Effects of bean seed treatment by the entomopathogenic fungi Metarhizium robertsii and Beauveria bassiana on plant growth, spider mite populations and behavior of predatory mites

Abstract

The fungal genera Metarhizium and Beauveria are considered as both entomopathogens and endophytes; they are able to colonize a wide variety of plants and can cause increased plant growth and protect plants against pests. In view of the need for new biological methods for plant protection and how promising and little studied candidates entomopathogens are, the aim of this research was to evaluate the potential of two isolates of Metarhizium robertsii (ESALQ 1622) and Beauveria bassiana (ESALQ 3375) to suppress spider mite Tetranychus urticae population growth and ability to promote growth of bean plants Phaseolus vulgaris after seed treatment, in order to develop an innovative strategy by using these fungi as inoculants to improve both spider mites control and plant growth and yield. In addition, behavioral responses and predation rates of the predatory mite Phytoseiulus persimilis towards fungal treated plants and spider mites from these plants were also evaluated in leaf disc assays to assess potential conflicting effects of the fungal inoculations on overall pest control at higher trophic levels. Seed inoculations by the two isolates of M. robertsii and B. bassiana were done individually and in combinations to evaluate potential benefits of co-inoculants. The results showed a significant reduction in T. urticae populations and improved plant development when inoculated with M. robertsii and B. bassiana individually and in combination. The predatory mite P. persimilis showed no difference in the predation rate on T. urticae from treated and untreated plants even though the predators were most likely to feed on spider mites from fungal treated plants during the first half of the trial, and on spider mites from control plants during the remainder of the trial. Overall, the two fungal isolates have potential as seed inoculants to suppress spider mites in bean and the strategy appears to have no conflict with use of predatory mites. Co-inoculation of both fungal isolates showed no additional benefits compared to single isolate applications under the given test conditions.

Fungal isolate and crop cultivar influence the beneficial effects of root inoculation with entomopathogenic fungi in strawberry

“…harzianum ESALQ 1306, the commercial products with B. subtilis + B. licheniformis and control after 60 and 120 days of root inoculation. Recently, Canassa et al . reported similar results, i.e.…”

“…These isolates should therefore be considered as promising candidates for plant protection in strawberry production in Brazil.A reduction in the oviposition of T. urticae females was observed in plants inoculated with almost all isolates of Metarhizium spp., B. bassiana and C. fumosorosea as compared to plants inoculated with T. harzianum ESALQ 1306, the commercial products with B. subtilis + B. licheniformis and control after 60 and 120 days of root inoculation. Recently, Canassa et al33 reported similar results, i.e. seed inoculation of bean plants, P. vulgaris, with the isolates M. robertsii (ESALQ 1622) and B. bassiana (ESALQ 3375) significantly reduced the population growth of T. urticae and resulted in increased root length, fresh and dry weight of roots and aerial parts, and also yield (number of string beans), compared to control plants.…”

“…Our results also demonstrate that the inoculation of strawberry roots with isolates of Metarhizium spp., B. bassiana and C. fumosorosea resulted in increased plant biomass and fruit yield compared to the commercial microbial products with growth promotion abilities. A number of studies have previously reported the growth promotion of plants inoculated with entomopathogenic fungi . Growth promotion observed in plants colonized by entomopathogenic fungi might be attributed to the production of organic acids, phytohormones or siderophores, which can change the bioavailability of several nutrients .…”

“…A number of studies have previously reported the growth promotion of plants inoculated with entomopathogenic fungi. 14,15,33,46,[53][54][55][56] Growth promotion observed in plants colonized by entomopathogenic fungi might be attributed to the production of organic acids, phytohormones or siderophores, which can change the bioavailability of several nutrients. [57][58][59] Furthermore, studies on endophytic fungus-plant interactions revealed that the positive effects on plant growth could also be due to fixation of nutrients.…”

“…In addition, Jaber and Ownley highlighted the importance of studying the selection of most adapted endophytic entomopathogenic fungi isolates to specific host plants and cultivars to their further development as biocontrol agents. Below‐ground inoculation of plants with fungi can cause reduction in T. urticae populations above ground, but to our knowledge no studies have been reported on the potentially variable effect of root inoculations of different strawberry cultivars with different isolates of entomopathogenic fungi in the Hypocreales on plant productivity and above‐ground pests.…”