Agronomy and Crop Science

Abstract The African citrus triozid (ACT) Trioza erytreae (Del Guercio) (Hemiptera: Triozidae) and the Asian citrus psyllid (ACP) Diaphorina citri (Kuwayama) (Hemiptera: Liviidae) are primary vectors of the ‘ Candidatus ’ Liberibacter spp. ACT is associated with ‘ Candidatus Liberibacter africanus’ (CLaf) causal agent of the African citrus greening disease (ACGD), whereas ACP vectors ‘ Candidatus Liberibacter asiaticus’ (CLas) and ‘ Candidatus Liberibacter americanus’ (CLam), the Asian and the American strains, respectively, associated with huanglongbing. A preliminary survey in Kenyan citrus groves after the invasion of ACP had revealed that lime‐green sticky traps (Asacp) were not effective in detecting and monitoring ACT and ACP. Therefore, this study compared eight differently coloured double‐sided sticky traps to evaluate, which colour was most effective for detecting ACT and ACP, particularly at low densities. The traps were coded Red, Blue, Asgreen1, GLMgreen, Asacp, Asyellow, White, and Black. Asyellow and GLMgreen traps captured more ACTs and ACPs than any other trap type. However, since there was no clear difference in the efficiency of Asyellow and GLMgreen in trapping ACP, any of the two trap types would be useful in the presence of both pests. Among the eight traps, Asyellow and GLMgreen traps are potentially promising options for monitoring and detecting the pests in areas where they coexist. Our findings will guide biosecurity agencies in decision‐making and designing ecologically friendly integrated pest management (IPM) strategies for citrus greening vectors in Africa as well as serve as an early warning to safeguard against pests' invasion into unaffected areas.

Huanglongbing (HLB) reduces the growth and development of citrus and induces changes in secondary metabolites such as flavonoids, limonoids, and polyamines. Likewise, infected plants have a deficient absorption of nutrients such as zinc, potassium, manganese, and copper. Therefore, the objective of this study was to evaluate the influence of different rootstocks on morphology and mineral changes of Citrus limon and Citrus sinensis cv. Valencia plants inoculated with Candidatus Liberibacter asiaticus. In a greenhouse of the Experimental Station-Autonomous University of Tamaulipas, the Candidatus Liberibacter asiaticus bacteria were inoculated to Citrus limon plants (growing on Citrus volkameriana, Citrus macrophylla, and Citrus aurantium rootstocks) and Citrus sinensis cv. Valencia (growing on Citrus volkameriana and Citrus aurantium rootstocks). The experiment was established under a completely randomized design with 45 graft/rootstock repetitions. In each graft/rootstock combination, the plant height and stem diameter were determined using a tape measurer and a Vernier, respectively. In addition, the nutrient content of foliar samples was determined by an X-ray fluorescence spectrometer. In both citrus species, the C. aurantium rootstock promoted a higher concentration of the bacteria. On the other hand, the rootstock that showed the best agronomical results after inoculation with the bacteria was C. volkameriana, presenting the least variation in mineral content and conferring greater plant height (15%) and stem diameter (23%). In contrast, the presence of Ca. Liberibacter asiaticus decreased S content and increased Cu concentration in C. lemon plants. Similarly, plants infected with C. sinensis presented higher Fe content. Finally, in both species, no significant differences were observed for Mn, P, and Zn concentration.

AbstractPathogens which need a vector for their transmission can alter the vectors’ behaviour to favour their spread. We used the electrical penetration graph technique to investigate this hypothesis by using the tomato potato psyllidBactericera cockerelliinfected or not with the plant pathogenCandidatusLiberibacter solanacearum (CLso) on African boxthorn and tomato. Probing was not affected by the host type but there was a significant effect on probing due to the infection status of the psyllid. More psyllids carried out probing activities in the sieve elements when infected with CLso, and more probing activities were observed from CLso-infected psyllids by comparison to the non-infected groups. Specifically, significant increases in salivation, phloem ingestion and number of probes, before and after reaching the sieve elements, were noticed in the infected groups. Furthermore, time elapsed to reach the sieve elements was significantly shortened by 2 h in the infected group. Remaining probing activities in xylem tissues were not different between all psyllid groups. The observed changes in feeding behaviour by pathogen-infected psyllids may well ensure further spread of the pathogen as greater salivation has the potential to increase transmission, highlighting at the same time the important role that crop and non-crop hosts play in disease epidemiology.

AbstractThe control of carrot psyllid Trioza apicalis Förster largely relies on chemical measures, and the current integrated pest management (IPM) strategy is based on pest scouting. The number of active ingredients available for psyllid control will further decline in the coming years. The need for alternative control measures is therefore urgent. In this work the efficiency of different control programs including a kaolin particle film and plant‐derived, crude saponin extract, chemical control, insect net and IPM (kaolin in combination with insecticides) programs to reduce psyllid feeding damage, reproduction and the spread of “Candidatus Liberibacter solanacearum” (CLso) in carrots was studied. Each year in 2016–2018, four replicates of each treatment were randomised in a row‐column design on a commercial carrot farm. After the treatments were executed, the psyllid nymphs and eggs were counted. At the harvest, the carrot roots and shoots were weighed, damages assessed, and samples were taken for CLso detection from 50 plants at each replicate. Kaolin treatment alone and in combination with insecticides effectively reduced the number of psyllid nymphs and eggs in all the years studied. Saponin was applied only on the first year, since it significantly increased the number of T. apicalis eggs compared to other treatments. The insect net was superior to other treatments in all the studied aspects in all the years. Under normal weather conditions, the highest root weight was harvested under the insect net, followed by the chemical control program consisting of pyrethroids, kaolin treatment, untreated control and saponin treatment. During extreme weather conditions, chemical control programs were not effective at protecting the carrots from psyllid feeding, which was reflected as low root yield. In all years, the carrot leaf damage percent negatively correlated with the root weight. Similarly, the shoot:root ratio increased as the leaf‐curling percentage rose, which was even pronounced under drought stress. The effect of climatic stress should be considered when developing IPM strategies.