Abstract
Late blight, caused by the oomycete Phytophthora infestans, is the most devastating disease of potato. In organic farming, late blight is controlled by repeated applications of copper-based products, which negatively impact the environment. To find alternative solutions for late blight management, we have previously isolated a large collection of bacteria from the phyllosphere and the rhizosphere of potatoes. Here we report the antagonistic potential of these strains when co-cultivated with P. infestans as well as with other potato pathogens. We then focused on three Pseudomonas strains and compared their protective impact against late blight to that of well-known biocontrol strains in planta using a high-throughput leaf disk assay with automated picture analysis. When sprayed on the leaves of potatoes in the greenhouse, the strains were able to survive for at least 15 days. Under field conditions, populations decreased faster but all tested strains could still be retrieved after 8 days. The most active strain in vitro, P. chlororaphis R47, was also the best protectant on leaf disks from plants grown in the greenhouse experiment, but its protection potential could not be verified in the field due to unfavorable infection conditions. However, its protective effect against P. infestans in planta, its survival in the phyllosphere as well as its ability to colonize the potato rhizosphere in very high population densities, suggest a potential for field application, e.g., in the form of tuber treatment or leaf spray.
Highlights
Over the last decades, the need to move from intensive agriculture to a more sustainable way of food production has risen in the awareness of growers and consumers
This differential reaction of the target organisms to the bacterial strains was further illustrated, e.g., by the fact that R. solani was more inhibited in its growth by Pseudomonas strains than by the other strains, while Bacillus strains R73 and R54 were able to drastically reduce the growth of both P. infestans and H. solani and the Streptomyces strain S01 was the best inhibitor of F. oxysporum (60% of its control growth) (Table 1)
Using microorganisms as biocontrol agents seems an appealing strategy for sustainable crop protection: in the last decades, much effort has been made to isolate, characterize and use microbial strains to this end, yet the bacterial antagonists available on the market are so far only few (Velivelli et al, 2014)
Summary
The need to move from intensive agriculture to a more sustainable way of food production has risen in the awareness of growers and consumers. Numerous studies have tested the effect of microbial inoculants on late blight (reviewed in Axel et al, 2012), none has so far demonstrated a protection against late blight in the field This might be at least partly due to the fast spreading of Phytophthora during humid conditions, which is mediated by the production of sporangia that, depending on temperature, can either directly infect plant tissues or release motile zoospores, which in turn infect new leaves (Fry, 2008). In an attempt to find such a biocontrol agent against potato late blight, we have previously isolated bacterial strains from the phyllosphere and rhizosphere of potato, which we hypothesized to be adapted to the host plant (Hunziker et al, 2015) In this earlier work, we reported the ability of potato-associated bacteria to inhibit growth and sporulation of Phytophthora infestans through the emission of volatile organic compounds (Hunziker et al, 2015). The strains’ survival and efficiency under field conditions was monitored in a microplot experiment
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