Phytopathology最新文献

Oomycetes in the genus Hyaloperonospora cause severe downy mildew that threatens the quality and yield of leafy vegetables in the Brassicaceae family. During 2022 and 2023, infected plant samples were collected from multiple fields across California's Central Coast, and a total of 26 downy mildew pathogen isolates were obtained. Morphological characteristics and molecular analyses based on ITS rDNA and cox2 mtDNA gene sequences identified the isolates as Hyaloperonospora brassicae, H. diplotaxidis, and H. erucae. To further evaluate the host specificity of these pathogens, one representative isolate from each species was selected and inoculated onto a group of commonly grown Brassicaceae crops. H. brassicae, originally isolated from Brassica oleracea, caused 45.8% disease severity in B. oleracea, while disease severity was significantly lower on wild arugula (Diplotaxis tenuifolia, 1.5%) and cultivated arugula (Eruca vesicaria, 0.0%). Similarly, H. diplotaxidis, originally isolated from wild arugula, caused 26.3% disease severity in wild arugula, compared to 2.0% on cultivated arugula and 0.3% in B. oleracea. H. erucae, derived from cultivated arugula, caused 10.4% disease severity on cultivated arugula, 1.4% on wild arugula, and only 0.6% on B. oleracea. This study underscores the host specificity of downy mildew pathogens while also suggesting a potential risk of cross-infection under favorable conditions. These findings may help growers optimize their cropping systems to enhance downy mildew management in the field.

Bioregenerative Life Support Systems (BLSS) are proposed for the production of food crops, water recycling, and air revitalization in future microgravity, Moon, and Mars habitats. The alternative hypothesis (Ha) for the current study was that simulated microgravity would increase phytopathogen growth rates potentially leading to increased levels of disease in space-based BLSS modules. Squash cotyledon leaf discs, and full canopies of Arabidopsis thaliana (At) plants, were dusted with conidia from 14-d-old colonies of Golovinomyces cichoracearum (Gc). Leaves were positioned into one of five gravity treatments including: (1) adaxial leaf surfaces pointed upward (1g-up control), (2) adaxial surface oriented 90° to Earth's 1g down vector (1g-90° control), (3) adaxial surfaces pointed down (1g-down control), (4) rotated on a 2D clinostat, or (5) randomly rotated on a 3D random positioning machine (RPM). Inoculated squash leaf discs and At canopies were incubated on agar media for 2- or 3-days post inoculation (dpi), fixed with 3% glutaraldehyde, and imaged with a SEM. The fastest growth rates on squash leaf discs were observed for germ tubes on squash leaf discs exposed to the 2D clinostat (mean = 94.2 µm at 3 dpi) and 3D RPM system (65.3 µm at 3 dpi); all three 1g controls were similar (~38-45 µm long). Growth rates of germ tubes on At leaves under similar gravity treatments were ~33% of the growth rates observed on squash leaves. Results suggest that Gc conidia may germinate more rapidly and grow faster in altered gravity conditions compared to 1g controls, supporting the Ha.

Stripe rust and powdery mildew are serious diseases that significantly reduce wheat yield. Distant hybridization between wheat and its related species to develop disease-resistant translocation lines is an effective method for enhancing wheat's disease resistance. However, many newly developed disease-resistant wheat-related species translocation lines are difficult to be used in wheat breeding because of their poor agronomic traits. Therefore, developing translocation lines that are resistant to diseases and have good agronomic traits is key for further use in wheat breeding programs. In this study, five novel T2AS.2RL translocation lines were developed from a cross between a high-yield wheat cultivar, CN25, and a Chinese rye landrace Qinling. The results of cytogenetics and molecular analyses indicated that all five lines contained a pair of T2AS.2RL translocation chromosomes. These T2AS.2RL lines were highly resistant to stripe rust and powdery mildew. Genetic analysis indicated that resistance to stripe rust and powdery mildew was conferred by the 2RL chromosome arms derived from Qinling. In addition, compared with their high-yielding wheat parent CN25, these translocation lines presented good agronomic traits and no significant difference in yield. These results indicate that these new T2AS.2RL lines have great potential for use in future wheat breeding programs.

The LuxI/LuxR system, that produces and perceives N-acyl homoserine lactones, plays a significant role in regulating pathogenesis and communication in gram-negative bacteria. A homologous system exists in Pseudomonas syringae pv. tomato DC3000, which is encoded by the single copy genes psyI/psyR. We created a double knockout mutant of the AHL synthase (psyI) and the AHL receptor (psyR) genes in Pst DC3000 and a corresponding complemented strain to gain insights into their role in plant-pathogen interactions and the metabolic processes associated with this system in vitro. The mutant strain psyI-R- overcomes stomatal immunity as the parental strain Pst DC3000. However, the psyI-R- apoplastic population is significantly smaller than that of Pst DC3000 at three days post inoculation. Furthermore, PsyI/PsyR are required for necrosis but not chlorosis in infected tomato leaves, evidenced by the observed pharmacological and genetic complementation of the psyI-R- strain. By comparison, the coronatine deficient mutant DB29 is unable to cause neither necrosis nor chlorosis on leaves. The reduced virulence of psyI-R- is associated with an intermediate induction level of the tomato PR2b marker gene for salicylic acid signaling and reduced expression of bacterial virulence genes in infected leaf tissue. Transcriptomic analysis of psyI-R- and DB29 mutants showed misregulation of genes related to bacterial secretion system, chemotaxis, flagellar motility, ABC transporters, and iron transporters and a partial overlap between metabolic processes regulated by PsyI/PsyR and coronatine. Altogether, these findings indicate that the PsyI/PsyR is required for full bacterium virulence at the later stages of infection in tomato leaves.

Xanthomonas arboricola pv. pruni (XAP) causes bacterial spot in Prunus and copper sprays have been widely used to manage this disease. Copper tolerance [≥ 150 µg/ml of copper sulfate pentahydrate (CSP)] is commonly found in XAP populations, but copper resistance (> 200 µg/ml of CSP) has not been previously reported. This study reports and characterizes the first copper-resistant strain of XAP (XAPCuR), which was isolated from diseased leaves of Prunus laurocerasus in North Carolina in 2017. Whole genome sequence analysis of XAPCuR revealed a ~247 kb plasmid carrying a duplicated 17 kb cluster containing copper resistance candidate genes copL, copA, copB, copC, copD, copM, copG, copF, cusA and cusB. The two copies of the copper resistance cluster did not increase the level of copper resistance compared to a single copy, but deletion of both copies led to the loss of resistance. Functional analysis of the cluster revealed that copL-D is the major contributor to copper resistance, allowing XAP to grow on nutrient agar containing up to 750 µg/ml of CSP. Removing copL from copL-D decreased the resistance level to 300 µg/ml of CSP. The copF and cusAB genes alone did not confer copper resistance, however, adding copF-cusB to copL-D increased the resistance level of XAP to 1000 µg/ml of CSP. The resistance genotype and phenotype were able to be transferred from XAP to Xanthomonas perforans via conjugation. This plasmid has up to 99% identity to other copper resistance plasmids of closely related xanthomonads, indicating horizontal transfer is driving its spread.

Lettuce big-vein disease (LBVD) is a major disease affecting lettuce cultivation worldwide. LBVD is caused by two unrelated negative-stranded RNA viruses, that is, Mirafiori lettuce big-vein virus (MiLBVV; Ophiovirus mirafioriense; Aspiviridae) and Lettuce big-vein associated virus (LBVaV; Varicosavirus lactucae; Rhabdoviridae) both vectored by the soilborne fungus Olpidium virulentus. Despite extensive research, a synergistic effect between the two viruses has not been observed, while both viruses individually have been suggested to be the causal agent for the disease. By performing lettuce infections using a large soil sample collection carrying LBVD infested O. virulentus spores, the presence of LBVaV was consistently established in diseased lettuce heads, while MiLBVV infections were apparently less prevalent. Yet, aboveground infections with MiLBVV corresponded with strong disease symptoms. Strikingly, the spread of LBVaV from the root to shoot always preceded that of MiLBVV. The LBVaV systemic spread was highly synchronized between the plants, while MiLBVV spread was always delayed and less synchronized. A pan-genome analysis revealed independent segment reassortments for both viruses indicative of mixed field infections over the sampled period. Yet, RNA segment abundance was highly conserved for both viruses between all re-infections, suggesting that segment abundance has a regulatory role for the two individual viruses, but that segment abundance is not impacted by the presence of the other two viruses. The pan-genome analysis also revealed different evolutionary rates of the viral ORFs suggesting that mutagenesis of certain ORFs compromises viral fitness and thus revealing a potential weak spot for both viruses.

Banana bunchy top disease (BBTD) has been reported in southwestern Rwanda since the 1980s. Recent sporadic observations of BBTD in western and central regions, made by extension services and the Rwanda Agriculture and Animal Resources Development Board, merited a comprehensive field survey across the country. This survey was performed in 2022 and 2023 to confirm and map the presence of BBTD in suspected areas in Rwanda. Combining the Rwandan survey with BBTD surveys in neighboring countries (eastern DR Congo, Burundi, and Uganda), a BBTD probability model was developed for Rwanda, taking into account environmental, land-use/land-cover, and socioeconomic variables, from which a country-wide vulnerability map was compiled. Field surveys confirmed that BBTD has spread to the Nyamasheke and Rubavu districts along Lake Kivu, all districts of Kigali, and the bordering districts of Rulindo and Kamonyi. Predictive risk mapping confirmed that these regions are the highest-risk areas of the country. Southeastern regions of Rwanda are identified as additional areas at risk for BBTD, mainly areas extending from Kigali to Gisagara in the south and to Kirehe in the east. Although BBTD is currently not widespread across Rwandan banana production zones, key recommendations are made to prevent further incursions. Specifically, Kigali was identified as a new hotspot for the spread of BBTD in Rwanda, and here, coordinated and rigorous eradication is key. Farmers' interviews revealed a critical need for awareness campaigns and training on BBTD. Most farmers were unaware of how the disease spreads, effective disease recognition, how to prevent its introduction and establishment, and how to manage the disease at both initial and advanced stages, making a comprehensive management approach imperative.

Wheat yellow rust, caused by Puccinia striiformis f. sp. tritici (PST), is one of the most important wind-borne diseases in all wheat-growing regions, and its occurrence can lead to devastating yield losses in wheat. In China, the wheat fields in Gansu act as an inoculum reservoir during summer periods and provide PST spores for wheat in the fall. The wheat fields in Qinghai provide large amounts of oversummering inocula. The exchange and migration of spores between the two regions are essential to ensure the persistence of PST. To confirm this relationship, we studied the genetic diversity, seasonal population dynamics, role of recombination, and gene flow between PST populations in different oversummering areas of Gansu and Qinghai using molecular markers combined with a spatiotemporal sampling strategy. Shared genotypes provide molecular evidence of migration between the pathogen populations in the two regions. The distribution of genotypic frequencies indicates that the pathogen mainly flows from Qinghai to Gansu in the autumn, whereas the opposite direction of movement occurs in the spring. The inoculum source from spring wheat can be directly transmitted to autumn seedlings, not necessarily through volunteer wheat. Therefore, the bridging effect of spring wheat may play a more important role than the off-season pathogen surviving on volunteer wheat plants. Furthermore, linkage disequilibrium tests indicate that sexual recombination continues throughout the year in the Tianshui and Dingxi regions of Gansu.

Anthracnose, caused by Colletotrichum spp., significantly threatens rubber trees globally. The nucleotide-binding and leucine-rich repeat (NLR) protein family constitutes one of the largest and most widespread classes of plant immune receptors. However, the specific relationship between NLR genes and anthracnose resistance in rubber trees remains poorly understood. This study comprehensively identified all NLR family members in the reference genome of the wild rubber tree accession MT/VB/25A 57/8. From 39,340 annotated proteins, 253 were classified as NLRs, including 155 coiled-coil NLR (CNL) and 35 Toll/interleukin-1 receptor NLR (TNL) proteins. Conserved motifs were identified in both CNL and TNL proteins. Notably, TNL proteins had a higher occurrence of conserved motif 8 in C-terminal leucine-rich repeat domains than CNL proteins. Chromosomal mapping indicated that CNL and TNL protein-coding genes largely localized on chromosomes 1, 3, 8, 12, and 14. The observed NLR distribution and diversity likely resulted from tandem, ectopic, and segmental gene duplications, with tandem duplications being the most common. Comparative transcriptome and quantitative RT-PCR analyses revealed two specific CNL genes (LOC110667987 and LOC110668014) preferentially expressed in a resistant genotype upon infection by Colletotrichum siamense. Identification of these genes provides insight into the molecular mechanisms of anthracnose resistance and will facilitate the development of resistant rubber trees.

Oil palm clonal propagation seeds (ramets) often give rise to somaclonal variation, which is a cause of hypomethylation in the Karma sequence as a part of the EgDEF1 gene that can lead to abnormalities in flowers and "mantled" fruits, causing a decrease in oil production. The loss of beneficial endophytic bacteria due to aseptic conditions in tissue culture is one of the causes of hypomethylation. This study analyzed the effect of inoculating beneficial endophyte bacteria in hypomethylated oil palm ramets and their impact on plant growth through measurement of the methylation level in the Karma sequence using methylation-specific restriction enzymes and quantitative PCR analysis. Our findings via inoculation of endophyte bacteria indicated that the most significant alteration in methylation occurred in ramets treated with the mixed isolates (3AK, PsJN, R6, GS 4.4) (62.5%), and the 3AK isolate showed a 44.4% increase in hypomethylation to normal methylation. In addition, all inoculation treatments showed more significant growth of oil palm ramets than the control group. These results demonstrate that inoculating appropriate beneficial microbes could significantly enhance the hypomethylated Karma sequence, leading to normal methylated oil palm ramets and promoting their growth.