Antifungal and antioomycetal properties of California-native Collimonas isolates and their use in synergy-based biocontrol of Fusarium plant of tomato

In a continued effort to discover bacteria with novel bio-based services for sustainable crop protection, we characterized a collection of bacterial isolates belonging to the genus Collimonas, for their ability to inhibit in vitro the growth of a wide range of plant pathogenic fungi and oomycetes. We identified the Californian isolate C. arenae Cal 35 as the one that showed the best all-round performance in these confrontation assays. It was the single best inhibitor of a number of fungi, including Fusarium oxysporum f. sp. lycopersici (Fol), which is the causal agent of Fusarium wilt of tomato. In greenhouse trials, neither C. arenae Cal35 nor our control treatment, i.e. the Bacillus-based biofungicide Serenade Soil, was able to prevent symptom formation on tomato plants. However, a mixture of the two (which we refer to as Collinade) significantly reduced vascular discoloration and prevented loss of shoot dry weight in Fol-challenged plants. In replicated field experiments, the application of Collinade produced the same reduction in vascular discoloration as was observed in the greenhouse. This finding was consistent with the lower levels of F. oxysporum that we detected in the root endosphere of Collinade-treated Fol-challenged plants compared to untreated Fol-challenged plants. Also, compared to Cal35 alone or Serenade Soil alone, Collinade was the only treatment to lower the loss of fruit due to sunburn in both years. We discuss the synergy-based emerging property of ‘biocombicontrol’ between Collimonas and Bacillus in the context of existing and upcoming formulations for the protection of crops against fungal pathogens.