PS 13-171 - Do raspberry (Rubus ideaus) growers need to manage arbuscular mycorrhizal fungal communities?

Abstract: Promoting arbuscular mycorrhizal (AM) fungal communities in agricultural fields could be an important component of an integrated-pest management system, particularly for perennial crops which are susceptible to a build-up of host-specific pathogens. Raspberries (Rubus ideaus) are one such crop as producers typically wait 8+ years before turning over fields, and plants are susceptible to Phytophthora rubi, an oomycete which causes root rot. Some producers apply commercial bioinoculants to increase AM fungi and other helpful biota in their fields. Yet, whether AM fungi increase raspberry’s resistance to P. rubi is unknown. Furthermore, it is not clear if management practices reduce the abundance of AM fungi in fields. In a greenhouse experiment and a field study we gathered data to determine if AM fungi from commercial or field sources increase raspberry’s resilience to P. rubi and if AM fungal abundance is reduced in newly-turned fields.

We grew ‘Meeker’ raspberry plants in the greenhouse with farm soil biota, a commercial bioinoculant, or no soil biota. At 4 weeks, we exposed half to P. rubi (3 soil biota treatments x 2 pathogen treatments x 10 reps = 60 pots). Survivorship and plant height was measured weekly until harvest (8.5 weeks). Roots were stained and AM fungal structures including hyphae, vesicles, and arbuscules were counted at 200X. Raspberry plants exposed to P. rubi were four times as likely to survive when grown with farm soil biota versus other treatments. Furthermore, plants receiving farm soil biota were the same height regardless of P. rubi exposure, which was not true for plants receiving other treatments. Finally, plants receiving farm soil biota had twice as much AM fungal colonization as those receiving the commercial bioinoculant. Second, we measured AM fungal spore abundance in newly-turned, 1-year, and 2-year fields. We separated spores from soil using the sucrose-gradient method and counted spore abundance at 60x (3 fields x 25 reps = 75). Spore density was lowest in the newly-turned field and greatest in the 2-year field.

Together, these results suggest AM fungal abundance is reduced after field turnover, but likely increases each subsequent year. Furthermore, AM fungi from fields, in concert with other soil biota, can increase raspberry’s resistance to P. rubi, but commercial bioinoculants may not confer the same resistance. In summary, AM fungi is likely an important component of an integrated-pest-management plant for raspberries, but commercial bioinoculants may not be the right answer for growers.