| Summary: | Root diseases have become an emerging biotic threat to Jak (<em>A. heterophillus</em>), grown in Sri Lanka. Isolation of the causal agents, identification and determination of the effect of selected microenvironmental factors and the crossinfection ability of the causal agents on other perennial crops towards development of effective management measures were the objectives of the present study. Two Rigidoporous microspores isolates and a Fusarium oxysporum and a F. solani isolate were identified as causal agents of root rot disease of Jak. Colony growth of the two R. microsporus isolates and the Fusarium isolates responded differently to pH, temperature and light intensity. Differences of colony growth responses were identified even between the two isolates of R. microporus. Among the perennial tree species used, <em>Atocarpus nobilis</em> was highly susceptible to all four fungal isolates and the lowest infection ability was shown on <em>Persea americana</em>. <em>Artocarpus altilis</em> (Rata Del), <em>Hevea brasiliensis</em> (Rubber), <em>Cinnammomum zeylanicum</em> (Cinnamon), <em>Durio zibethinus</em> (Durian), <em>Psidium guajava</em> (Guvava) and <em>Nephellium lapaceum</em> (Rambutan) were equally-susceptible to the infection by the four fungal isolates, used. Findings of the study are informative when developing an integrated management programme against root diseases of Jak.
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