Effect of arbuscular mycorrhizal fungus infection on the incidence of fusarium root rot in asparagus seedlings

The incidence of fusarium root rot, caused by Fusarium oxysporum f. sp. asp aragi (Foa), was investigated in seedlings of asparagus (Asparagus officina lis L., cv. Mary Washington 500W) by inoculation with three species of arbu scular mycorrhizal (AM) fungi; Gigaspora margarita (GM), Glomus fasciculatu m (GF), and Glomus sp. R10 (GR). Ten weeks after AM fungal inoculation, the inoculated plants were taller; t hey produced more shoots and roots and accumulated more dry matter in the s hoots and roots than noninoculated ones. The infection levels in a root sys tem differed with fungal species. Six weeks after Foa inoculation, 90% of the noninoculated plants exhibited symptoms of fusarium root rot, whereas 20-50% of the inoculated plants did. The effect was more pronounced in GR, GM, and GF, in that order. As for th e disease indices, it was lower in the inoculated plants than in the nonino culated ones. The indices differed among the AM fungal species; it was sign ificantly low in GR. The number of Foa hyphae invading feeder roots decreas ed in the inoculated plants, compared with the noninoculated ones. In addit ion, AM fungal hyphae preferentially elongated into short cells in the exod ermis of feeder roots, while the Foa hyphae also elongated into short cells as well as AM fungus. However, no short cells became infected with both AM fungus and Foa. These results reveal that tolerance to fusarium root rot was conditioned by AM fungal infection in asparagus seedlings, although the effect differed w ith the AM fungal species. It seems that the tolerance to fusarium root rot was partially caused by AM fungal pre-infection in short cells which suppr essed invasion by Foa in feeder roots.