Plant diseases caused by, or associated with, phytoplasmas occur in hundreds of commercial and native plants, causing minor to extensive damage. Insect vectors, primarily leafhoppers, planthoppers, and psyllids, have been identified for relatively few phytoplasma diseases, limiting the capacity of managers to make informed decisions to protect crops and endangered indigenous plants. In the past two decades our knowledge of insect vector-phytoplasma interactions has increased dramatically, allowing researchers to make more accurate predictions about the nature and epidemiology of phytoplasma diseases. These better-characterized systems also may provide clues to the identity of insect vectors of other phytoplasma-associated diseases. We review the literature addressing the ecology of insect vectors, phytoplasma-insect ecological and molecular interactions, vector movement and dispersal, and possible management strategies with an emphasis on research from the past 20 years.

Insect vectors of phytoplasmas

In mammals, cadmium is widely considered as a non-genotoxic carcinogen acting through a methylation-dependent epigenetic mechanism. Here, the effects of Cd treatment on the DNA methylation patten are examined together with its effect on chromatin reconfiguration in Posidonia oceanica. DNA methylation level and pattern were analysed in actively growing organs, under short- (6 h) and long- (2 d or 4 d) term and low (10 mM) and high (50 mM) doses of Cd, through a Methylation-Sensitive Amplification Polymorphism technique and an immunocytological approach, respectively. The expression of one member of the CHROMOMETHYLASE (CMT) family, a DNA methyltransferase, was also assessed by qRT-PCR. Nuclear chromatin ultrastructure was investigated by transmission electron microscopy. Cd treatment induced a DNA hypermethylation, as well as an up-regulation of CMT, indicating that de novo methylation did indeed occur. Moreover, a high dose of Cd led to a progressive heterochromatinization of interphase nuclei and apoptotic figures were also observed after long-term treatment. The data demonstrate that Cd perturbs the DNA methylation status through the involvement of a specific methyltransferase. Such changes are linked to nuclear chromatin reconfiguration likely to establish a new balance of expressed/repressed chromatin. Overall, the data show an epigenetic basis to the mechanism underlying Cd toxicity in plants.

/pdf/in-posidonia-oceanica-cadmium-induces-changes-in-dna-3cu7uj83mr.pdf

In Posidonia oceanica cadmium induces changes in DNA methylation and chromatin patterning

Plants can be induced to develop enhanced resistance to pathogen infection by treatment with a variety of abiotic and biotic inducers. Biotic inducers include infection by necrotizing pathogens and plant-growth-promoting rhizobacteria, and treatment with nonpathogens or cell wall fragments. Abiotic inducers include chemicals which act at various points in the signaling pathways involved in disease resistance, as well as water stress, heat shock, and pH stress. Resistance induced by these agents (resistance elicitors) is broad spectrum and long lasting, but rarely provides complete control of infection, with many resistance elicitors providing between 20 and 85% disease control. There also are many reports of resistance elicitors providing no significant disease control. In the field, expression of induced resistance is likely to be influenced by the environment, genotype, and crop nutrition. Unfortunately, little information is available on the influence of these factors on expression of induced resistance. In order to maximize the efficacy of resistance elicitors, a greater understanding of these interactions is required. It also will be important to determine how induced resistance can best fit into disease control strategies because they are not, and should not be, deployed simply as "safe fungicides". This, in turn, will require information on the interaction of resistance elicitors with crop management practices such as appropriate-dose fungicide use.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-95-1368

Induced Resistance for Plant Disease Control: Maximizing the Efficacy of Resistance Elicitors

Priming for stress resistance: from the lab to the field.

Normal 0 14 false false false IT ZH-TW X-NONE MicrosoftInternetExplorer4 The oomycete Plasmopara viticola is native to North America and was accidentally introduced into Europe at the end of the 19th century, where it caused widespread damage to the grape industry. Since that time, the damage caused by this plant pathogen has generally been controlled with multiple fungicide applications. Modern fungicides applied as prescribed by weather-based warning systems can effectively prevent any damage that might be caused. Alternatives to chemical treatments, such as the use of biocontrol agents or resistant cultivars, currently play a marginal role in controlling this disease. Until the middle of last century, research efforts were mainly concentrated on optimizing the application of copper fungicides and developing new molecules and formulations for controlling the disease. During the second half of the last century, highly efficient products for control were available, so research efforts moved toward optimizing and minimizing chemical control, mostly through the use of weather-based warning systems based on complex biological models. In the last 20 years, parallel to the development of technologies for genomic and transcriptomic analyses, host-pathogen interactions and population genetics have captured the interest of researchers in this field. Breeding for resistance against downy mildew has always coexisted with chemical control. However, the results of these breeding programmes have traditionally been cultivated only in marginal areas and organic production systems. This review traces the history of European knowledge of P. viticola .

/pdf/plasmopara-viticola-a-review-of-knowledge-on-downy-mildew-of-4f66wjt01p.pdf

Plasmopara viticola: a review of knowledge on downy mildew of grapevine and effective disease management

A secondary sexual character may act as an honest signal of the quality of the individual if the trait bears a cost and if its expression is phenotypically condition dependent. The cost of increasing the trait should be tolerable for individuals in good condition but not for those in a poor condition. The trait thus provides an honest signal of quality that enables the receiver to choose higher quality mates. Evidence for sex pheromones, which play a major role in shaping sexual evolution, inflicting a signaling cost is scarce. Here, we demonstrate that the amount of the major component of the pheromone in glands of Lobesia botrana (Lepidoptera) females at signaling time was significantly greater in large than in small females, that male moths preferred larger females as mates when responding to volatile signals, and small virgin females, but not large ones, exposed to conspecific pheromone, produced, when mated, significantly fewer eggs than nonexposed females. The latter indicates a condition-dependent cost of signaling. These results are in accordance with the predictions of condition-dependent honest signals. We therefore suggest that female signaling for males using sex pheromones bears a cost and thus calling may serve as honest advertisement for female quality.

Fitness cost of pheromone production in signaling female moths

Hyalesthes obsoletus Signoret (Homoptera: Cixiidae) is a polyphagous planthopper that transmits stolbur phytoplasma (a causative agent of “yellows” disease) to various weeds, members of the Solanaceae, and wine grapes (Vitis vinifera L.) in Europe and the Middle East. Planthoppers were collected by hand vacuuming eight native plant species. Vitex agnus-castus L., a shrub in the Verbenaceae, hosted the largest number of H. obsoletus, although Olea europaea L. also served as a host for adults. Using a Y-olfactometer, we compared the planthoppers relative preference for V. agnus-castus, Convolvulus arvensis, and V. vinifera. V. agnus-castus was more attractive to both male and female H. obsoletus than the other plants. H. obsoletus antennal response was stronger to volatiles collected from V. agnus-castus than from Cabernet Sauvignon variety of V. vinifera. To determine if V. agnus-castus would serve as a reservoir for the pathogen, H. obsoletus were collected from leaf and stem samples of native V. agnus-castus, and were tested by polymerase chain reaction (PCR) for the presence of phytoplasma DNA. While 14% and 25% (2003 and 2004, respectively) of the insects tested positive for phytoplasma DNA, none of the plant samples tested positive. To determine if V. agnus-castus could serve as a host plant for the development of the planthopper, we placed emergence cages beneath field shrubs and enclosed wild-caught H. obsoletus in a cage with a potted young shrub. We found adult H. obsoletus in the emergence cases and planthopper nymphs in the soil of the potted plant. We concluded that V. agnus-castus is attractive to H. obsoletus, which seems to be refractory to phytoplasma infections and warrants further testing as a trap plant near vineyards.

Vitex agnus-castus is a preferred host plant for Hyalesthes obsoletus.

Two foliar sprays of BABA (DL-3-amino-n-butanoic acid, DL-β-aminobutyric acid), or a mixture of BABA and different fungicides at reduced rates, effectively controlled (>90%) downy mildew, caused byPlasmopara viticola, in the foliage of field-grown grapevines. In five field trials BABA sprays resulted in a significant reduction of infectious leaf area and fungal sporulation, and of necrosis of oilspots. The level of disease control for BABA in four trials was similar to that achieved by metalaxyl-Cu or ‘Acrobat Plus’ (dimethomorph + mancozeb). Two-way tank-mixtures of BABA + fosetyl-AI, BABA + folpet, or BABA + Bion (benzothiadiazole), each at half the recommended rate, provided an additive effect againstP. viticola, performing as well as the full rate of each fungicide alone. BABA was not phytotoxic and did not affect pH, total titratable acids, or °Brix of the juice, as determined by commercial fungicidal standards. The results indicate that foliar applications of BABA can efficiently be integrated into a downy mildew control program in vineyards.

Controlling downy mildew ( Plasmopara viticola ) in field-grown grapevine with β-aminobutyric acid (BABA)

Summary

Species from three subfamilies of Cicadellidae and one species of Cixiidae, all potential vectors of phytoplasmas, were trapped in vineyards over the course of two years These insects were caught on yellow sticky traps during the spring; virtually none were trapped during the summer months Molecular analysis (PCR) of selected samples of the leafhoppers and planthoppers for presence of phytoplasma DNA was conducted on most species Neoaliturus fenestratus was the most abundant known vector species and was positive for stolbur (Stol) and aster yellows (AY) phytoplasma Circulifer haematoceps complex, which were also positive for Stol and AY, were about 10-fold fewer than N fenestratus Hyalesthes obsoletus, also a known phytoplasma vector, occurred in substantial numbers only late in the season when the vines and leaves were dehiscing and turning brown, but was positive for Stol and AY A species whose vector status is unknown (Megophthalmus scabripennis) was also caught in large numbers and was shown, by PCR analysis, to be positive for AY Other known vector species trapped included: Anaceratagallia laevis (positive for AY), Austroagallia sinuata, Dryodurades sp (dlabolai?) (positive for AY), Macrosteles quadripunctulatus (positive for Stol), and Orosius orientalis The spatial dispersion pattern of the four most abundant species were investigated by using Moran's spatial statistic; N fenestratus and M scabripennis showed significant spatial patterns, whereas C haematoceps and H obsoletus did not

Spatial dispersion patterns of potential leafhopper and planthopper (Homoptera) vectors of phytoplasma in wine vineyards

Israeli vine growers have been reluctant to adopt the mating disruption technique for control of the European vine moth, Lobesia botrana Den. & Schiff. Since the chemically controlled honeydew moth, Cryptoblabes gnidiella Mill., coexists with the European vine moth, growers have maintained that the use of mating disruption would fail to bring about a significant reduction in pesticide use. In this study, the efficacy of mating disruption techniques against C. gnidiella was tested, as well as the effect of these methods on pesticide use and damage to clusters when the method was employed against both of the pests in wine grapes. Comparisons were made between plots treated with (1) L. botrana mating disruption pheromone, (2) L. botrana and C. gnidiella mating disruption pheromones and (3) control plots. A significant difference in the number of clusters infested with the developmental stages of the moths was seen between pheromone-treated plots and controls, while no such difference was observed between plots treated with one versus two pheromones. A similar pattern was observed in the number of insecticide applications; the greatest number of applications was used in control plots, followed by plots treated with L. botrana mating disruption pheromone and by plots treated with pheromones against both pests, in which no pesticides were applied.

Tirtza Zahavi

Papers

Fitness cost of pheromone production in signaling female moths

Vitex agnus-castus is a preferred host plant for Hyalesthes obsoletus.

Controlling downy mildew ( Plasmopara viticola ) in field-grown grapevine with β-aminobutyric acid (BABA)

Spatial dispersion patterns of potential leafhopper and planthopper (Homoptera) vectors of phytoplasma in wine vineyards

Pest management programmes in vineyards using male mating disruption.