Phytoplasmas

Huanglongbing (HLB), also known as citrus greening, is one of the most destructive
diseases of citrus worldwide. HLB is associated with three species of ‘Candidatus
Liberibacter’ with ‘Ca. L. asiaticus’ (Las) being the most widely distributed around
the world, and the only species detected in Thailand. To understand the genetic
diversity of Las bacteria in Thailand, we evaluated two closely-related effector
genes, lasAI and lasAII, found within the Las prophages from 239 infected citrus and
55 infected psyllid samples collected from different provinces in Thailand. The
results indicated that most of the Las-infected samples collected from Thailand
contained at least one prophage sequence with 48.29% containing prophage 1
(FP1), 63.26% containing prophage 2 (FP2), and 19.38% containing both
prophages. Interestingly, FP2 was found to be the predominant population in Lasinfected
citrus samples while Las-infected psyllids contained primarily FP1. The
multiple banding patterns that resulted from amplification of lasAI imply extensive
variation exists within the full and partial repeat sequence while the single band
from lasAII indicates a low amount of variation within the repeat sequence.
Phylogenetic analysis of Las-infected samples from 22 provinces in Thailand
suggested that the bacterial pathogen may have been introduced to Thailand from
China and the Philippines. This is the first report evaluating the genetic variation of
a large population of Ca. L. asiaticus infected samples in Thailand using the two
effector genes from Las prophage regions.

Phytoplasmas causing diseases of woody plants in native environments and agricultural systems are responsible for considerably economic and environmental damage. In Central and Southern Europe, phytoplasma diseases of apple (Malus domestica: Apple proliferation, AP) and grapevine (Vitis vinifera: Bois noir, BN and Flavescence dorèe, FD) are widespread and impact quantity and quality of the fruits. The ecology of phytoplasma diseases is complex and involves one or more insect vectors and in some cases alternative host plants. Phytoplasma densities in infected plants and expression of symptoms can vary considerably among seasons, and remission of symptoms occurs frequently. Disease control by pesticide application generally is not very efficient, therefore a polyphasic control strategy using a mix of agronomical, chemical and biological strategies needs to be developed for each disease. In the case of Bois noir e.g., the population density of the vector Hyalesthes obsoletus is related to the presence of its main host plants Urtica dioica and Convulvulus arvensis in the understory of the vineyards, which can be managed by agronomical methods. Application of foliar fertilizer showed no significant effect on BN-infected grapevines. AP-infected apple trees were treated with four bio-active compounds (Acibenzolar-S-Methyl, Harpin protein, Prohexadione-Ca and Cyanamide) over a

The effects of four commercially available bio-active compounds on the infection rates, symptom expression and growth rates of apple trees (Malus x domestica Borkh.) cv. Golden Delicious infected with ‘Candidatus Phytoplasma mali’ (the so-called Apple Proliferation phytoplasma or AP) were tested over a three-year period under controlled conditions. Post-infection treatments using Bion® (active ingredient: Acibenzolar-S-Methyl), Messenger® (Harpin protein), Regalis® (Prohexadione-Ca) and Dormex® (Cyanamide) had no significant effect on infection rates. Terminal growth of apple trees (grown as one-shoot pruned trees) was increased significantly by AP infection; Prohexadione-Ca was the only compound which had a significant (inhibiting) effect on the growth of both infected and non-infected apple trees. Acibenzolar-S-Methyl and Harpin had no significant effects on symptom expression. AP symptoms were masked during summer by Prohexadione-Ca, which caused severe growth abnormalities. Cyanamide changed the seasonal appearance of AP symptoms: while symptoms were delayed compared to the untreated control the first two years (2008 and 2009), symptoms appeared earlier the third year (2010). Differences in symptom expression levelled off later in the vegetative season, and no significant difference was found in October.

Reports on phytoplasma diseases in plant wild species are rare. Particularly interesting is the case in an Austrian forest, in the region Rosalia, Lower Austria, where a high number of plants, including Euonymus europaea, Sorbus aucuparia, Fraxinus excelsior, Fagus sylvatica, Betula alba, Sambucus nigra, Pyrus sp. and Picea abies, appeared with typical symptoms attributed normally to phytoplasma infection. Plants of Rubus ideaus, Rubus fruticosus and Vaccinium myrtillus, which represent small fruit species, showed clear symptoms. In preliminary test to verify phytoplasma presence in V. myrtillus the presence of phytoplasmas belonging to ribosomal group 16SrVI were identified after nested PCR on 16S ribosomal gene and restriction digestion with the appropriate enzymes. These plants were introduced in vitro to assure the conservation of the phytoplasma isolate for further analysis. Since in this area only a few home gardens are present in the neighbourhood of the forest land, the way ...

Although jute (Corchorus olitorius L.) is treated as a weed in Turkey, it is cultivated and harvested for its fiber, tender shoots, and leaves in Africa and Asia. We report the occurrence and symptomatology of phyllody disease in jute observed for the first time in 2010 during our studies focusing on sesame phyllody in an experimental field at the Akdeniz University Campus, Antalya, Turkey. The disease was also observed in the following two years, 2011 and 2012. In the top of the infected jute plant, the internodes were shortened which resulted a cluster of leaves in smaller size than the normal ones, and the leaves were crinkled as well as turned to yellowing and leathery-looking. Additionally, the large leaves accumulated more anthocyanin in their margins. The floral organs abnormally developed into leafy structures; and ovaries at the symptomatic part enlarged but stamens and filaments did not show any symptoms. There was neither proliferation of the branches nor needle-like shape of the leaves in our case. Jute and sesame seeds started germination synchronously, and looked similar at the cotyledonary stage. Wild plants or weeds deserve a particular attention for disease development or inoculum build-up in cultivated crops. Considering the voluntary nature, jute may be an alternative for biofuel production. Also, the similarity in developmental stages of jute and sesame suggests that they might be affected by the same phytoplasma. To verify this, molecular analyses have been started.

Potential insect vectors and alternative host plants of the phytoplasmas associated with grapevine yellows (GY) were surveyed in the Fynbos and Succulent Karoo biomes in the Western Cape, South Africa. Aster yellows phytoplasma (16SrI-B), which has been reported infecting grapevine in three regions in the Western Cape, was identified in a plant species belonging to the Aizoaceae. Other phytoplasmas were detected from species belonging to the Brassicaceae, Montiniaceae, Proteaceae and Zygophyllaceae and in a few insect specimens. The information will be used to confirm the insect vector status and the role of the plant species identified as alternative host plants in controlled transmission experiments.

The incremental incidence of Aster Yellows (AY) disease
caused by phytoplasma (a non-cellular bacteria-like organism)
in North America is of great concern. Although there is
no prescribed control measure for AY, monitoring and phytosanitary
activities can prevent its spread. Knowledge of
phytoplasma distribution across plant parts is important to
optimize sampling for diagnosis required by phytosanitary
regulations, but such information is not available for AYdiseased
Brassica plants. The numbers of phytoplasmas
present in different locations (root, stem above 10 cm of
soil, stem above 35 cm of soil, older leaf, younger leaf,
petiole, flower and pod) of field-collected naturally AYdiseased
Brassica plants (B. napus L., B. alba, B. carinata,
Camelina sativa and Thlaspi arvense), and of artificially
inoculated laboratory-grown canola (B. napus) plants over
the growing period were quantified by droplet digital PCR
(ddPCR). Phytoplasmas were detected in every part of
infested plants. The numbers of phytoplasmas varied
among parts of both field-collected and artificially inoculated
canola plants, with more phytoplasmas in the lower
parts (root) of younger plants and in the reproductive parts
(pod) of older plants. The severity of AY disease symptoms
during bolting, flowering and seed-set related directly to the
numbers of phytoplasmas in the plant tissues. This result
will be useful for early season monitoring of AY in canola
fields.