Agronomy and Crop Science

‘Candidatus Liberibacter solanacearum’ (Lso), transmitted by the potato psyllid (Bactericera cockerelli), is the putative causal agent of potato zebra chip disease. The bacterial pathogen infects a wide range of solanaceous plants (both wild and cultivated species), among which are peppers, potatoes, and tomatoes. Currently there are two commonly detected, genetically distinct haplotypes of Lso (A and B) identified from potatoes in the United States. To determine whether there are interactions between Lso haplotypes and different solanaceous hosts, experiments were conducted in the greenhouse in which pepper, potato, and tomato plants were infested with psyllids carrying Lso A, B, or an A and B mix (AB) or with psyllids free of Lso. Host plants were grown in pots in cages on the greenhouse benches and infested with six psyllids per plant. In addition, eight pepper cultivars were similarly infested for deeper understanding of host–haplotype interactions. Approximately 7 weeks after infestation, adult psyllids in each cage were counted to determine the impact of Lso haplotype–host interactions on psyllid survival and plants were sampled and tested molecularly for Lso. Individual psyllids carrying haplotypes B or AB and those free of Lso copiously reproduced on all three hosts, and leaf tissue from each plant tested positive for the respective Lso except those infested with Lso-negative psyllids. However, psyllids carrying Lso A did not survive on peppers but survived and abundantly reproduced on potatoes and tomatoes. In addition, samples from peppers infested with psyllids carrying Lso A tested negative for Lso. However, peppers infested with individual psyllids carrying Lso AB tested positive for Lso A, indicating that the presence of B may be required for infection by Lso A and psyllid survival on peppers. The different pepper cultivars infested with psyllids carrying Lso A showed similar results to the haplotype–host interaction tests, suggesting that cultivar may not be a factor in Lso A–pepper host interactions. Results from these studies suggest that Lso A may affect host selection by psyllids either for nutrition or laying of eggs. Mechanisms involved in preventing psyllid reproduction on peppers, once identified, will have significant implications for potential psyllid management.

AbstractIn Argentina, amaranth is a promising crop due to high nutritional quality and ability to grow in a diversity of environments. In areas cultivated with amaranth, were observed plants exhibiting slow growth, deformed leaves, proliferation of shoots and malformed lateral panicles. Field survey revealed up to 96% disease incidence and 92% of the seeds collected from mother plants produced diseased seedlings. A phytoplasma was detected in association with seedlings and adult plants using nested PCR assays. Molecular identification by computer‐simulated RFLP and phylogenetic analysis evidenced the occurrence of a ‘Candidatus Phytoplasma hispanicum’‐related strain, affiliated with 16SrXIII‐A subgroup. The findings implicate amaranth as a new host for this subgroup and as a potential reservoir of the pathogen for other cultivated species. In addition, to the best of our knowledge, this study reports for the first time the presence of 16SrXIII‐A phytoplasma in Argentina and in South America. Furthermore, transmission assays pointed that naturally infected seed is an important vehicle of dissemination of the pathogen, threatening the expansion of the crop for new geographical areas.

Murraya paniculata and Swinglea glutinosa are aurantioid hosts of the Asian citrus psyllid (ACP) Diaphorina citri, the principal vector of ‘Candidatus Liberibacter asiaticus’ (Las). Las is the pathogen associated with huanglongbing (HLB), the Asian form of which is the most devastating disease of Citrus species and cultivars (Rutaceae: Aurantioideae). M. paniculata is a common ornamental and S. glutinosa is grown as an ornamental, a citrus rootstock, and a hedgerow fence plant. Because of the uncertain status of these plants as reservoirs of Las, a series of cross-inoculation bioassays were carried out in different environments, using infected Valencia sweet orange (Citrus × aurantium) infected shoot tops as a source of inoculum and D. citri nymphs and adults reared on M. paniculata and S. glutinosa to inoculate pathogen-free Valencia orange plantlets. In contrast to sweet orange, Las was more unevenly distributed and reached much lower titers in M. paniculata and S. glutinosa. Infections in M. paniculata and S. glutinosa were also transient. Very few insects that successfully acquired Las from M. paniculata and S. glutinosa were able to transmit the pathogen to healthy citrus. Transmission rates were low from M. paniculata (1.0%) and S. glutinosa (2.0%) and occurred only in a controlled environment highly favorable to Las and ACP using 10-day-old adults that completed their life cycle on Las-positive plants. Our study showed that in HLB-endemic areas, M. paniculata and S. glutinosa can be deemed as epidemiologically dead-end hosts for Las and are not important alternative hosts of the pathogen for transmission to citrus. However, under a combination of conditions highly favorable to Las infection and transmission and in the absence of effective quarantine procedures, these plants could eventually serve as carriers of Las to regions currently free from HLB.