Hybrid vigor in Eucalyptus increases resistance against Phytophthora root rot

Abstract

Eucalyptus nitens is a cold-tolerant eucalypt that is native to Eastern Australia. Pure E. nitens as well as its hybrids, such as Eucalyptus grandis × Eucalyptus nitens, is propagated commercially in various regions of the southern hemisphere, including South Africa. In a plantation environment, E. nitens is susceptible to a variety of native and invasive pathogens, including Phytophthora alticola and P. cinnamomi. Recently, there have been increasing reports of root and collar rot in E. nitens in South Africa. The severity of this disease was substantially lower among interspecific hybrids of E. grandis × E. nitens compared to purebred E. nitens. In South Africa, the susceptibility of commercially propagated provenances of pure E. nitens and varieties of hybrid E. grandis × E. nitens to Phytophthora species is unknown. Therefore, we conducted greenhouse trials to evaluate the pathogenicity of P. alticola and P. cinnamomi to two families of pure E. nitens, one self-fertilized and the other outcrossed, as well as a single clonal variety of the most widely planted interspecific hybrid, E. grandis × E. nitens. The outcomes from these trials revealed that both self-fertilized and outcrossed families of E. nitens were highly susceptible to the tested Phytophthora species. The severity of root rot was greatest among plants inoculated with P. cinnamomi. The tested interspecific hybrid was tolerant to both Phytophthora species and developed new lateral and fine roots to offset the effects of root rot.