VARIATION IN MYCORRHIZAL DEVELOPMENT AND GROWTH-STIMULATION BY 20 PISOLITHUS ISOLATES INOCULATED ON TO EUCALYPTUS-GRANDIS W HILL EX MAIDEN

Twenty isolates of Pisolithus, covering a range of hosts, basidiocarp types and geographic locations were compared on their ability to form mycorrhiza in vitro with Eucalyptus grandis and to stimulate seedling growth in vivo. There was a large variation between isolates in the rate of mycorrhizal development and this was used to categorize isolates into six types. Mycorrhiza of seedlings inoculated with type I isolates were the most advanced with a dual-layered mantle, a fully-developed Hartig net and radially elongated epidermal cells. Mycorrhiza of seedlings inoculated with types II-V isolates were progressively less advanced so that mycorrhiza of type V isolates had a thin single-layered mantle, the Hartig net was only partially developed and the epidermal cells were not elongated. Root tips of seedlings inoculated with type VI isolates were non-mycorrhizal. Inoculated seedlings were transplanted directly into undrained pots containing a yellow sand deficient in phosphorus. Growth stimulation of E. grandis seedlings in the glasshouse varied greatly between Pisolithus isolates, with dry weight increases ranging from two to 45 times that of the non-inoculated control seedlings. Extent of mycorrhizal development was positively correlated to growth stimulation in the glasshouse and it is proposed that this could be used as an indicator of isolate aggressiveness and consequently the potential of an isolate to promote tree growth.