Growth and behaviour of Armillaria mellea rhizomorphs in soil

Abstract

The number and dry weight of rhizomorphs produced from woody inocula containing Armillaria mellea (Vahl ex Fr.) Kummer and buried in soil varied with soil type and incubation temperature. In the same soil fewer rhizomorphs were initiated at 15°C than 25° but the total dry weight of rhizomorphs was approximately the same, thus systems produced by individual initials were larger at 15° than at 25°. The form and branch pattern of these systems also differed markedly. It is tentatively suggested that within as yet undefined limits the growth rate of individual rhizomorph systems may be inversely proportional to the total number of systems supported by a food base. Similarly, the same relationship may hold within a system: the greater the number of growing tips the slower the growth rate of each. Unbranched rhizomorphs growing from mineral soil into a layer of peat branched at the point of entry into the peat and, by regular dichotomous branching, formed complex systems within it. Distribution of rhizomorphs in soil varies with depth, but the main concentration occurs between 2·5 and 20·0 cm below the surface. They are rarely found below 30·0 cm. On one site, rhizomorph growth rate was estimated at 1 m per year. For a variable distance behind the growing tip rhizomorphs are hollow but farther away the centre becomes filled with a compact mass of cells. Up to 20 rhizomorphs were formed from both of the cut ends of severed rhizomorphs in the field and from both ends of 6 cm lengths of excised rhizomorphs. In old mixed or broadleaved woodlands infested by A. mellea there is often little evidence of its presence, although rhizomorphs may occur epiphytically on the root systems of most trees and many roots may have localized lesions. It is suggested that felling, and rhizomorph severing following soil disturbance caused by timber extraction or ploughing, may cause a rapid increase in activity by A. mellea and any newly planted trees would be at maximum risk.