Chapter 12 - Updates on phytoplasma diseases associated with sandalwood in Asia

Abstract

The Indian sandalwood, Santalum album, is the most appreciated Santalum species globally, mainly for its fragrant heartwood and oil used for incense, soaps, creams, perfumes, carvings, paintings, and religious worships for over 4000 years. Sandalwood diseases include seedling disease (Phytophthora sp. and Fusarium oxysporum), leaf spot disease (Ascochyta santali, Macrophomina phaseoli, Asterina congesta, and Sphaceloma santali), leaf curl disease (virus), and sandal spike disease (SSD, Candidatus Phytoplasma). SSD is the prime factor for the decline of the S. album population in India, and S. album is now listed in the IUCN ‘Red List’ of threatened species. Earlier, SSD detection was often tricky due to the uneven spread of the phytoplasma in tree samples. These detections were grounded on the observation of symptoms, insect or graft transmission to the host plant, microscopy observation of ultrathin sections of phloem tissues, staining, and serological techniques. Today, the accurate detection and diagnosis of SSD is carried out by the PCR followed by the sequencing of targeted amplicons of the 16S rRNA gene. The SSD phytoplasma was found associated with the ‘aster Yellows’ group belonging to the ‘Ca. P. asteris’. Insect vectors play a vital role in spreading phytoplasma diseases, their epidemiology, and the extent of loss they cause. No insect vectors have been confirmed spreading the SSD yet. This chapter involves various aspects of SSD identification, diagnosis, disease control, and aims to draw interest in protecting the sandalwood bioresource in its natural habitats.