Improving the colonization capacity and effectiveness of ectomycorrhizal fungal cultures by association with a host plant and re-isolation

The ability of an ectomycorrhizal fungus to colonize plant roots (colonization capacity) and to increase plant growth at a deficient supply of P (effectiveness) declines after repeated subculture on agar media. We attempted to revitalize selected isolates of ectomycorrhizal fungi by inoculating them onto a compatible host ( Eucalyptus globulus) and reisolating them from ectomycorrhizas and basidiomes. The growth on agar of the reisolated fungal cultures and the original fungal cultures was measured over 14 d. Seedlings of E. globulus were also inoculated with either the original fungal isolates or their reisolates and were grown in pots containing a P-deficient sand, in a temperature-controlled glasshouse. Seedlings were harvested 49 and 93 d after planting and were assessed for mycorrhizal colonization, dry weights and P concentrations. The reisolates generally grew at a faster rate on agar, and colonized plant roots more quickly, than the original isolates. Plants inoculated with the reisolates also had increased dry weights by day 93, which could be attributed to increased P uptake by the plant. We concluded that reisolating ectomycorrhizal fungi from mycorrhizas and basidiomes can increase the colonization capacity and effectiveness of isolates which have been grown on agar media for extended periods. This result, and the high cost of maintaining cultures emphasizes the need to examine alternative methods of storage of fungal isolates.