Trapping of root-knot nematodes by the adhesive hyphae-forming fungus Arthrobotrys oligospora.

The present study addresses the ecology of a particular isolate of Arthrobotrys oligospora (CBS 289.82) in relation to its efficacy in controlling the root-knot nematode, Meloidogyne hapla.This isolate was selected because it differs from most nematode-trapping fungi in that it captures nematodes with adhesive hyphae without having to form complex trapping devices. This characteristic may make it a very useful biological control agent. An attempt was made to gain insight into the factors controlling its capture ability.In vitro experiments demonstrated that the A. oligospora (CBS 289.82) isolate was very effective in capturing M.hapla and M. incognita, compared to the ability of other fungi with other trapping devices. Mobile juveniles were all caught by the hyphae within one hour and in some cases attachment occurred within the very first contact (chapter 2).Electron microscopic observations revealed that attachment of juveniles of Meloidogyne spp. to hyphae is mediated by a layer of extracellular material, about 0.1 gm thick, on the hyphae (chapter 3). Such a layer was never observed in hyphae of fungal cultures to which nematodes were not added, suggesting that its presence depends on an interaction of the fungus with the nematode. The attachment of Meloidogyne second-stage juveniles was not affected by temperatures between 5 and 3O°C. However, at 15°C ring structure development and growth of trophic hyphae were strongly hampered, which suggest that under prevailing soil temperatures in temperate regions, ring structure development and growth of trophic hyphae may proceed slowly whereas trapping would continue to occur (chapter 4).Furthermore, the nutritional conditions during growth of the fungus did not correlate with the rapidity of nematode-hypha attachment. The results also provide evidence that the trapping ability of the isolate tested continued for over more than 70 days (chapter 4).Arthrobotrys oligospora (CBS 289.82) covered dead, ruptured nematodes with a dense mycelium, whereas dead but otherwise intact nematodes were penetrated through the buccal cavity by a corkscrew-like structure and were subsequently colonized by trophic hyphae. Colonization of living second-stage juveniles by trophic hyphae following attachment and penetration was faster than colonization of dead second-stage juveniles. The addition of dead juveniles to a fungal colony prior to the addition of living juveniles did not affect attachment or the development of trophic hyphae through the live juveniles. However, one day after the addition of live juveniles, the proportion of live nematodes with ring structures was higher than when living and dead juveniles were added at the same time. The development of trophic hyphae in dead second-stage juveniles was delayed in the presence of live second-stage juveniles. The results refute the commonly held assumption that poor possibilities for saprophytic growth are a prerequisite for the formation of trapping devices and the predacious mode of feeding in the fungus (chapter 5).An important quality of fungi as potential biological control agents is their ability to form mycelium and capture structures in the soil at the place where their activity is desirable.The establishment and capture activity of this isolate in a simple microcosm system at 20°C, was compared to that of other fungi from the Dactylaria -complex. Direct microscopic observations in microcosms confirmed the attachment of mobile juveniles of M. hapla to hyphae of A. oligospora (chapter 6). Application of about 30 mm hyphal fragments of A. oligospora (CBS 289.82) per gram soil resulted in 100-170 in of hyphae per gram oven-dry soil at 20°C within 12 days, a reduction of 90% in the number of living nematodes of M. hapla within one day after addition and the extermination of the nematodes within 10 days. In non-sterilized soil, the hyphae reached a total length of 10 in per gram oven-dry soil. This amount of hyphal mass was sufficient to reduce the number of nematodes by 70% as compared to the control within 10 days after nematodes were added to the soil. At 13°C, similar results were obtained. Even at low densities, this isolate is effective. Notwithstanding their ability to form the most extensive mycelial mats of all fungi tested and despite the fairly large amounts of M. hapla added, nematode capture in both adhesive ring-forming fungi A. conoides (CBS 265.83) and A. oligospora (ATCC 24927) was zero or low. This supports other observations that the adhesive ring-forming fungi are inefficient (chapter 6).In chapter 7 the results on the ability of A. oligospora (CBS 289.82) to capture root-knot nematodes presented in the previous chapters are evaluated. The mechanism of nematode trapping is also discussed from the perspective of screening nematode-trapping fungi and using the adhesive hyphae-forming fungi as biological control agents