Root Temperature Effects on the Growth ofPhytophthora cinnamomiin the Roots ofEucalyptus marginataandE. calophylla

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Abstract
Colonization of the roots of seedling jarrah (E. marginata) and marri (E. calophylla) by the cinnamon fungus (P. cinnamomi) was studied under controlled temperatures and light intensity. Clear differences in pathogen development were in the 2 spp. In marri (field-resistant), invasion was restricted after 20-120 mm of the root tissue was colonized. The extent of the invasion within a temperature range of 14-28.degree. C varied, but the growth of the pathogen was always limited in this species. P. cinnamomi could be recovered from the zone of healthy tissue adjacent to the lesion for up to 30 days after hyphal growth was contained, but there was no infection of roots not in direct contact with the diseased tissue. In jarrah, a field-susceptible species, a wider variation in the degree of root colonization was observed, although the extension of the pathogen was still dependent upon root temperature. At 14.degree. C, the invasion was restricted to between 30 and 50 mm of the root. At 28.degree. C the fungus spread through the root system, although the total extension varied between seedlings. At intermediate temperatures, the reaction varied from containment of hyphae to systemic invasion of the root system. These differences in the capacity of seedling roots of laboratory grown plants to restrict the invasion by P. cinnamomi parallel differences in the sensitivity to the disease shown by mature trees of the same species in the field. They suggest that even in a susceptible species such as jarrah, the degree to which root tissue is colonized is temperature dependent.