Horticulture

The aim of this work was to assess the effects of a combined inoculum of a rhizobacterium and an arbuscular mycorrhizal (AM) fungus on plant responses to phytoplasma infection, and on phytoplasma multiplication and viability in Chrysanthemum carinatum plants infected by chrysanthemum yellows phytoplasma (CY). Combined inoculation with Glomus mosseae BEG12 and Pseudomonas putida S1Pf1Rif resulted in some resistance to phytoplasma infection (about 30%), delayed symptom expression in nonresistant plants, improved growth of the aerial part of the infected plants (+68·1%), and altered root morphology (root tip number: +49·9%; branching degree: +82·8%). Combined inoculation with the two beneficial microorganisms did not alter CY multiplication and viability. In inoculated and infected plants, phytoplasma morphology was typical of senescent cells. A more active and efficient root system in double‐inoculated plants probably mediated the effects of the two rhizospheric microorganisms in the infected plants. The practical application of rhizospheric microorganisms for mitigating phytoplasma damage, following evaluation under field conditions, represents an additional tool for the integrated management of phytoplasmosis.

Incidence and severity of Huanglongbing (HLB) disease were assessed in Apr. 2010 among eight citrus cultivars representing diverse scion types growing in commercial groves in Florida's Indian River region, an area with a high incidence of HLB. In each grove, 20 trees of each cultivar were rated for visual HLB symptoms and leaves were collected for quantitative polymerase chain reaction quantification of Candidatus Liberibacter asiaticus (CLas), the presumptive causal agent of HLB. There was a strong correlation between HLB rating and CLas titer (titer represented by Ct, r2 = 0.37 and 0.40, for whole tree and leaf sample, respectively, both with P < 0.0001) across all cultivars and groves. Although incidence and severity of HLB varied considerably among the groves, scion-specific differences were apparent, even when analyses excluded potentially confounding grove effects. ‘Temple’ tangor showed the most consistently low incidence of HLB symptoms and CLas titer; in contrast, ‘Murcott’ tangor and ‘Minneola’ tangelo had the highest incidence of HLB symptoms and highest CLas titer. These results suggest useful resistance to HLB with reduced symptoms and reduced CLas titer may be found in conventional scion cultivars and further work is needed to assess this potential and its commercial value.

The diseases huanglongbing [HLB, associated with Candidatus Liberibacter asiaticus (CLas)] and Asian citrus canker [ACC, caused by Xanthomonas citri (Xcc)] are widespread in Florida and many other citrus-growing areas, presenting unprecedented challenges for citrus breeding. Because HLB and ACC weaken trees and compromise cropping, breeding is much less efficient using seed parents that have been exposed to these diseases. Therefore, it would be highly desirable to use unique disease-exposed selections only as pollen parents with pollen applied to disease-free trees. However, there may be a risk of introducing these diseases using such pollen sources. To assess this potential, abundance of the pathogens associated with these diseases was assessed in anthers and flowers using quantitative polymerase chain reaction. Because CLas is systemic, levels on mature leaves from the flower source trees were assessed to see if the presence of CLas in flowers was associated with leaf levels. Disease-exposed trees were tested in 10 genotypes from each of three broad genotypic categories, which reflect different levels of susceptibility to the diseases associated with the pathogens studied: Poncirus trifoliata hybrids (most resistant to HLB), Citrus maxima and hybrids (susceptible to both diseases), and C. reticulata and hybrids (considerable resistance to ACC). Of the 30 samples of each tissue type analyzed for CLas, 88% of mature leaves, 69% of flowers, and 88% of anthers had one or more CLas bacterium per sample. The trifoliate genotypic group had significantly lower levels of CLas than the pummelo and mandarin groups in mature leaf samples, but CLas levels were more similar between groups in anther and flower samples, and the pathogen was present in most of the trifoliate hybrids tested. Mean numbers of CLas detected per nanogram nucleic acid were 100 to 800 times higher in mature leaf samples, most characteristic of HLB symptoms, compared with anther samples. Xcc DNA was detected in 30% of flower samples and 23% of anther samples. No significant differences in Xcc levels were found between tissue type or genotypic group. However, regressions between Xcc levels in flowers and percent of plant pedigree derived from mandarin had a negative correlation and an r2 of 0.159 (P = 0.029). The biology of CLas is consistent with the pathogen being present in anthers from unopened flowers, whereas the ACC pathogen detected inside flowers was likely the result of contamination despite great care in sample collection and handling. Where exceptional diligence to exclude HLB and ACC is appropriate, results suggest that there may be a risk of spreading these pathogens through use of pollen from trees on infected farms.

Elm yellows phytoplasmas (EY) belonging to the 16SrV‐A subgroup were recently proposed as a new candidate species ‘ Candidatus Phytoplasma ulmi’. These pathogens infect elm trees, causing leaf yellowing and premature drying. In this study, 25 isolates originating from localities in northeast, east and southwest Serbia were characterized by means of RFLP analysis and DNA sequencing of four genomic loci: 16S rRNA, ribosomal protein rpl22 ‐ rps3 , secY and map . In total, five different genotypes were identified based on collective sequencing of all four genes. Four of these genotypes showed significant nucleotide changes compared with the EY1 T reference strain. Phylogeny based on parsimony analyses of ribosomal protein, secY and map genetic loci indicated a single monophyletic origin of EY1 T and the new ‘ Ca . Phytoplasma ulmi’ strains. Unlike phylogenetic clustering, DNA sequence comparison of EY1 T and the novel strains revealed mutations in oligonucleotide signature sequences for all three genes (16S, rpl22 ‐ rps3 and secY ) used for the characterization and assignment of 16SrV‐A phytoplasmas to the ‘ Ca.  Phytoplasma ulmi’ species in the original description. Based on their high degree of genetic variability, the Serbian strains were assigned to four different subtypes of ‘ Ca.  Phytoplasma ulmi’ (EY‐S1, EY‐S2, EY‐S3 and EY‐S4). New diagnostic enzymes for practical use in ‘ Ca.  Phytoplasma ulmi’ identification are proposed for the 16S rRNA, ribosomal protein and secY genes. The implications of genetic variability within signature sequences for taxonomy and identification of ‘ Ca . Phytoplasma’ species, as well as the importance of geographic variability and number of strains characterized for species description, are discussed.