Effects of Isolate and Time of Inoculation on Invasion of Secondary Phloem of Eucalyptus spp. and Banksia grandis by Phytophthora spp.. B. L. Shearer, Research Station, Department of Conservation and Land Management, Dwellingup, Western Australia 6213. B. J. Michaelsen, and P. J. Somerford, Research Station, Department of Conservation and Land Management, Dwellingup, Western Australia 6213. Plant Dis. 72:121-126. Accepted for publication 7 July 1987. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1988. DOI: 10.1094/PD-72-0121.

Inoculated stems were used to compare the growth of isolates of Phytophthora cactorum, P. cambivora, P. cinnamomi, P. citricola, P. cryptogea A₁ and A₂, P. megasperma var. sojae, P. nicotianae var. nicotianae, P. n. var. parasitica, and Phytophthora species identified as unknown by the Commonwealth Mycological Institute in the secondary phloem of Banksia grandis and Eucalyptus marginata. Only isolates of P. cinnamomi grew faster in the phloem of B. grandis than E. marginata. In E. marginata, there were no significant differences in mean extension rates between P. cactorum, P. cinnamomi, P. citricola, P. cryptogea A₁ and A₂, P. n. var. nicotianae, and P. n. var. parasitica. In contrast, the mean lesion extension rate of 4.98 mm/day for P. cinnamomi in B. grandis was significantly (P = 0.05) greater than the mean rates of 0.11–2.44 mm/day for the other Phytophthora species. In E. marginata, variation in lesion extension was greatest between isolates of unknown Phytophthora species and P. m. var. sojae and least between isolates of P. cinnamomi and P. cryptogea A₂. Linear growth rate of the Phytophthora species in the two hosts was correlated with tangential growth. Stems of B. grandis, E. marginata, and E. calophylla were inoculated with P. cinnamomi and P. citricola in summer, autumn, and winter, and lesion size was assessed 6 wk and 6 and 12 mo after inoculation. Linear and tangential growth of P. cinnamomi and P. citricola in E. marginata was greatest in summer and least in winter. Except for the second assessment after summer inoculation, lesion extension of P. citricola in E. marginata and E. calophylla was consistently, though not always significantly, greater than that for P. cinnamomi, with greatest differences between the two Phytophthora species occurring in stems assessed in winter. In B. grandis, lesions of P. cinnamomi were consistently greater than those of P. citricola. Lesions of P. citricola in B. grandis were confined as were lesions of P. cinnamomi in the moderately resistant host E. calophylla. Infection of secondary tissue of B. grandis provides a mechanism for survival and spread of P. cinnamomi in the E. marginata forest. The Phytophthora species with slow rates of growth in B. grandis are likely to be confined, and infection of this host is unlikely to favor their survival and spread as much as it does that of P. cinnamomi.