Agronomy and Crop Science

AbstractBACKGROUND: Vector‐borne plant disease management can be enhanced by deployment of antifeedants in addition to the use of broad‐spectrum neurotoxic insecticides. The effects of pymetrozine on Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, feeding behaviour, survival and transmission of Candidatus Liberibacter asiaticus (Las), the presumed causal pathogen of huanglongbing, were investigated.RESULTS: Pymetrozine applied at 52 and 104 µg mL−1 to citrus plants [Swingle citrumelo (X Citroncirus webberi Ingram and Moore)] modified the feeding behavior of ACP and increased the amount of time spent performing non‐penetration behaviors while decreasing the time spent performing ingestion behaviors compared with the controls 1 day after treatment. However, the antifeedant effect of pymetrozine subsided 5 days after application. Pymetrozine reduced the survival of both adults and nymphs on treated plants compared with the control. However, it had a greater impact on survival of nymphs than on survival of adults. Pymetrozine applied at 52 and 104 µg mL−1 on Las‐infected ‘Valencia’ sweet orange plants [Citrus sinensis L. (Osbeck)] reduced acquisition (12 and 21% respectively) and transmission (11 and 18% respectively) of Las by feeding ACP adults compared with the controls; however, these reductions were not statistically significant.CONCLUSIONS: Pymetrozine exhibited moderate antifeedant effects by modifying the feeding behavior of ACP adults with short residual activity. The impact of pymetrozine on survival of nymphs was greater than on adults at the higher concentrations tested. Pymetrozine also reduced the acquisition and transmission of Las by feeding ACP adults up to 21 and 18%, respectively, compared with untreated controls. Copyright © 2010 Society of Chemical Industry

AbstractBACKGROUND: In the present investigation, the effect of Candidatus Liberibacter asiaticus (Las), a bacterium considered to be responsible for causing huanglongbing (HLB) disease in citrus, on the physiology of its vector, the Asian citrus psyllid (ACP) Diaphorina citri Kuwayama, was determined. Specifically, the effects of Las infection on the susceptibility of ACP to selected insecticides were determined. Furthermore, total protein content and general esterase activity were quantified in Las‐infected and uninfected ACP to gain insight into the possible mechanism(s) responsible for altered susceptibility to insecticides owing to Las infection.RESULTS: LC50 values were significantly lower in Las‐infected than in uninfected ACP adults for chlorpyrifos and spinetoram. Furthermore, there was a general trend towards lower LC50 values for three other insecticides for Las‐infected ACP; however, the differences were not statistically significant. Total protein content (µg mL−1) was significantly lower in Las‐infected (23.5 ± 1.3 in head + thorax; 27.7 ± 1.9 in abdomen) than in uninfected (29.7 ± 2.1 in head + thorax; 35.0 ± 2.3 in abdomen) ACP. Likewise, general esterase enzyme activity (nmol min−1 mg−1 protein) was significantly lower in Las‐infected (111.6 ± 4.5 in head + thorax; 109.5 ± 3.7 in abdomen) than in uninfected (135.9 ± 7.5 in head + thorax; 206.1 ± 23.7 in abdomen) ACP.CONCLUSION: Susceptibility of ACP to selected insecticides from five major chemistries was greater in Las‐infected than in uninfected ACP. The lower total protein content and reduced general esterase activity in Las‐infected than in uninfected ACP may partly explain the observed higher insecticide susceptibility of Las‐infected ACP. Copyright © 2010 Society of Chemical Industry

Changes in carbohydrate metabolism in sweet orange ( Citrus sinensis ) infected with ‘ Candidatus Liberibacter asiaticus’, a purported cause of citrus Huanglongbing (HLB), were investigated. Starch levels in HLB‐infected leaves with and without symptoms increased 3·1‐ and 7·9‐fold, respectively, compared to healthy controls. In symptomless leaves, sucrose and fructose accumulated significantly ( P  <   0·05) in both midribs and lobes, and glucose only in the midribs (greater than fivefold); whereas maltose levels were reduced to 64·6% and 86·8% in the midribs and foliar lobes, respectively, of the values in healthy leaves. In leaves with symptoms, sucrose and glucose remained at high levels compared to healthy leaves, whilst no accumulation of fructose was observed; by contrast, the maltose content decreased to as low as 49·6% of that in healthy leaves. Fourfold induction of cell‐wall‐bound invertase activity was detected in both types of leaves on diseased plants. Additionally, the expression profiles of starch breakdown genes suggested that the transcription of DPE2 and MEX1 was down‐regulated. Together with the reduction of maltose, it is suggested that the impairment of starch breakdown may contribute to the starch accumulation in infected leaves. The imbalance of carbohydrate partitioning and its relation to disease physiology are discussed.