Chapter 2 - Update on phytoplasma diseases associated with vegetable crops in Asian countries

Abstract

Phytoplasmas are phloem-limited bacteria associated with diseases in different vegetable crops from tropical to temperate regions of the world. The phytoplasma groups associated with vegetable crops have a major economic relevance all over Asia and leading to considerable economic losses. Phytoplasmas affect numerous vegetable crops belonging mostly to Solanaceae, Cucurbitaceae, Fabaceae, Cruciferae, Brassicaceae, Asteraceae, and Apiaceae families. Of the ribosomal groups documented globally, at least 11 groups have been detected in vegetables crops in Asia, and this highlights the need to adopt measures to prevent their introduction to new areas in various Asian countries. The majority of the phytoplasma reports on vegetables are from South and West Asia, however phytoplasmas in subgroup 16SrII are widely distributed throughout Asia suggesting the potential spread of these pathogens. The 16SrI (aster yellows) group is the most frequently detected in the East Asian countries mainly infecting solanaceous crops such as potato, tomato, brinjal, and chili. Conversely, the 16SrII (peanut witches' broom) group is the most widespread in the West Asian countries where it was associated with 17 different vegetable crop species followed by the phytoplasmas classified in the 16SrVI group. Several leafhoppers and weed species are reported in different Asian countries as reservoirs of phytoplasmas in different vegetable crops. The management of these phytoplasma-associated diseases relies on integrated approaches involving the use of resistant genotypes and cultural practices such as rogueing, use of clean propagating materials, and control of insect vectors. This chapter provides an overview of phytoplasma diseases infecting vegetable crops in Asia, with emphasis on symptoms, classification, geographic distribution, genetic diversity, mixed infection with other pathogens and management.