Since 2009 extensive dieback and mortality of Nothofagus obliqua, associated with bleeding cankers on stems and branches, has been observed in the UK. The causal agent was identified as Phytophthora ...pseudosyringae, based on morphological and analysis of the internal transcribed spacer (ITS) sequences. Between 2011 and 2013, a survey assessed the frequency and nature of these P. pseudosyringae infections. Mature trees of Nothofagus with stem lesions caused by P. pseudosyringae were found across England, Scotland and Wales. Additional symptoms such as twig blight and leaf necrosis indicated that aerial infection was occurring. Besides N. obliqua, other hosts regularly encountered included Nothofagus alpina, Fagus sylvatica and Vaccinium myrtillus. In pathogenicity tests involving inoculation of logs, P. pseudosyringae was shown to be an aggressive bark pathogen of N. obliqua and F. sylvatica, but significantly less aggressive on N. alpina. Foliage susceptibility and sporulation tests showed marked differences between the six host species tested. Leaves of N. obliqua and V. myrtillus were highly susceptible. Leaves of N. alpina were moderately susceptible, those of Rhododendron ponticum slightly susceptible and those of F. sylvatica not susceptible at all. High levels of sporulation were observed only on inoculated N. obliqua and V. myrtillus leaves. This suggests that P. pseudosyringae may sporulate heavily on N. obliqua foliage in the field and that this inoculum initiates the aerial lesions observed on the shoots, branches and stems. The results also suggest that P. pseudosyringae has the potential to pose a serious threat to N. obliqua and other Nothofagus species in their Southern Hemisphere native ranges.
New and emerging Phytophthora-related diseases in small trees, shrubs and herbaceous plants typical of subalpine vegetation have recently been observed in Italy and Slovenia. Diseased plants showed a ...complex symptomatology including foliar necrosis, fruit rot, shoot blight and branch bleeding cankers. Since little information is available about the aetiology of these aerial Phytophthora diseases, from 2019 to 2022, field surveys were conducted in 54 sites to define the occurrence, distribution and impact of the Phytophthora species on mountain vegetation. A total of 360 Phytophthora isolates were obtained from 397 samples collected from 33 herbaceous and woody host species. Based on phylogenetic analysis and morphometric data, 17 Phytophthora species were identified: P. pseudosyringae (201 isolates), P. plurivora (54), P. gonapodyides (21), P. ilicis (20), P. alpina (17), P. acerina (11), P. cactorum (7), P. pseudocryptogea (6), P. cambivora (5), P. idaei (4), P. psychrophila (3), P. bilorbang (2), P. chlamydospora (2), P. hedraiandra (1), P. kelmanii (1), P. rosacearum (1) and P. syringae (1). In addition, three isolates of a new putative Phytophthora species obtained from Alnus viridis, Juniperus communis and Rhododendron ferrugineum are described here as Phytophthora pseudogregata sp. nov. Overall, the results highlighted an unexpectedly high diversity of Phytophthora species in mountain areas, with many species able to cause aerial infections due to the production of caducous sporangia.
Abstract The Rice stripe virus (RSV) pc4 has been determined as the viral movement protein (MP). In this study, the pc4 gene was cloned into a movement-deficient Tobacco mosaic virus (TMV). The ...resulting hybrid TMV-pc4, in addition to spreading cell to cell in Nicotiana tabacum , moved systemically and induced foliar necrosis in Nicotiana benthamiana , indicating novel functions of the RSV MP. A systematic alanine-scanning mutagenesis study established the region K122 –D258 of the pc4 substantially associated with cell-to-cell movement, and mutants by replacement of KGR122–124 , D135 , ED170–171 , ER201–202 , EFE218–220 or ELD256–258 with alanine(s) no longer moved cell to cell. However, only one amino acid group KGR122–124 was linked with long-distance movement. The region D17 –K33 was recognized as a crucial domain for leaf necrosis response, and mutagenesis of DD17–18 or RK32–33 greatly attenuated necrosis. The overall data suggested manifold roles of the pc4 during the RSV infection in its experimental host N. benthamiana.
Although bioactive metabolites capable of causing oxidative photo-necrosis in plant tissues have been identified in fungi, little is known about this type of mechanism in bacteria. These metabolites ...act as photosensitizers that generate reactive oxygen species (ROS) capable of causing damage to cells. In addition, these metabolites can pass into an energetically excited state when they receive some luminous stimulus, a condition in which they interact with other molecules present in the environment, such as molecular oxygen (O2), also known as triplet oxygen (3O2), generating ROS.
The suspension of the bacterial culture of Pseudomonas cedrina was shown to produce foliar necrosis in papaya leaves (Carica papaya L.) only in the presence of sunlight, which is evidence of photosensitizing mechanisms that generate singlet oxygen (1O2). From the chemical study of extracts obtained from this bacteria, 3-(4-(2-carboxipropyl) phenyl) but-2-enoic acid (1) was isolated. This compound, in the presence of light and triplet oxygen (3O2), was able to oxidize ergosterol to its peroxide, since it acted as a photosensitizer producing 1O2, with which it was corroborated that a photosensitization reaction occurs, mechanism by which this bacterium could prove to cause oxidative foliar photo-necrosis.
P. cedrina was able to induce oxidative foliar photo-necrosis because of its potential ability to produce photosensitizing metabolites that generate singlet oxygen in the plants it colonizes. Based on the above, it can be proposed that some bacteria can cause oxidative foliar photo-necrosis as an important mechanism in the pathogenesis of host species.
How to cite: Mendoza G, Sánchez-Tafolla L, Trigos A. Oxidative foliar photo-necrosis produced by the bacteria Pseudomonas cedrina. Electron J Biotechnol 2020;44. https://doi.org/10.1016/j.ejbt.2020.01.007
Umbellularia californica is one of the key infectious hosts of the exotic Phytophthora ramorum, which causes sudden oak death (SOD) in California and Oregon forests. This study provides a ...comprehensive analysis of the epidemiologically relevant parameters for SOD in California and southern Oregon, including potential differences between the two states. Experimental infection of U. californica leaves was optimal when leaves were wet for 6–12 h, temperature was approximately 19°C and pathogen concentration was approximately 2·7 × 104 zoospores mL−1. Seasonal variation in host susceptibility and disease incidence was examined for two populations by inoculating detached leaves at 12 dates and by monitoring naturally infected leaves, respectively. Susceptibility of U. californica and disease incidence varied significantly in time and the variation was highest for both in spring. Susceptibility of trees from 17 natural populations from California and southern Oregon was assessed in detached leaf inoculations. One California and three southern Oregon populations had significantly and repeatable lower average susceptibility in artificial inoculations, but differences among three selected California and Oregon populations were not significant in inoculations of seedlings grown from seed in a common garden. This study concludes that U. californica susceptibility has a large environmental component, yet still predicts potential disease severity in different sites especially where infestations are young or the pathogen has not yet arrived. The accuracy and utility of predictive risk models for P. ramorum will be enhanced by the inclusion of both the environmental and host susceptibility components.
Powdery mildew (Leveillula taurica) became more severe on tomatoes in California’s Central Valley during the 2007 through 2013 seasons. Eighteen uniform field trials evaluating chemical control ...programs were conducted over a four-year period to identify economical and effective control programs, but also to better understand the impact of powdery mildew on yield and fruit quality in processing tomatoes. Of the three trial years, disease pressure was highest in 2009 and 2012; lighter in two intervening years. Disease severity and foliar necrosis at harvest were always significantly reduced by the fungicide programs. Mildew reduced fruit yield by 52% in one trial; in all other trials there was no yield effect. Soluble solids were increased by an average of 0.6 °Brix in sulfur-treated plots relative to non-treated plots; the greatest increase was 1.4 °Brix in one trial. Disease increasing only one month prior to harvest may affect soluble solids without affecting yield, while earlier high disease pressure may significantly reduce yields.
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