Fungal pathogens and arbuscular mycorrhizal fungi of abandoned agricultural fields: potential limits to restoration.
Little is known about impacts of soilborne pathogen legacies on reestablishment of native plant species in abandoned agricultural fields. We tested whether pathogens found in abandoned citrus orchards affect growth of native and invasive plant species in a controlled greenhouse experiment. In previous research, we identified several species of ascomycete (Fusarium spp.) and oomycete (Pythium spp.) pathogens from field roots and soils. The invasive annual grass, ripgut brome [Bromus diandrus (Roth.)], and native forbs, common fiddleneck [Amsinckia intermedia Fisch. & C.A. Mey.], coastal tidytips [Layia platyglossa (Fisch. & C.A. Mey.) A. Gray], and California goldfields [Lasthenia californica (DC. ex Lindl.)], were grown together in four different field soil treatments. Using pesticides on soils collected from abandoned citrus fields, we created four soil treatments that excluded different groups of potential pathogens: (1) untreated control (2) metalaxyl (oomyceticide) (3) fludioxonil (fungicide), and (4) steam-sterilized. Fludioxonil increased aboveground biomass of L. platyglossa (P = 0.005) and L. californica (P= 0.02) compared with sterile and metalaxyl-treated soils. Lasthenia californica had decreased arbuscular mycorrhizal colonization with metalaxyl, suggesting metalaxyl has non-target effects on mycorrhizae. Fludioxonil decreased potential pathogens in L. californica roots while having no effect on mycorrhizal colonization. Bromus diandrus had higher biomass in sterile and fludioxonil-treated soils than untreated soils (P = 0.0001), suggesting a release from soilborne pathogens. The release from soilborne pathogens with the use of fludioxonil in both native forbs and B. diandrus, combined with overall higher biomass across treatments in B. diandrus, suggests that pathogen impacts in a field setting are insufficient to reduce success of this invasive grass, and use of a fungicide would not benefit native species in mixed stands with B. diandrus.