Further investigations on the putative causal agent/s of the emerging Volkamer lemon disease threatening the citrus industry in Egypt
Résumé
Volkamer disease is an emerging serious rootstock disease appeared to affect Volkamer lemon rootstock in the Mediterranean basin when tolerant rootstocks were introduced to control citrus tristeza disease. The disease is devastating citrus grown in the Egyptian newly reclaimed land. Diseased trees are usually stunted, and scraping the bark reveals gum deposits, phloem discoloration, and stem pitting. These symptoms resemble those of cachexia on mandarin and gummy bark disease on sweet orange. The disease incidence was correlated with hot climate and transmission potency, which has drawn the attention to consider citrus viroids and/or phytoplasmas as a putative causal agent. Molecular detection and characterization of several citrus viroids were performed on previously collected samples belonging to the newly reclaimed land in Egypt.
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Meanwhile, a biological assay of infected material was included in this investigation. Interestingly, all the assays disclosed and confirmed the absence of phytoplasmas. Whereas, HSVd and CBCVd were the most prevalent detected viroids in the symptomatic samples. Molecular characterization and biological indexing confirmed the severity of both viroids, indicating that the volkamer diseasecould be associated with single or mixed infection involving HSVd and CBCVd.