Improving the efficacy of biocontrol agents against soilborne pathogens

Abstract

Technical, economical and environmental factors are forcing to adopt new sustainable methods, such as the use of microbial antagonists, for the control of soilborne pathogens. Research has mainly focused on antagonistic fungi and bacteria, often not providing consistent or satisfying results. Biocontrol agents can be combined with other chemical products, with physical methods (solarization or steam sterilization) or agronomical practices, such as enhancement of suppressive soils, use of amendments or microbial optimization in the case of soilless systems. Different biocontrol strategies should be developed for different pathogens. The use of microorganisms can play an important role in a more complex vision of crop protection, as a key element of IPM programs. Mixtures of antagonists with complementary activities could be developed, but the need for multiple registration is critical. Genetic manipulation could result in new biocontrol strains with increased production of toxic compounds or lytic enzymes, improved space or nutrient competence, wider host range or enhanced tolerance to abiotic stresses. The potential risks related to the environmental release of genetically modified microorganisms must be carefully assessed. Genes and enzymes involved in the biocontrol mechanism could be applied directly or transferred to crops. Finally, the production system, formulation and methods of application (soil and seed treatment or microbial colonization of the hydroponic nutrient solution) are crucial to maintain and improve the efficacy of microbial antagonists.

Introduction

Meeting the goal of improving soilborne disease control for efficient and sustainable production systems requires a reduction of the chemical inputs in agriculture. Various movements or parties see increased pesticide application as a threat to public health and the environment. Agronomists experience growing problems with the buildup of resistance to the pesticides in target pathogen populations (Dekker, 1976; Brent and Hollomon, 1998) and the withdrawal of pesticides from the market (Gullino and Kuijpers, 1994). Of special relevance, in the case of many vegetable and ornamental crops, is the loss of methyl bromide as soil fumigant (Katan, 1999; Gullino et al., 2003). Furthermore, the private industry is coming across increasing costs in discovering and developing new molecules for disease control application. Consequently, there is a growing awareness that integrated pest management (IPM) strategies can provide more environmentally sound and economically feasible alternatives for soilborne disease control. Biological control with introduced microorganisms has been identified as an important component of IPM strategies (Cook, 1993).

Many studies have been devoted to the identification of microorganisms able to reduce the activity of soilborne pathogens during the past four decades (Baker and Snyder, 1965).

Many laboratories around the world have developed their own microorganisms and this allowed the collection of important contributions about the biology of pathogens and antagonists. However, in spite of the initial great optimism and extensive later research efforts, progress in achieving commercial, large-scale usage of biological pest control has been slow.

Most experiences were on a laboratory scale, thus avoiding the problems related to antagonist mass production and biofungicide formulation, and disease control trials were performed in simplified environments such as growth chambers, experimental greenhouses or small field plots, thus avoiding the risk of large-scale experiments. When trials move towards the farm scale, many antagonists fail to confirm their activity or, when tested in different environments, behave unpredictably, and consequently lack reliability (Mathre et al., 1999).

Biofungicides presently meet many constraints and it is not a straightforward task to find, develop and implement practically feasible biocontrol products against soilborne diseases. Nevertheless, there are also reasons for stating careful optimism. There are many possibilities for combining various biocontrol agents, with each other, or with agronomical, physical or chemical control methods. In particular, by combining different methods of control, the aim is to obtain a synergistic effect, rather than additive. For that reason, a complete comprehension of the mode of action and mechanism of control is needed. A good combination is obtained when both partners benefit from the combined treatment. For example, combining a biocontrol agent (BCA) with a fungicide improves the control by the BCA and, at the same time, it enables to reduce the fungicide dosage, rendering its use more acceptable. Moreover, the combination of control methods provides a wider spectrum of control, which is especially needed to replace the wide-spectrum fumigants, such as methyl bromide. Finally, combining different control methods enables the use of less effective control methods which cannot stand by themselves, but in combination they can contribute to the control.

Other possibilities to improve the antagonist effectiveness include the genetic manipulation of the microorganisms and the enhancement of the mass production, formulation and methods of application The main possibilities to improve the efficacy of the biocontrol agents against soilborne pathogens are the topic of this brief review.