Agronomy and Crop Science

AbstractInfection with the bacterium ‘Candidatus Liberibacter solanacearum’ (Lso) is suspected to cause severe damage in carrot leading to high carrot weight loss. This study investigates three main aspects: (i) whether there is a reduction of carrot root weight under field conditions due to Lso infection; (ii) the correlation between Lso infection rate in carrot plants and occurrence of the psyllid Trioza apicalis as the known vector for Lso in carrot, and (iii) the comparison between symptoms described in literature and observed symptoms associated with Lso infection. Therefore, field surveys were conducted from 2018 to 2021 on organically managed carrot fields in Lower Saxony, Germany. Two Rebell orange sticky traps were placed per field replaced on a weekly basis. Captured T. apicalis were morphologically identified and counted. Carrot plant samples were collected from T. apicalis infested fields. Discolouration of foliage and further symptoms on carrots were investigated. For detection of Lso in carrot plants and psyllids, the samples were analysed using PCR. The infection rate of carrot plants varied between 2.5% and 80% per field with low abundance of T. apicalis. Of the asymptomatic plants, 39.4% were infected with Lso, while 80.9% of the symptomatic plants were infected with Lso. The weight of Lso-positive carrot samples was not significantly reduced compared to Lso-negative samples. No economically relevant losses of carrot yield were reported by farmers participating in this study. This indicates that there was no effect of Lso infection on carrot root weight during this study. Of the symptoms associated with a Lso infection in carrots, only leaf discolouration could be confirmed. Overall, the bacterium is present in organically grown carrot plants in Lower Saxony but no major carrot root weight losses were observed.

Citrus Huanglongbing, one of the most devastating citrus diseases, is caused by Candidatus Liberibacter asiaticus (CLas). Polyamines are aliphatic nitrogen-containing compounds that play important roles in disease resistance. However, the role of polyamine metabolism in the tolerance of citrus to infection with CLas have not been extensively studied. We used HPLC and UPLC-Q/TOF-MS to detect the contents of nine polyamine metabolism-related compounds (PMRCs) in six citrus cultivars with varying levels of tolerance to CLas. Moreover, we systematically detected the changes of PMRC and H2O2 contents, and compared the gene expression levels and activities of enzymes involved in the polyamine metabolic pathway among healthy, asymptomatic, and symptomatic leaves of Newhall navel oranges infected with CLas. The tolerant and moderately tolerant varieties showed higher PMRC levels than those of susceptible varieties. Compared with the healthy group, the symptomatic group showed significantly increased contents of arginine, ornithine, γ-aminobutyric acid, and putrescine by approximately 180, 19, 1.5, and 0.2 times, respectively, and up-regulated expression of biosynthetic genes. Arginase and ornithine decarboxylase enzyme activities were the highest in the symptomatic group, while arginine decarboxylase and agmatine deiminase enzyme activities were the highest in the asymptomatic group. The two polyamine biosynthetic pathways showed different trends with the increase of the titer of CLas, indicating that polyamines were mainly synthesized through the arginine decarboxylase pathway in the asymptomatic leaves, and were synthesized via the ornithine decarboxylase pathway in symptomatic leaves. These findings provide new insight into the changes of polyamine metabolism in citrus infected with CLas.

Prophages/phages are important components of the genome of ‘Candidatus Liberibacter asiaticus’ (CLas), an unculturable alphaproteobacterium associated with citrus Huanglongbing (HLB) disease. Phage variations have significant contributions to CLas strain diversity research, which provide critical information for HLB management. In this study, prophage variations among selected CLas strains from southern Texas were studied. The CLas strains were collected from three different CLas inhabitant environments: citrus leaf, citrus root, and Asian citrus psyllid (ACP), the vector of CLas. Regardless of the different habitats and time span, more than 80% of CLas strains consistently had both Type 1 and Type 2 prophages, the same prophage type profile as in CLas strains from Florida, but different to those reported in California and China. Further studies were performed on within prophage type diversity. Analyses on Type 1 specific PCR amplicon sequences (encoding an endolysin protein) revealed the presence of two groups: Type 1-A, clustered around prophage SC1 originated from Florida and Type 1-B, clustered with prophage P-SGCA5-1 detected in California. Type 1-B strains were mostly from ACP of nearby citrus orchards. On the other hand, analyses on Type 2 specific PCR amplicon sequences (encoding a putative hypothetical protein) showed a single group clustering around prophage SC2 originated from Florida, although a different Type 2 prophage has been reported in California. The presence of two distinct Type 1 prophage groups suggested the possibility of two different CLas introductions in southern Texas. Results from this study provides an initial baseline information on genomic and population diversity of CLas in Texas.

The potato/tomato psyllid, Bactericera cockerelli (Šulc), is among the most important pests of solanaceous crops as a vector of the pathogen ‘Candidatus Liberibacter solanacearum’ (Lso). Lso-infected psyllids often arrive in crop fields from various wild species of Solanaceae and Convolvulaceae, especially those that provide early-season hosts for the vector. Physalis species are perennial plants within the family Solanaceae with often broad geographical distributions that overlap those of B. cockerelli, yet the status of many Physalis species as hosts for B. cockerelli or Lso remains unknown. Our objective was to determine whether wild Physalis species that occur in the potato-growing region of Galeana, Nuevo León, Mexico, host B. cockerelli populations and whether they also are susceptible to Lso. Sampling was carried out in the potato-growing zone of Galeana, Nuevo León, Mexico, where unidentified Physalis spp. are common. In March to October 2021, a wild plant identified as Physalis virginiana was observed; eggs, nymphs, and adults of B. cockerelli were observed on these plants throughout the growing season, and nymphs completed development on these plants under laboratory conditions. Lso also was detected in 22 of the 93 (23.7%) wild P. virginiana plants using conventional PCR, while 13.3% of B. cockerelli adults that emerged from P. virginiana cuttings harbored the pathogen. This is the first report that P. virginiana is a host for B. cockerelli and for Lso. These results suggest that P. virginiana is a likely source of Lso-infected psyllids colonizing solanaceous crops in northeastern Mexico. The importance of P. virginiana and other wild hosts on the population dynamics of the vector and pathogen should be investigated to assist in pest management decision-making.

AbstractIncreasing incidences of phytoplasma infestations in Almond trees warrants the better management approach to prevent yield losses. Disease management rely on identification of the pathogen based on molecular profiling. The present study aimed, to identify the phytoplasma agent in almond trees and to measure the biochemical responses it causes in the host. Direct and Nested PCRs performed using phytoplasma specific primer pairs 16S rRNA, detected the presence of phytoplasma agent in symptomatic trees but not in symptomless trees. Phylogeny based on the sequence analysis revealed that the infecting agent was closely related to ‘Candidatus Phytoplasma phoenicium’ (16SrIX‐B subgroup). Then the study was carried out to determine the responses of physiological and biochemical mechanisms in almond trees infected with phytoplasma. Total chlorophyll and protein contents of infected almond trees were lower compared to healthy control, on the other hand, the levels of catalase and peroxidase activity increased in infected trees. Higher levels of stress‐related metabolites such as proline (20.53–39.23 μmol/g), phenol (3.00–4.44 mg GAE/g), salicylic (94.96–138.22 ng SA/mg protein) and jasmonic acid (965.86–1465.10 ng JA/mg protein) were observed in infected trees compared to asymptomatic trees, respectively in healthy and infected trees. The results showed that the Ca. P. phoenicium, which was detected for the first time in Türkiye, was able to change the physiological and biochemical mechanisms. The pathogenic agent could possess a potential danger in almond production areas. It is crucially important that this agent should be considered in certification programs.