Citrus Huanglongbing correlated with incidence of Diaphorina citri carrying Candidatus Liberibacter asiaticus and citrus phyllosphere microbiome.

In China, citrus Huanglongbing (HLB) disease is caused by the Candidatus Liberibacter asiaticus bacterium, which is carried by the Asian citrus psyllid Diaphorina citri Kuwayama. It was hypothesized that the epidemic of the HLB may related with the rate of bacterium presence in the insect vector and bacterium content in plant tissues, as well as the phyllosphere microbe communities changes. This study systematically analyzed the presence or absence of Ca. L. asiaticus in citrus tree leaves and in the insect vector D. citri over a 6-year period using real-time PCR. In addition, changes in the number of bacteria carried by D. citri over 12 months were quantified, as well as the relationship between the proportion of D. citri carrying Ca. L. asiaticus and the proportion of plants infected with Ca. L. asiaticus were analyzed. Results showed that the proportion of D. citri carrying bacteria was stable and relatively low from January to September. The bacteria in citrus leaves relatively low in spring and summer, then peaked in December. The proportion of D. citri carrying bacteria gradually declined from 2014 to 2019. The proportion of D. citri carrying Ca. L. asiaticus showed a significant positive correlation with the proportion of diseased citrus. The phyllosphere bacterial and fungal communities on the healthy citrus leaf were significantly different with the disease leaf in April and December. Pathogenic invasions change the citrus phyllosphere microbial community structure. It could be summarized that citrus Huanglongbing correlated with incidence of Diaphorina citri carrying Candidatus Liberibacter asiaticus and citrus phyllosphere microbiome.

Control efficacy and joint toxicity of thiamethoxam mixed with spirotetramat against the Asian citrus psyllid, Diaphorina citri Kuwayama.

BACKGROUND: The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, is one of the most devastating pests in citrus orchards, and has caused huge economic losses worldwide. Chemical control is the most effective way for psyllid control. Herein, the toxicity of nine insecticides to ACP adults and the joint action of thiamethoxam + spirotetramat were determined by a topical application method in the laboratory; field plot experiments were conducted to evaluate the control efficacy of one self-made thiamethoxam + spirotetramat 40% suspension concentrate (SC) comparing with thiamethoxam 21% SC, spirotetramat 22.4% SC, tolfenpyrad 15% SC and bifenthrin 100 g/L emulsifiable concentrate against ACP using foliar sprays in 2018-2019. RESULTS: The highest toxicity to ACP adults was achieved by beta-cyfulthrin, bifenthrin, thiamethoxam and acetamiprid, with median lethal doses of 0.247 to 1.382 ng/adult at 24 h after treatment. High toxicity was observed by chlorpyrifos, spirotetramat and tolfenpyrad, but moderate toxicity by pyriproxyfen and buprofezin. For mixutres of thiamethoxam and spirotetramat, a 25:15 mass ratio showed the highest synergistic effect, with a co-toxicity coefficient (CTC) of 246.52; while a 10:30 mass ratio exhibited an additive effect, with a CTC of 109.84. Thiamethoxam + spirotetramat 40% SC at 60-80 mg/kg can effectively control ACP with a control efficacy of 72.92 to 99.29% during 3-30 days. Moreover, foliar sprays of all tested insecticides at the tested rates had no phytotoxic effects on citrus trees. CONCLUSION: A one-time foliar spray of thiamethoxam + spirotetramat 40% SC at 80 mg/kg could be recommended to control ACP during its infestation period in citrus groves.

Field Efficacies and Joint Actions of Beta-cyfluthrin Mixed With Thiamethoxam or Tolfenpyrad Against Diaphorina citri (Hemiptera: Liviidae).

The Asian citrus psyllid, Diaphorina citri Kuwayama, is the most serious pest of citrus because it is a vector for the highly destructive citrus greening disease (huanglongbing, HLB). Currently, insecticide applications are being used widely to control psyllid populations, thereby suppressing the spread of HLB. In the present study, topical application bioassays were performed to detect the joint actions of beta-cyfluthrin and thiamethoxam or tolfenpyrad against D. citri adults in the laboratory. In 2019, a field plot experiment was conducted to evaluate the control efficacies of beta-cyfluthrin+thiamethoxam 22% capsule suspension and beta-cyfluthrin+tolfenpyrad 30% microemulsion against D. citri using foliar sprays. For the former, a 9:13 mass ratio had the highest synergistic effect, with a cotoxicity coefficient of 188.64. For the latter, a 5:25 mass ratio had the highest synergistic effect, with a cotoxicity coefficient of 153.94. A one-time foliar spray of the former at 30-40 mg/kg or of the latter at 40-60 mg/kg effectively controlled D. citri, with control efficacies varying from 80.1 to 99.4% or 80.4 to 100.0%, during the 3-30 d after treatment, respectively. Moreover, field observations indicated that these foliar sprays at the tested rates had no negative effects on citrus trees. Thus, foliar sprays of beta-cyfluthrin+thiamethoxam or beta-cyfluthrin+tolfenpyrad under the given conditions may control D. citri.