Almond witches’ broom phytoplasma : development of detection methods, epidemiology and management of the disease -

Abstract

In Lebanon since the early 1990s, a severe disease of stone fruits characterized by proliferation and appearance of witches’ broom symptoms devastated almond, peach and nectarine plants; over 200,000 trees were affected. The disease was named almond witches’ broom (AlmWB) and the causal agent was identified as “Candidatus Phytoplasma phoenicium”, a phytoplasma belonging to sub-group 16SrIX-D. So far, pathogen detection relied on PCR and management relied mainly on eradication of infected trees. This research focused on development of serological detection methods which are normally less expensive and require less experience than PCR methods. Phytoplasma are phloem limited and cannot be cultured in vitro; therefore, recombinant DNA technology was successfully used to amplify, clone and express two integral membrane protein genes of “Ca. P. phoenicium”. However, the levels of expression are low and optimizations of protein expression protocols are in progress in order to produce enough antigen for antibody production. Development of rapid, sensitive and specific serological detection method will provide an efficient tool in surveys aiming at early detection of the pathogen for eradication purposes. Since no resistant almond cultivars have been identified, grafting experiments were conducted in field and in greenhouse trials. The two grafting trials showed promising results. In the field trial, the growth from apricot or plum scions grafted on AlmWB-infected almond trees was symptomless for over a year. Similarly grafting AlmWB-infected scions on seedlings of plum and apricot, grown in the greenhouse, resulted in growth of symptomless shoots. An interesting recovery phenomenon was observed with three varieties; shoots developing from apricot Farclo grafted on AlmWB-infected trees in the field showed severe symptoms two months post-grafting but recovered three months later and remained symptomless to present; about two and a half years. in the greenhouse trial, the growth of AlmWB