Phloem Limited Citrus Pathogens in the 21st Century: Distribution, Diagnostics and Management Strategies

Abstract

Citrus is regarded as a fruit crop which is economically relevant in the whole world as a source of nutrition, job creation and overall agricultural trade in the world. However, the agricultural productivity of citrus is being limited by a set of devastating diseases which are born by the phloem affirming or phloem confined pathogens commonly known as the plasma diseases. Diverse biological agents such as phloem-limited bacteria including Candidatus Liberibacter spp. responsible of Huanglongbing, Spiroplasma citri responsible of citrus stubborn disease, phytoplasmas causing symptoms of decline and witches’ broom and a variety of citrus viroid like citrus Exocortis viroid or Hop stunt viroid cause these diseases. These pathogens inhabit the phloem tissues, interfere with the movement of photo assimilates and compromise root systems and eventually lead to a decline in yields, decline in fruit quality and mortality of the trees. These phloem restricted citrus diseases are known to be systemic, highly latent and variably symptomatic and are ascribed to effectively be transmitted by insect vectors or non-sterile propagation media. Over the last few decades changes in the incidence and the spread geographically of these diseases have risen steeply owing to increased global trade of plants, proliferation of vectors and alterations in the ecology of vectors caused by climate change. The systems of weak certification, informal nurseries, scanty diagnostic infrastructure as well as unsatisfactory management of vectors have also made citrus orchards in Asia including Pakistan susceptible to these pathogens. The present review synthesizes the existing knowledge of the world and regions on the citrus phloem associated diseases with specific emphasis on the issue of pathogen diversity and disease development and its increasing menace to the citrus production in Pakistan and South Asia. There is a focus on disease drivers to facilitate proper surveillance and diagnosis and combined management plans.