Phenolic Chemistry of Coast Live Oak Response to Phytophthora ramorum Infection

Since the mid 1990s, Phytophthora ramorum has been responsible for the widespread mortality of tanoaks, as well as several oak species throughout California and Oregon forests. However, not all trees die, even in areas with high disease pressure, suggesting that some trees may be resistant to the pathogen. In this study, the chemical basis of host resistance was investigated. Three field experiments were carried out in California between December 2004 and September 2005. The levels of nine phenolic compounds (gallic acid, catechin, tyrosol, a tyrosol derivative, ellagic acid, and four ellagic acid derivatives) extracted from the phloem of trees that had been either artificially inoculated with P. ramorum or trees putatively infected with P. ramorum (based on canker symptoms) were quantified by high-performance liquid chromatography (HPLC). Significant differences in phenolic profiles were found between phloem sampled from the active margins of cankers, healthy phloem from asymptomatic trees, and phloem sampled 60 cm away from canker sites, although the magnitude and direction of the responses was not consistent across all experiments. Concentrations of gallic acid, tyrosol, and ellagic acid showed the greatest differences in these different tissues, but varied considerably across treatments. Gallic acid and tyrosol were tested in in vitro bioassays and showed strong dose-dependent inhibitory effects against P. ramorum, P. cinnamomi, P. citricola, and P. citrophthora. These results suggest that phloem chemistry varies in response to pathogen infection in California coast live oak populations and that changes in phloem chemistry may be related to apparently resistant phenotypes observed in the field. © 2007 Springer Science+Business Media, LLC

Genetic tools and strategies for citrus breeding aiming at resistant rootstocks to gummosis disease

Phytophthora nicotianae Breda de Haan (syn. Phytophthora parasitica Dastur), causal agent of citrus gummosis disease, has caused great damage to citrus orchards throughout the world. While chemical and horticultural measures do not guarantee the preventive or curative control of citrus gummosis, the use of resistant rootstocks is the most reliable management strategy against the disease. Aiming at the development of citrus rootstocks resistant to gummosis and to better elucidate the Phytophthora-citrus pathosystem, citrus breeding programs have been ongoing worldwide, mostly employing directed crosses. These studies have succeeded in identifying differences in symptom development between resistant and susceptible rootstocks, as well as in the progeny of their crosses. In addition, differentially expressed genes were assessed, which ultimately should lead to the identification of markers involved in resistance to P. nicotianae. In this review we summarize the current knowledge of the molecular basis of citrus gummosis and the main strategies employed to obtain genetically resistant rootstocks.Biotechnology Lab Instituto Agronômico de Campinas Centro APTA Citros Sylvio Moreira, Caixa Postal 04Departamento de Genética Instituto de Biociências UNESP, Caixa Postal 510Department of Entomology and Plant Pathology Auburn UniversityDepartamento de Genética Instituto de Biociências UNESP, Caixa Postal 51