Abstract
The incidence of fusarium root rot, caused by Fusarium oxysporum f. sp. asparagi (Foa), was investigated in seedlings of asparagus (Asparagus officinalis L., cv. Mary Washington 500W) by inoculation with three species of arbuscular mycorrhizal (AM) fungi ; Gigaspora margarita (GM), Glomus fasciculatum (GF), and Glomus sp. R10 (GR). Ten weeks after AM fungal inoculation, the inoculated plants were taller ; they produced more shoots and roots and accumulated more dry matter in the shoots and roots than noninoculated ones. The infection levels in a root system 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 the disease indices, it was lower in the inoculated plants than in the noninoculated ones. The indices differed among the AM fungal species ; it was significantly low in GR. The number of Foa hyphae invading feeder roots decreased in the inoculated plants, compared with the noninoculated ones. In addition, AM fungal hyphae preferentially elongated into short cells in the exodermis 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 with 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 suppressed invasion by Foa in feeder roots.
