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Ciencia e investigación agraria

On-line version ISSN 0718-1620

Cienc. Inv. Agr. vol.39 no.3 Santiago Dec. 2012

http://dx.doi.org/10.4067/S0718-16202012000300016 

Cien. Inv. Agr. 39(3):569-576. 2012
www.rcia.uc.cl

CROP PRODUCTION
RESEARCH NOTE

 

Sensitivity of wild-type and mutant Trichoderma harzianum strains to fungicides

Sensibilidad de cepas silvestres y mutantes de Trichoderma harzianum a fungicidas

 

Rodrigo Herrera1, David Núñez1, Natalia Romero2, Ximena Besoain3, Luz María Pérez4, and Jaime Montealegre1

1 Departamento de Sanidad Vegetal, Facultad de Ciencias Agronómicas, Universidad de Chile, Casilla 1004, Santiago, Chile.
2 Facultad de Ciencias Médicas y Enfermería. Universidad Pedro de Valdivia. Huérfanos 1546, Santiago, Chile.
3 Facultad de Agronomía, Pontificia Universidad Católica de Valparaíso. Casilla 4059, Valparaíso, Chile.
4
Asesorías e Inversiones Biostrategy Ltda., Hernando de Aguirre 1372, Providencia, Santiago, Chile.

Corresponding to:


Abstract

The germination of conidia in wild-type (Th11, Th12 and Th650) and mutant (Th11A80.1, Th12A10.1 and Th650-NG7) strains of Trichoderma harzianum that were exposed to different commercial fungicides was studied. All wild-type and mutant Trichoderma strains were germinated in the presence of 1,700 mg L-1 of pencycuron. The wild-type strains Th12 and Th650 and the corresponding mutant strains Th12A10.1 and Th650-NG7 were sensitive to all concentrations of iprodione and metalaxil + mancozeb. The EC50 (Median Effective Concentration) values for the different fungicides were between 10-1 and 10-4 less than the concentrations recommended for field applications; one exception was Phyto-fos on Th650-NG7, where this ratio was 0.72. These results establish whether some of these fungicides can be used in combination with the biocontrol agents evaluated.

Key words: biocontrol, biocontrol agents, control of tomato root diseases, fungicides, fungicide tolerant Trichoderma, integrated management.


Resumen

Se estudió la germinación de conidias de cepas silvestres de Trichoderma harzianum (Th11, Th12 y Th650) y de sus correspondientes cepas mutantes (Th11A80.1, Th12A10.1 y Th650-NG7) sometidas a dosis crecientes de distintos fungicidas comerciales. Tanto las cepas silvestres de Trichoderma como las mutantes germinaron a 1.700 mgL-1 de pencycuron, mientras que las cepas silvestres Th12 y Th650 y sus correspondientes mutantes Th12A10.1 y Th650-NG7 fueron sensibles a todas las concentraciones de iprodione y metalaxil+mancozeb. La mayoría de los valores de CE50 (Concentración Efectiva Media) para los diferentes fungicidas variaron desde 10-1 a 10-4 veces la concentración recomendada para uso a nivel de campo, excepto para Phytofos sobre la cepa mutante Th650-NG7, donde este valor fue de 0,72. Los resultados permiten establecer que algunos de estos fungicidas podrían utilizarse en combinación con agentes biocontroladores.

Palabras clave: Biocontrol, fungicidas, control de enfermedades de la raíz del tomate, manejo integrado, Trichoderma tolerante a fungicidas.


Introduction

The Trichoderma species are fungi commonly found in some suppressive soils (Weller et al., 2002) and are some of the most important microorganisms employed as biological control agents of plant diseases. The ability of these fungi to parasitize and antagonize several phytopathogenic fungi is accomplished via different mechanisms: competence for space and nutrients, production of antibiotics and lytic enzymes, modification of environment conditions and promotion of the growth of plants and their defense mechanisms (Howell, 2003; Howell, 2005; Harman, 2006). Beneficial microorganisms in the soil and within the plant are also important for plant production. The maintenance of these microorganisms can be affected by conventional disease control practices (Tilman et al, 2002). In this study, we investigated alternative agricultural practices and fungicide substitutes that minimize negative effects of some agricultural practices, prevent environmental pollution and contribute to produce innocuous foods.

Although biocontrol agents (BCAs) can delay the onset of a disease or partially suppress it, they rarely are as effective as chemical fungicides. This is a challenge in the development of new disease control alternatives. The simultaneous use of BCAs and fungicides is one alternative for the control of plant diseases. To be used as complementary tools in biocontrol, fungicides must be used at concentrations lower than those used for controlling plant diseases so that BCAs and/or plant beneficial microorganisms are not negatively impacted.

The use of integrated biological and chemical control methods is an established sustainable strategy for plant disease control in the field (Elmer and McGovern, 2004). Fungal strains may differ in their sensitivity to fungicides, as previously described in fungicide-resistance studies of microorganisms (Figueras-Roca et al., 1996). More efficient BCAs have improved antagonistic activity and/or the spectrum of this activity and have remained active in the presence of fungicides (Arias et al., 2006).

The tomato root diseases caused by Fusarium oxysporum f. sp. lycopersici, Fusarium solani, Phytophthora nicotianae, Pyrenochaeta lycopersici and Rhizoctonia solani are controlled in Chile by several fungicides (AFIPA, 2006). Previous experiments have shown that certain wild-type and mutant Trichoderma strains can control some of these pathogens in vitro and in greenhouse conditions (Pérez et al., 2002; Arias et al., 2006; Besoain et al., 2007). Formulations containing these strains are stable; i.e., they retain full R. solani biocontrol activity after six months without reductions in the CFU count of the formulation (Montealegre et al., 2009). It is not known whether these Trichoderma strains are sensitive to the fungicides used to control those plant pathogens.

Here, we describe the in vitro sensitivity of the wild-type and mutant strains of Trichoderma previously obtained by our research group (Be-soain et al, 2007; Pérez et al., 2007) to different fungicides commonly used to control F. oxyspo-rum f. sp. lycopersici, F. solani, P. nicotianae, P. lycopersici and R. solani. We focused on select fungicide-tolerant Trichoderma strains used as part of an integrated plant management and control program that included the simultaneous use of BCAs and fungicides.

Materials and methods

Fungal strains

The fungal strains used were the T. harzianum mutant strains Th650-NG7, Th11A80.1 and Th12A. 10.1, which had been obtained previously (Besoain et al, 2007; Pérez et al., 2007), and their parental strains Th650, Th11 and Th12. All isolates were cultivated in potato-dextrose-agar, and conidia were produced as described previously (Pérez et al., 2002).

Fungicides

The fungicides used for evaluating the sensitivities of the T. harzianum strains were obtained from commercial companies and tested at the concentrations indicated in Table 1. The highest active ingredients (a.i.) concentrations tested were the highest concentrations recommended (AFIPA, 2006) for the control of root and crown rot pathogens of tomatoes in the field.


Table 1. The commercial fungicides used to test conidial germination sensitivity in wild-type and mutant strains of Trichoderma harzianum strains.

Conidial germination in fungicide containing media

Conidial germination was established by cultivating 1 x 105 conidia of each Trichoderma strain on sterile Petri dishes containing 2.25% of glucose-agar and increasing doses of each fungicide. For the nematicide-fungicide sodium tetrathiocarbamate, Trichoderma conidia were cultivated in liquid dextrose (18 g L-1) and boiling potato extract (200 g L-1); sealed Petri dishes were used to prevent loss during incubation and to provide an adequate availability of the chemical product. The fungicide concentration was increased as required in a second test to calculate the corresponding EC50 (50% effective concentration) value.

Plates were incubated at 22 °C for 24 h, and the percentages of germinated conidia were determined using a microscope. Germinated conidia had a germinating tube length five times the conidia diameter or greater. The results for each Trichoderma strain are the mean of four replications of two independent experiments and are expressed as an EC50 (effective mean concentration) value. The EC50 values were calculated using the Probit regression of

the germination inhibition percentage versus the logarithm of the fungicide concentration (Figueras-Roca et al., 1996). Conidia were considered sensitive if the EC50 value of a given fungicide was lower than the highest recommended concentration used in the field. Conidia were considered resistant if the EC50 value of a given fungicide was higher than the recommended field concentration. Ratios of the EC50 values of different fungicides against mutant and wild-type strains were calculated; these ratios were used to determine whether the mutant strain had improved resistance compared to their corresponding wild-type strain.

Results and discussion

The germination rates of all strains in the presence of different agrochemicals are discussed in relation to each specific fungicide. Variability is based on the specific characteristics of each strain, which agrees with the results obtained by Figueras-Roca et al. (1996).

Conidial germination in the presence of fungicides

The inhibitory effect of the different fungicides were evaluated on conidial germination of wild-type and mutant Trichoderma strains and are described as EC50 values, as shown in Table 2.


Table 2. Median effective concentration (EC50) values of different fungicides against the conidial germination of wild-type and mutant Trichoderma strains.

Pencycuron [N-((4-chlorophenyl)-methyl)-N-cyclopentyl-N'-phenyl urea] was the only fungicide that did not inhibit the conidial germination of any Trichoderma wild-type or mutant strains. No effect on Trichoderma wild-type or mutant strains was observed in vitro with this fungicide at a.i. concentrations of 1,375 mgL-1 and 1,700 mg L-1 (higher than the commercial doses). Pencycuron has been described as a non-systemic phenyl urea fungicide with specific action against diseases caused by R. solani and Pellicularia spp. in several horticulture plants. The mode of action of pencycuron on R. solani is not understood, although the destruction of microtubules in the hyphal tips of the fungus has been previously described (Ueyama and Kurahashi, 2007). The non-inhibitory effect on the fungal β-tubulin assembly and the change in the osmotic stability and fluidity of the plasma membrane in pencycuron-sensitive R. solani isolates has been reported. The inhibition of giant protoplast formation and late membrane lipid peroxidation are secondary effects of pen-cycuron (Kim et al., 2001).

Conidial germination in the wild-type strains Th11, Th12, Th650 and their corresponding derived mutants Th11A80.1, Th12A10.1 and Th650-NG7 was inhibited by all other fungicides (Table 2) at concentrations lower than those recommended for use at the field level (AFIPA, 2006), with one exception. The strain Th11 was not inhibited by Phytofos at any of the tested concentrations. Phyto-fos is a fertilizer containing neutralized phosphorous acid, a known fungicidal compound (Guest and Grant, 1991), and is recommended for the control of Phytophthora spp. and Oomycotas fungi (AFIPA, 2006). The mechanisms of growth inhibition in Phytophthora spp. are related to key phosphorylation reactions and the inhibition of adenylate synthesis, as well as the alteration of nucleotide pools and the metabolism pentose phosphate (Varadarajan et al., 2002).

The dicarboximide fungicide iprodione [3-(3,5 diclorophenyl)-N-isopropil-2,4-dioxoimidazoli-dine-1-carboxamide] controls Rhizoctonia spp., Sclerotium spp., Sclerotinia spp., Botrytis cinerea, Penicillium spp. and Alternaria spp.(AFIPA, 2006). It is described as an inhibitor of lipid and membrane synthesis. Iprodione inhibited conidial germination in the wild-type Trichoderma strains Th12 and Th650 at all concentrations tested. The derived mutant strains were less sensitive to this fungicide. The EC50 of iprodione against the wild-type T. harzianum strain Th11 was 522.4 mg L-1, which is significantly lower than the 2,448 mg L-1 that is recommended for field use. This strain was classified as sensitive to the fungicide. The strain Th11 could be combined with non-inhibitory concentrations of iprodione in a complementary biocontrol strategy. The EC50 value of iprodione against Th11 correlates with concentrations used by other authors in combined treatments with T. harzianum for the control of cucumber fruit and stem mold. Other authors have identified some Trichoderma strains resistant to iprodione, with EC50 values even lower than those obtained in our study (Kay and Stewart, 1994).

All wild-type and mutant Trichoderma strains were sensitive to the mixture of metalaxil [N-(methoxyacetyl)-N-(2,6-xylil)-DL-alaninate] and mancozeb [Mn Ethylene-bis- dithiocarbamate complexed to Zn salts]. Metalaxil is an inhibitor of RNA synthesis (Papavizas and Lewis, 1983; Wollgiehn et al, 1984). Mancozeb causes DNA damage and induces apoptosis (Calviello et al., 2006). These results suggest that the simultaneous use of any of these wild-type or mutant strains with this mixture of agrochemicals is unfeasible. The concentration of metalaxil utilized for the control of Phytophthora spp. in the field (AFIPA, 2006) is approximately 103-fold greater than the EC50 values obtained for the wild-type Trichoderma strains.

The EC50 values obtained for thiabendazol [(2-(4-thiazolyl)-1H-benzimidazole)] against Trichoderma strains were ca. 103 fold lower than the 1,220 mg L-1 concentration recommended for the control of Rhizoctonia spp., Sclerotium spp. and Fusarium spp. in the field (AFIPA, 2006). Thiabendazole is a mitotic and meiotic inhibitor that prevents β-tubulin assembly. This fungicide can induce mutations in the β-tubulin gene in phytopathogens when used intensively, producing resistant strains in species such as F. sambucinum (González et al., 2002). The low thiabendazole concentration necessary to reduce conidial germination by 50% in all Trichoderma strains tested suggests that its use in combination with any of the BCAs evaluated is unfeasible.

Hymexazol [5-methyl-1,2-oxazol-3-ol] is a fungicide with a DNA/RNA synthesis inhibitor activity (Kato et al, 1990). It is recommended for the control of Fusarium oxysporum f. sp. lycopersici, Pythium and other fungi that cause damping-off (AFIPA, 2006). Hymexazol has an inhibitory effect similar to thiabendazol on the conidial germination of T. harzianum wild-types Th11 and Th12. The EC50 values of hymexazol against strains Th11 and Th12 were 509 and 506 mg L-1, respectively. These fungicide concentrations are close to the 700 mg L-1 required to control plant pathogens in the field; thus, there is potential use of these strains in combination with non-inhibitory concentrations of the fungicide. The EC50 values for hymexazol against the corresponding mutant strains were lower, which would make their use in combined strategies difficult. The EC50 value of hymexazol against the mutant strain Th650-NG7 was more than 10-fold greater than the EC50 value obtained against the wild-type strain Th650 (Table 2). Thus, Th650-NG7 appears to be a good candidate for combined use with hymexazol.

The fungicides sodium tetrathiocarbonate, aluminum tris-o-ethyl-fosfonate and [N-(trichloromethylthio) cyclohex-4-ene-1,2-dicarboximide] plus iprodione all inhibited conidial germination of wild and mutant Trichodermaat concentrations lower than those recommended for use in the field. While all mutants were sensitive to sodium tetrathionate, they were less sensitive to aluminum tris-o-ethyl-fosfonate compared to the wild-type strains. Th11 showed a ca. 66-fold increase in sensitivity towards [N-(trichloromethylthio) cyclohex-4-ene-1,2-dicarboximide] plus iprodione compared to iprodione alone. This increase in sensitivity was not observed in the mutant Th11A80.1, where the EC50 value of the mixture was only 1.4-fold greater than that of iprodione alone. The EC50 values of the mixture against Th12 and Th650 were similar; conidia germination in both strains was completely inhibited in the presence of iprodione. The EC50 values of the mixture or iprodione alone against their corresponding mutants had similar orders of magnitude.

EC50 ratios

The ratio of EC50 values between mutant and wild-type strains are shown in Table 3. Values greater than 1 indicate that the conidial germination was inhibited less in the mutant strain than in the corresponding wild-type strain at the same fungicide concentration; i.e., the mutants are less sensitive than their corresponding parental strains.


Table 3. Mediam effective concentration (EC50) ratios of different fungicides tested against the conidial germination of mutant Trichoderma strains related to wild-types Trichoderma strains.

The EC50 ratios demonstrate that mutations produced by UV light in Th11 (Besoain et al., 2007) did not affect the sensitivity of Th11A80.1 towards thiabendazol. The mutations induced in Th12 (Besoain et al, 2007) and in Th650 (Pérez et al., 2007) decreased sensitivity; this is similar to the decreased sensitivity of all mutants towards aluminum tris-o-ethyl-fosfonate. Each wild-type strain and mutant strain, independent of the genus or the mutagenic agent used, must be tested for fungicide sensitivity before any recommendations for their use in combined practices with chemical fungicides can be made.

The above results lead to the following conclusions: All wild-type and mutant Trichoderma strains analyzed could be used in combination with pencycuron as a potential strategy to control R. solani, based on their resistance to this fungicide.

The wild-type and/or mutant Trichoderma strains could be used in combination with thiabendazol or iprodione to control R. solani, but only fungicide concentrations that do not inhibit conidial germination can be used.

The combined use of different fungicide molecules with any of these BCAs is feasible only if the fungicide concentration is not inhibitory to conidial germination. If positive biocontrol is obtained, a wider spectrum of phytopathogens could be controlled by a combination of Tricho-derma strains and low-concentration fungicides.

Further studies of the complementary use of fungicide resistant BCAs must be performed to reduce fungicide concentrations or the number of applications needed to control phytopathogens. This strategy, in addition to the persistence of these BCAs in the soil (Montealegre et al, 2009), should have a significant and positive impact on the environment.

Acknowledgements

This work was financed by Project FONDECYT 1040531-04.

 

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Received December 20, 2011. Accepted July 13, 2012.

Corresponding autho: rherrera@uchile.cl

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