currently the most used management strategy to reduce HRD. Biological control was proposed as a more sustainable strategy in plant disease management to minimize the detrimental effects of chemical biocides for the environment [5]. The genus Paenibacillus consists of many soil-borne bacteria with plant growthpromoting or disease-protective traits [6]. Several studies showed antagonistic properties of Paenibacillus spp. or demonstrated their potential as biological control organisms (BCOs) for the management of several plant diseases caused by pathogenic bacteria, nematodes, fungi, and oomycetes [7,8,9,10]. A number of Paenibacillus-based products were introduced as commercial biological control products [11], such as “Topsid (AC-1)” (Greenbiotech, South Korea), which is used against fungal diseases in pear and apple [12,13]
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