Advances in researches on the occurrence and control of Asia citrus psyllid

Abstract: The citrus yellow shoot disease (CYSD) , a phloem-limited bacteria [Candidatus Liberibacter spp., notably Ca. L. Asiaticus (LAS) ] disease, is a most destructive and devastating disease of citrus in Asia, Africa, North and South America. All commercially cultivated citrus is susceptible and varieties tolerant to the disease is not yet available. It causes substantial economic losses by reducing fruit production, shortening the lifespan of the tree, but no effective cure has been found. The CYSD-infected trees develop symptoms that include chlorotic leaves, twig dieback, fruit drop, abnormal and small fruits, lower internal fruit quality and eventual death of the whole plant. Three species of Candidatus Liberibacter (Ca. Liberibacter asiaticus, Ca. Liberibacter africanus and Ca. Liberibacter americanus) are associated with CYSD And Ca. Liberibacter asiaticus is the most important one. CYSD can be transmitted by psyllid and grafting. The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae) , is the most efficient vector to CYSD. Although the rate of LAS transmission by D. citri individuals is usually low, onset of CYSD occurs following a longer latent period after inoculation, and thus the pathogen can spread widely prior to detection. D. citri is attracted to yellow and yellowish-green colors that mimic reflectance spectra of host plants. And it is attracted both to natural host plant odors prevalent in the headspace collection of citrus and to a synthetic terpene mixture modeled on the principal volatiles collected from M. paniculata. The upper and lower thresholds for oviposition were estimated to be 16 ℃ and 41.6 ℃, respectively. D. citri females are prolific and can develop rapidly, laying up to 800 eggs per lifetime, which are deposited only on young tissue, particularly on newly expanded leaves growing from 1 to 5 days after budbreak. More recent investigations were made by using either conventional polymerase chain reaction (PCR) or real-time PCR (qPCR) to detect Las of psyllid. Worldwide, control of D. citri to reduce its role as a vector has been one of three citrical components of CYSD man-agement, in addition to planting pathogen-free nursery stocks and removing inoculums by destroying infected trees. It remains to be seen if a number of areas will be explored, notably on D. citri-LAS-plant interactions, on host plant resistance to D. citri, and on molecular methods of silencing D. citri genes to induce mortality or to block its ability to transmit CYSD-causing bacteria. Chemical control is the primary management strategy currently employed, but recently documented decreases in susceptibility of D. citri to several insecticides illustrate the need for more sustainable tools. But very little information on insecticidal control againse D. Citri was reported in the literature prior to the arrival of CYSD in the Americas in the mid-2000 s. Phylogeographic and genetic studies on D. citri populations in the Americas indicated that two founding events of D. citri probably occurred, one in South America and one in North America. More recent studies on D. citri's response to insecticides indicate sensitivity to a number of different insecticide classes, including pyrethroids, organophosphates, carbamates, neonicotinoids, some isect growth regulators (IGRs) , horticulturl oil, the lipid synthesis inhibitor spirotetramat, spinetoram, abamectin, and sucrose octanoate. A number of gungal entomopathogens are reported to infect D. citri especially under conditions of high humidity, including Isaria (Paecilomyces) fumosorosea (Wize) , Lecanicillium lecanii, and so on. The ectoparasitoid Tamarixia radiata (Waterston) (Eulophidae) and the endoparasitoid Diphorencyrtus aligarbensis (Shafee, Alam and Argarwal) (Encyrtidae) are generally accepted as the only currently known primary parasitoids of D. citri. There is general agreement that the major predators of D. citri are lady beetles, lacewings, syrphids, and spiders. However, the relative importance of each group is less certain due in part to the difficulty in evaluating their individual contributions to mortality. Hundreds of thousands of trees in Florida that probably were infected before the management program was implemented were removed from groves. The extent to which the three-tiered management problem negates incidence and spread of the disease may depend largely on levels of inoculum within and around a grove. Within less than 7 years of the discovery of CYSD in Florida, most growers found the three-tiered program too expensive, were reluctant to remove infected trees that were productive and consequently abandoned efforts to remove trees unless they were nonproductive. More recently, a number of growers have implemented three nutritional programs hoping to sustain the producticity of infected trees, but there has been no published research supporting this idea.Advances in these and other research areas may depend greatly on a better understanding of basic D.citri biology and vector-pathogen-host plant interactions at the molecular, cellular, and community levels. Here, we present an updated review of D. citri and CYSD with an emphasis on the problem in the world. Currently considering D. citri, the world's most serious disease of citrus, is transmitting quickly, it has been a hot topic of citrus researches. In this paper, the distribution, host plants, mechanism of transimission, life cycle, forecasting and prevention methods were summarized. This review will lay a sound foundation for better control of CYSD.