Account of an invasion: emerging pests and diseases impacting cassava fields in Southeast Asia

Cassava is a major staple, bio-energy and industrial crop in many parts of the developing world, and a crucial crop in Southeast Asia, where it provides income for 8 million small-scale farmholds. While no limiting biotic threats occurred until the turn of the century, a complex of invasive arthropod pests and plant pathogens is now impacting Southeast Asian cassava crops. We present the results of a region-wide survey of cassava fields, covering Thailand, Laos, Cambodia, Myanmar and Vietnam. We evaluated pest and diseases distribution, field-level incidence and infestation pressure. Mealybugs and mites were most commonly found, affecting 70% and 54% of fields, and infesting 27% and 16% of plants respectively. The mealybug complex was composed by 6 taxa, all of which non-native. The cassava mealybug Phenacoccus manihoti (Homoptera: Pseudococcidae) was the most common (37% fields), followed by Paracoccus marginatus (29%) and Pseudococcus jackbeardsleyi (25%). Furthermore, plant-level infestation pressure of the recent invader P. manihoti was the highest among mealybug taxa. Cassava witches' broom (CWB), a systemic phytoplasma disease, was reported from 64% of plots at field-level incidence of 32%. Although the main pests and diseases are all exotics, we hypothesize that accelerating intensification of local cropping systems, increased climate change and variability, and deficient crop management are aggravating both organism activity and crop susceptibility. Future efforts should address all these potential drivers, and promote improved pest monitoring an detection, quarantine systems, locally-appropriate integrated management approaches, and ecologically-sound, sustainable crop cultivation techniques.