Horticulture

Citrus Huanglongbing (HLB, also known as “citrus greening”), an important disease worldwide, is associated with three species of phloem-limited Candidatus liberibacter, of which Candidatus L. asiaticus (CLas) is the predominant one that has severely affected citrus production. TaqMan real-time polymerase chain reaction (PCR) (TM) has been the standard and very efficient method to diagnose several strains of Candidatus Liberibacter in citrus; however, it detects total bacteria and is unable to differentiate dead from live Liberibacter. The detection of only live bacteria is essential for testing methods of control for this important citrus disease. It is well known that ethidium monoazide and propidium monoazide (PMA) are compounds that supposedly enter only dead or membrane-damaged bacteria, intercalate the DNA strand, and make the DNA unavailable for amplification by PCR. These compounds are widely used when extracting the plant DNA to detect only live bacteria. In this research, we tested primers amplifying products from 79 to 1160 bp in TM and SYBR Green real-time PCR (SG) and PMA as DNA intercalating compound. Specifically, primers amplifying a 500-bp amplicon in SG provided the most reliable live-only detection, whereas those producing a smaller amplicon were unable to distinguish between live and dead. This is the first report of testing primers amplifying various amplicon sizes for the detection of only live CLas cells in citrus.

Abstract “ Candidatus Liberibacter solanacearum” (Lso), transmitted by the potato psyllid ( Bactericera cockerelli ), is the causal agent of potato zebra chip, but can also infect other solanaceous plants, including peppers. Studies were conducted to investigate whether Lso could be transmitted to the next generation of plants through seeds from infected pepper plants. In 2014, jalapeno pepper plants were infested with psyllids carrying a mixture of Lso A and B (AB) at the AgriLife Research Station at Bushland. The study was again conducted in 2019 and pepper plants were infested with psyllids carrying Lso B or Lso AB. In each of the studies, noninfested plants served as controls. At harvest, fruits were collected and tested for the presence of Lso using quantitative PCR. Seeds from infected fruits were then tested for Lso. Overall, the percentage of seeds that tested positive for Lso ranged from 33% to 70%. However, Lso detection in embryos ranged only from 0% to 8%. Seed samples from Lso‐positive fruits were planted in the greenhouse to determine the impact of Lso on emergence and the incidence of Lso in emerged plants. Although plant emergence differed between some of the seeds obtained from Lso‐positive and ‐negative fruits, the overall impact of Lso on plant emergence was not consistent. However, of the 182 plants that emerged from seeds collected from infected fruits, none was positive for Lso, suggesting that seeds are unlikely to serve as sources for new Lso infections and their impact on disease epidemiology is negligible.

AbstractPear decline, induced by the phytoplasma 'CandidatusPhytoplasma pyri', transmitted by pear psyllids, is one of the most devastating diseases onPyrus communisin Europe and North America. Investigations of pear psyllids in 4 pear orchards in lower Austria showed the presence ofCacopsylla pyri,C. pyricolaandC. pyrisugaat all locations. PCR analyses revealed overall phytoplasma infection rates forC. pyriof 5.4%, forC. pyricola,of 4.6%, forC. pyrisugaremigrants of 9.6% and forC. pyrisugaemigrants of 0%. The rates of PCR-positiveC. pyriandC. pyricolaindividuals varied greatly in the course of the year, and the highest infection rates were observed in late summer, autumn and in late winter. In transmission experiments with healthy pear seedlings, winterform individuals ofC. pyri and C. pyricolatransmitted the pathogen to 19.2% (5 out of 26) and 4.8% (2 out of 41) of the test plants, respectively. The vectoring ability ofC. pyrisugawas experimentally proven for the first time, and in transmission experiments with remigrants, 9.5% (2 out of 21) of the pear seedlings were infected. Our data indicate a significant risk of pathogen transmission in pear orchards during the greater part of the year, especially in late winter, early spring and autumn. Multilocus sequence analysis by aid of the genesaceFandimpallowed the discrimination between 15 phytoplasma types. Three so far undescribedaceFgenotypes and four undescribedimpgenotypes were identified.

‘Candidatus Phytoplasma spp.’ are pathogenic bacteria that infect many plant species. ‘Candidatus Phytoplasma pyri’, one of the members of the 16SrX group causes pear decline disease that adversely affects pear crops. To describe the prevalence of ‘Ca. P. pyri’ genotypes in the Czech Republic, 143 pear samples were collected from 41 locations including commercial orchards as well as trees along roads. Phytoplasma was detected by PCR in 115 samples, and it was possible to determine imp gene genotype in 84 samples. The most frequent genotypes were A1, B1, and C, which were identified in 71% of phytoplasma positive samples. ‘Ca. P. pyri’ was present either alone or as a mix of two populations in 88% of genotyped samples, and in another 6% of samples it was found in a mixed infection with ‘Ca. Phytoplasma mali’. A sole infection with ‘Ca. Phytoplasma mali’ was observed in 6% of samples. As for symptoms, 19% of symptomatic samples were found to be phytoplasma negative, and 74% of asymptomatic samples proved to be phytoplasma positive; leaf roll was more often observed in phytoplasma positive samples, while leaf narrowing rather indicated the absence of phytoplasma. The mildest symptoms were observed in samples infected with ‘Ca. P. pyri’ of the A1 imp genotype.

The devastating citrus disease huanglongbing (HLB) associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas) has caused a more than 70% reduction in citrus production since its discovery in Florida in 2005. Most citrus scion cultivars are sensitive to HLB, whereas some cultivars used as rootstocks are tolerant. Using such tolerant rootstocks can help trees to cope better with the disease’s impact. Evaluating rootstock effects on a grafted scion in the field takes many years, but shorter-term evaluation is imperative to aid in rootstock selection for an HLB-endemic production environment. In this study, we investigated grafted healthy and CLas-infected citrus trees under controlled greenhouse conditions. The objectives were to identify traits suitable for assessing grafted tree tolerance in advance of longer-term field studies and aiding in the selection of superior rootstock cultivars. We assessed 10 commercially important rootstocks grafted with ‘Valencia’ sweet orange scion and with known field performance. At 6, 9, 15, and 21 months after graft inoculation (mai), leaf CLas titers were determined and canopy health was evaluated. Plants were destructively sampled at 21 mai to assess plant biomasses and other physiological and horticultural variables. There was little influence of the rootstock cultivar on CLas titers. Surprisingly, few HLB foliar disease symptoms and no differences in soluble and nonsoluble carbohydrate concentrations were measured in infected compared with healthy plants, despite high CLas titers and significant reductions in plant biomasses. Most trees on rootstocks with trifoliate orange parentage were less damaged by HLB than other rootstocks, although results did not always agree with reported field performance. Among the different variables measured, leaf size appeared to be most predictive for grafted tree assessment of HLB sensitivity. The results of this study provide a better understanding of the strengths and weaknesses of assessing rootstock influence on grafted tree performance in a controlled greenhouse environment. Although such studies provide valuable information for cultivar tolerance to HLB, other rootstock traits will ultimately contribute to field survival and productivity in an HLB endemic production environment.

A survey of weeds was undertaken in a palm nursery affected by lethal bronzing (LB) to identify a reservoir host of the causal phytoplasma. Three common species were identified; Urochloa maxima (Guineagrass), Sporobolus indicus (smut grass), and Cyperus esculentus (yellow nutsedge) and sampled over a period of 2 years. Each species was sampled 36 times and all three species were negative for the LB phytoplasma. However, three specimens of C. esculentus tested positive for the phytoplasma species ‘Candidatus Phytoplasma brasiliense’. These findings represent the first documented case of ‘Ca. P. brasiliense’ in North America, specifically in Florida, U.S.A., as well as a new host record for the phytoplasma and the first monocot host documented. Because of the impact this phytoplasma has on papaya and hibiscus in South America, it presents a unique threat to ornamental and agricultural sectors in south Florida. An area-wide survey for the phytoplasma and potential vectors is recommended.

Grafting a scion onto a rootstock results in physical and physiological changes in plant growth and development, which can affect tree vigor, productivity, and tolerance to stress and disease. Huanglongbing (HLB) is one of the most destructive citrus diseases and has become endemic in Florida since its introduction in 2005. It is associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas), which cause severe metabolic disruptions in affected plants. Although most scion cultivars are highly susceptible, some rootstock cultivars are tolerant and allow the grafted tree to cope better with the disease. The objectives of this study were to identify rootstock traits that can be used to assess cultivars under controlled greenhouse conditions in advance of longer-term field trials. We used 10 commercially important rootstocks with different genetic backgrounds and known field performance in graft combination with ‘Valencia’ sweet orange scion. Trees were graft-inoculated with CLas and compared against mock-inoculated trees. Tree health and CLas populations were assessed regularly, and root growth was monitored using a minirhizotron imaging system. Plants were excavated and destructively sampled 21 months after inoculation to assess biomass distributions and other CLas-induced effects. We found significant differences between healthy and infected trees for most variables measured, regardless of the rootstock. In contrast to leaf CLas titers, root titers were significantly influenced by the rootstock, and highest levels were measured for ‘Ridge’ sweet orange and sour orange. Root growth and root biomasses were reduced upon infection but differences among rootstocks did not always agree with reported field performances. Despite severe biomass reductions plants maintained their relative distribution of biomass among different components of the root system, and no dead roots were observed. Root respiration was reduced by CLas infection and was overall higher in tolerant cultivars suggesting its potential as a physiological marker. This study improves our knowledge about the strengths and weaknesses of assessing rootstock traits of grafted trees in a controlled greenhouse setting. Results from the study suggest that in addition to HLB tolerance, other rootstock traits will ultimately have major contributions to field survival and productivity of the grafted trees in an HLB endemic production environment.