Evaluation of Streptomyces spp. for the biological control of corky root of tomato and Verticillium wilt of eggplant

Highlights

• Streptomyces isolates were obtained from the rhizospheres of vegetable crops.

• Pyrenocaheta lycopersici and Verticillium dahliae are sensitive to streptomycetes.

• Corky root and Verticillium wilt are partially controlled in the greenhouse.

• Three streptomycetes controlled corky root but not Verticillium wilt in the field.

Abstract

The effects of Streptomyces spp. isolates in the biological control of corky root of tomato and Verticillium wilt of eggplant was determined in in vitro, greenhouse and field trials. Twenty-six Streptomyces spp. isolates were obtained from the rhizospheres of different vegetable crops in southern Italy. In in vitro dual culture tests, mycelial radial growth of Pyrenochaeta lycopersici and Verticillium dahliae was reduced up to 18.6% and 30.1%, respectively. Radial growth of seven other fungal pathogens was variably reduced as well. The isolates StB-3, StB-6, StB-11 and StB-12 showed a good antagonistic effect against both P. lycopersici and V. dahliae, while the rest of isolates eventually showed antagonism against only one pathogen.

In pot-experiments in the greenhouse three of the four above-mentioned Streptomyces spp. isolates significantly reduced corky root up to 64.9% (StB-11), and all four isolates reduced foliar symptoms of Verticillium wilt (AUDPC) up to 48.3%, but none of them reduced the severity of vascular browning. In naturally infested field trials, StB-11 significantly reduced corky root severity in tomato by 48.2%, StB-12 by 35% and StB-6 by 32.6%, but none of the isolates were effective in controlling Verticillium wilt of eggplant. The effectiveness of the streptomycete AtB-42, successfully used in previous researches, was here confirmed as it reduced corky root of tomato in the field by 33.6%. In conclusion, our research demonstrates that under field conditions corky root of tomato, but not Verticillium wilt of eggplant, can be effectively controlled by the Streptomyces spp. isolates used in this study.

Introduction

After the phasing out of methyl bromide for soil fumigation in agriculture, the control of soil-borne plant pathogens has become a serious threat, in particular in those pathosystems lacking in resistant cultivars, such as corky root of tomato, caused by Pyrenochaeta lycopersici Schn. & Ger., and Verticillium wilt of eggplant, caused by Verticillium dahliae Kleb. (Polley, 1985, Ciccarese et al., 1994b). The currently allowed fumigants, such as metham-sodium, metham-potassium and dazomet, in general provide a lower control level of soil-borne pathogens compared to that achieved by methyl bromide (Martin, 2003). Several control measures for these two important diseases have been studied so far, including chemical, cultural and physical ones (reviewed by Bubici, 2006 and Bubici et al., 2006). Also, with the aim to find environmental-friendly control measures, several microorganisms have been widely studied as potential biological control agents (BCAs) of soil-borne pathogens (Paulitz and Bélanger, 2001). Information about biological control of P. lycopersici is limited to a few reports dealing with the use of Trichoderma spp. Pers., Bacillus subtilis (Ehrenberg) Cohn and Streptomyces spp. Waksman & Henrici (Whipps, 1987, Vanachter et al., 1988, Ciccarese et al., 1994a, Colella et al., 2001, Perez et al., 2002). In contrast, more efforts have been dedicated for searching BCAs against V. dahliae, including fungi (mainly Gliocladium spp. Corda, Talaromyces flavus (Klöcker) Stolk & Samson, and Trichoderma spp.), and bacteria [mainly Bacillus spp., Paenibacillus alvei Cheshire and Cheyne, Pseudomonas spp. Migula, Serratia plymuthica (Lehmann and Neumann) Breed et al., and Streptomyces spp.] (for reviews see Baker, 1981, Pegg and Brady, 2002, Bubici and Cirulli, 2008, Bubici and Cirulli, 2011). Many of these BCAs were isolated from suppressive soils and compost substrates (Berg et al., 2001, Berg et al., 2005, Tjamos et al., 2004). Also, mycorrhizal, fungal and bacterial endophytes have been studied as BCAs against V. dahliae (Narisawa et al., 2002, Tjamos et al., 2004, Goicoechea et al., 2010).

Streptomycetes are recognized as good producers of several secondary metabolites, antibiotics and lytic enzymes affecting the growth of fungal pathogens (Doumbou et al., 2001). Members of this family are consistently referred in the literature as potential BCAs against a large number of soil-borne pathogens, including P. lycopersici and V. dahliae (Chi, 1963, Azad et al., 1987, El-Abyad et al., 1993, El-Abyad et al., 1996, El-Quakfaoui et al., 1995, Aghighi et al., 2004). Also, proliferation of streptomycetes has been associated with natural suppressiveness of soil, as well as other cropping substrates such as rockwool, against several soil-borne fungi, including Pythium aphanidermatum (Edson) Fitzp., Rhizoctonia spp. DC. and Streptomyces scabies (Lambert and Loria) (Lorang et al., 1995, Liu et al., 1996, Ryan and Kinkel, 1997, Postma et al., 2005, Mazzola, 2002, Mazzola et al., 2007). The in vitro antimicrobial activity of streptomycetes is due to molecules, such as antibiotics and extracellular enzymes, diffused through the growth medium (Goodfellow and Williams, 1983, Korn-Wendisch and Kutzner, 1999). Several studies have elucidated some antibiotics involved in antifungal activity, and nigericin, geldamycin, a complex of macrocyclic lactone antibiotics, 1-propanone 1-(4-chlorophenyl) and strevertenes are just some examples (Rothrock and Gottlieb, 1984, Valois et al., 1996, Trejo-Estrada et al., 1998, Beauséjour et al., 2003, Ezziyyani et al., 2007, Kim et al., 2011). Other studies have demonstrated the role of chitinase and β-1,3-gluacanase in the antifungal activity (Singh et al., 1999, Prapagdee et al., 2008). Streptomycetes may act also by the fumigant activity of volatile compounds such as geosmin (Wan et al., 2008, Li et al., 2010).

Presently, several bio-fungicides for the control of soil-borne diseases are available as commercial products worldwide (Paulitz and Bélanger, 2001). In Italy, a few commercialized products specifically registered as biological fungicides for the control of soil-borne diseases contain Coniothyrium minitans W.A. Campb., species of Trichoderma, or Streptomyces griseoviridis Anderson strain K61, while other products are marketed as plant growth promoters, plant strengtheners, or soil conditioners. All these products are reported to target at several soil-borne pathogens, such as Fusarium Link, Phytophthora de Bary, Pythium Pringsheim, Rhizoctonia, Sclerotinia Fuckel, Thielaviopsis Went and others, but usually not at P. lycopersici or V. dahliae are not usually referred to as target pathogens. Informations are scarce on the control by streptomycetes of soil-borne diseases in the field.

Therefore, the research reported here was aimed at screening Streptomyces isolates for their effectiveness in controlling corky root of tomato and Verticillium wilt of eggplant. Streptomyces isolates were obtained from the rhizosphere of vegetable crops, and evaluated for: (a) their antagonistic activity in vitro against P. lycopersici and V. dahliae, (b) their effectiveness in the control of the diseases in the greenhouse, and (c) effectiveness of selected promising Streptomyces isolates for biological control of the diseases in the field.