Epiphytic bacteria to prevent maize foliar pathogens that impact on yield

Abstract

Maize (Zea mays L.) is one of the most important cereal crops in Argentina. However, a negative factor is the emergence and re-emergence of some foliar diseases. The common rust caused by Puccinia sorghi and the northern leaf blight caused by Exserohilum turcicum are two foliar diseases that affect the crop. The leaf blight becomes important in maize sown in late December and January after wheat harvest. The residues on the soil surface and intense rainfall during the summer months and moderate temperatures favor the disease development. At present there is a lack of information about the life cycle of this fungal pathogen. Therefore it was necessary to study the influence of abiotic factors such as temperature and water potential on the growth of E. turcicum on different crop residues used in rotations with maize under no-till system cultivation. According to our results, soybean residues allowed the pathogen growth in a wide range of conditions compared to wheat and maize residue, providing an inoculum source for maize infections in subsequent season. The expansion of emerging and reemerging diseases require the development of prevention and control strategies. Biological control is presented as an alternative aimed to minimize yield losses caused by foliar diseases. The use of microorganisms that antagonize foliar pathogens is risk-free when these organisms come from the same ecosystem. To achieve the selection of a potential biocontrol agent it is important to consider the relationship between biological interactions and environmental stress factors. Our research began by selecting microbial isolates from phyllosphere of maize and examining, in vitro, their antagonistic activity against E. turcicum. Of 111 microbial isolates, 11 potencial biocontrol agents against the pathogen were selected. Subsequently, potential biocontrol agents were evaluated in planta, in greenhouse, in order to assess their effectiveness in reducing disease severity of northern leaf blight. Foliar application of different bacteria significantly reduced the leaf blight between 30-78% and 39-56% at 20 and 39 days respectively. It was necessary to carry out studies on putative mechanisms of action and tolerance to the environmental stress conditions of the different biocontrol agents. The partial 16S rRNA gene sequencing of antagonistic bacteria had a high match with three bacterial genera: Curtobacterium, Pantoea and Bacillus. Bacillus strains showed the most relevant production of enzymes, volatile organic compounds and antibiosis against E. turcicum. Respect to tolerance to UV radiation and temperature, Pantoea spp. and Bacillus spp. strains were more tolerant. A preliminary evaluation of natural incidence of the common rust and the northern leaf blight in maize after application of bacteria antagonists was conducted at field level. Plants treated with biological control agents showed significant reductions in the incidence of foliar diseases. In northern leaf blight the reduction was higher than 50% in plants treated with Bacillus spp. Moreover, grain yield was significantly higher as compared to control treatments. We understand that these studies involves a long way, in order to achieve a broad knowledge of the agroecosystem and understand the behavior of the pathogen to achieve an effective preventive strategy.