Desirable Traits of a Good Biocontrol Agent against Verticillium Wilt.

Silke Deketelaere,Soraya C França,Lien Tyvaert,Monica Höfte

doi:10.3389/fmicb.2017.01186

Abstract

The soil-borne fungus Verticillium causes serious vascular disease in a wide variety of annual crops and woody perennials. Verticillium wilt is notoriously difficult to control by conventional methods, so there is great potential for biocontrol to manage this disease. In this study we aimed to review the research about Verticillium biocontrol to get a better understanding of characteristics that are desirable in a biocontrol agent (BCA) against Verticillium wilt. We only considered studies in which the BCAs were tested on plants. Most biocontrol studies were focused on plants of the Solanaceae, Malvaceae, and Brassicaceae and within these families eggplant, cotton, and oilseed rape were the most studied crops. The list of bacterial BCAs with potential against Verticillium was dominated by endophytic Bacillus and Pseudomonas isolates, while non-pathogenic xylem-colonizing Verticillium and Fusarium isolates topped the fungal list. Predominant modes of action involved in biocontrol were inhibition of primary inoculum germination, plant growth promotion, competition and induced resistance. Many BCAs showed in vitro antibiosis and mycoparasitism but these traits were not correlated with activity in vivo and there is no evidence that they play a role in planta. Good BCAs were obtained from soils suppressive to Verticillium wilt, disease suppressive composts, and healthy plants in infested fields. Desirable characteristics in a BCA against Verticillium are the ability to (1) affect the survival or germination of microsclerotia, (2) colonize the xylem and/or cortex and compete with the pathogen for nutrients and/or space, (3) induce resistance responses in the plant and/or (4) promote plant growth. Potential BCAs should be screened in conditions that resemble the field situation to increase the chance of successful use in practice. Furthermore, issues such as large scale production, formulation, preservation conditions, shelf life, and application methods should be considered early in the process of selecting BCAs against Verticillium.

Highlights

Vascular wilts caused by members of the genus Verticillium are among the most devastating fungal diseases worldwide
We focused on Verticillium wilt and included non-endophytic bacterial biocontrol agent (BCA)
Experiments with BCAs are often carried out in sterile soils using plants that have been artificially inoculated with Verticillium via root dipping in a conidial suspension or via soil drench with a conidial suspension

Summary

Introduction

Vascular wilts caused by members of the genus Verticillium are among the most devastating fungal diseases worldwide. Verticillium species produce long-lasting resting structures such as microsclerotia, chlamydospores, and resting mycelium in dead or dying plant tissues (Table 1). These resting structures serve as the primary inoculum from which hyphae are formed that directly penetrate the roots of host plants. Within the different Verticillium species non-pathogenic isolates can be found that do not cause symptoms upon inoculation of host plants. Several of these nonpathogenic Verticillium isolates show biocontrol efficacy against Verticillium wilt (Matta and Garibaldi, 1977; Davis et al, 2000; Robinson et al, 2007; Qin et al, 2008; García et al, 2011; França et al, 2013; Zhu et al, 2013; Tyvaert et al, 2014)

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