Susceptible Host Plants Research Articles

The timing of heat waves has multiyear effects on milkweed and its insect community.

Extreme heat events are becoming more frequent and intense as climate variability increases, and these events inherently vary in their timing. We predicted that the timing of a heat wave would determine its consequences for insect communities owing to temporal variation in the susceptibility of host plants to heat stress. We subjected common milkweed (Asclepias syriaca) plants to in-field experimental heat waves to investigate how the timing of heat waves, both seasonally and relative to a biotic stressor (experimental herbivory), affected their ecological consequences. We found that heat waves had multiyear, timing-specific effects on plant-insect communities. Early-season heat waves led to greater and more persistent effects on plants and herbivore communities than late-season heat waves. Heat waves following experimental herbivory had reduced consequences. Our results show that extreme climate events can have complex, lasting ecological effects beyond the year of the event-and that timing is key to understanding those effects.

Role of plant essential oil nanoemulsions on host colonization by the invasive ambrosia beetle Xylosandrus compactus

The development of effective control strategies targeting non-native ambrosia beetles (Coleoptera: Curculionidae: Scolytinae and Platypodinae) represents an emerging challenge. This is due both to their rapid spread into climatically suitable newly invaded regions rich in susceptible host plants and to the limited knowledge concerning sustainable management tools. In this context, the use of botanical insecticides, such as plant essential oils (EOs), is considered a promising option in Integrated Pest Management (IPM) strategies as an alternative to conventional insecticides for sustainable crop protection. Here, the impact of five EO nanoemulsions (Rosmarinus officinalis L. cultivar verbenone, Carlina acaulis L., Laurus nobilis L., Cupressus sempervirens L., and Betula alba L.) on the choice behavior, host colonization and progeny production by the invasive ambrosia beetle Xylosandrus compactus (Eichhoff) was evaluated under laboratory and semi-field conditions. The chemical composition of tested EOs was firstly evaluated by means of gas chromatography-mass spectrometry (GC-MS) analyses. Then, EO nanoemulsions were prepared and characterized through dynamic light scattering (DLS). Tested EO nanoemulsions showed single or bimodal droplet size distributions in the nanometric range. Results highlighted a significant repellent activity of R. officinalis cv. verbenone and C. acaulis EO nanoemulsions (at 3% EO concentration). However, the trend for repellence of the R. officinalis EO against the beetle was not confirmed at decreasing EO concentrations (i.e., 1% and 0.5%). Nanoemulsions based on C. acaulis EO significantly affected the beetle choices regardless of the tested EO concentration. Indeed, in no choice bioassays, a significantly lower number of individuals started infesting stems treated with these nanoemulsions. On the other hand, no effect of the EO treatment was found on the number of offspring produced by survived beetle females. The repellence by R. officinalis and C. acaulis EOs was confirmed in semi-field conditions. Overall, our findings provide new insights toward the use of R. officinalis and C. acaulis EOs as repellents of X. compactus, suggesting their future incorporation in non-chemical strategies, such as ‘push-pull’, as part of sustainable IPM packages targeting this invasive pest.

The root-knot nematode effector MiMSP32 targets host 12-oxophytodienoate reductase 2 to regulate plant susceptibility.

To establish persistent infections in host plants, herbivorous invaders, such as root-knot nematodes, must rely on effectors for suppressing damage-induced jasmonate-dependent host defenses. However, at present, the effector mechanisms targeting the biosynthesis of biologically active jasmonates to avoid adverse host responses are unknown. Using yeast two-hybrid, in planta co-immunoprecipitation, and mutant analyses, we identified 12-oxophytodienoate reductase 2 (OPR2) as an important host target of the stylet-secreted effector MiMSP32 of the root-knot nematode Meloidogyne incognita. MiMSP32 has no informative sequence similarities with other functionally annotated genes but was selected for the discovery of novel effector mechanisms based on evidence of positive, diversifying selection. OPR2 catalyzes the conversion of a derivative of 12-oxophytodienoate to jasmonic acid (JA) and operates parallel to 12-oxophytodienoate reductase 3 (OPR3), which controls the main pathway in the biosynthesis of jasmonates. We show that MiMSP32 targets OPR2 to promote parasitism of M. incognita in host plants independent of OPR3-mediated JA biosynthesis. Artificially manipulating the conversion of the 12-oxophytodienoate by OPRs increases susceptibility to multiple unrelated plant invaders. Our study is the first to shed light on a novel effector mechanism targeting this process to regulate the susceptibility of host plants.

Epicuticular wax content of western white pine is involved in Cronartium ribicola resistance

AbstractThe introduction of Cronartium ribicola, the causative agent of white pine blister rust, has been devastating for white pines in North America, including western white pine. Among the observed partial resistance responses to white pine blister rust is a mechanism referred to as difficult‐to‐infect (DI), which is characterized by lower frequency of infection spots on needles and a lower incidence of branch and stem cankers than susceptible host plants. Parents with the DI trait were selected and bred to produce nine full‐sib families. Progeny were propagated and cloned using in vitro techniques in controlled environmental conditions. Explants were inoculated with a single strain of C. ribicola and characterized by using a disease assessment index (DAI), which was used to compare DI full‐sib families to several other full‐sib families (from Idaho) selected for other partial resistance traits. Most DI families had significantly lower DAI scores and higher epicuticular wax content. When the wax was removed from the surface of needles, the DAI of all full‐sib families increased. Scanning electron microscopy revealed that stomata in DI full‐sib families are mostly occluded by epicuticular wax. The results of this study suggest that the DI resistance mechanism of western white pine is related to needle surface epicuticular wax, which likely hinders the penetration of stomata by fungal hyphae. The occurrence and magnitude of this trait in the breeding population remains unknown, and special assessment technique for large‐scale screening will have to be developed.

A conserved type III effector RipB is recognized in tobacco and contributes to Ralstonia solanacearum virulence in susceptible host plants

Ralstonia solanacearum, the causal agent of bacterial wilt, causes devastating diseases in a wide range of plants including potato, tomato, pepper and tobacco. The pathogen delivers approximately 70 type III effectors (T3Es) into plant cells during infection. In this study, we confirmed that a T3E RipB is recognized in tobacco. We further demonstrated that RipB is conserved among R. solanacearum isolates and five different ripB alleles are all recognized in tobacco. The ripB from GMI1000 was transformed into susceptible host Arabidopsis, and a defect in root development was observed in ripB-transgenic plants. Pathogen inoculation assays showed that ripB expression promoted plant susceptibility to R. solanacearum infection, indicating that RipB contributes to pathogen virulence in Arabidopsis. Expression of ripB in roq1 mutant partially suppressed reactive oxygen species production, confirming that RipB interferes with plant basal defense. Interestingly, ripB expression promoted cytokinin-related gene expression in Arabidopsis, suggesting a role of cytokinin signaling pathway in plant-R. solanacearum interactions. Finally, RipB harbors potential 14-3-3 binding motifs, but the associations between RipB and 14-3-3 proteins were undetectable in yeast two-hybrid assay. Together, our results demonstrate that multiple ripB alleles are recognized in Nicotiana, and RipB suppresses basal defense in susceptible host to promote R. solanacearum infection.

The cytosolic iron-sulphur cluster assembly mechanism in grapevine is one target of a virulent Crinkler effector from Plasmopara viticola.

Grapevine downy mildew is one of the most devastating diseases in grape production worldwide, but its pathogenesis remains largely unknown. A thorough understanding of the interaction between grapevine and the causal agent, Plasmopara viticola, is helpful to develop alternative disease control measures. Effector proteins that could be secreted to the interaction interface by pathogens are responsible for the susceptibility of host plants. In this study, a Crinkler effector, named PvCRN17, which is from P.viticola and showed virulent effects towards Nicotiana benthamiana previously, was further investigated. Consistently, PvCRN17 showed a virulent effect on grapevine plants. Protein-protein interaction experiments identified grapevine VAE7L1 (Vitis protein ASYMMETRIC LEAVES 1/2 ENHANCER 7-Like 1) as one target of PvCRN17. VAE7L1 was found to interact with VvCIA1 and VvAE7, thus it may function in the cytosolic iron-sulphur cluster assembly (CIA) pathway. Transient expression of VAE7L1 in Vitis riparia and N.benthamiana leaves enhanced the host resistance to oomycete pathogens. Downstream of the CIA pathway in grapevine, three iron-sulphur (Fe-S) proteins showed an enhancing effect on the disease resistance of N.benthamiana. Competitive co-immunoprecipitation assay showed PvCRN17 could compete with VvCIA1 to bind with VAE7L1 and VvAE7. Moreover, PvCRN17 and VAE7L1 were colocalized at the plasma membrane of the plant cell. To conclude, after intruding into the grapevine cell, PvCRN17 would compete with VCIA1 to bind with VAE7L1 and VAE7, demolishing the CIA Fe-S cluster transfer complex, interrupting the maturation of Fe-S proteins, to suppress Fe-S proteins-mediated defence responses.

Response of sunflower and soybean to infection with Sclerotinia sclerotiorum with addition of organic amendments

The addition of compost, green waste biochar, and wood biochar to a pathosystem consisting of Sclerotinia sclerotiorum (Lib.) de Bary and two susceptible host plants, sunflower (Helianthus annuus L.) and soybean (Glycine max L.), was investigated in a greenhouse study. Plant growth characteristics, disease incidence and the mycelial growth of S. sclerotiorum in root exudates of plants inoculated and not inoculated with this pathogen were determined. Both plants showed different responses in terms of shoot and root weight. Disease incidence in sunflower was lowest in the substrate consisting of green waste biochar and compost. The disease incidence in this substrate was approximately half that in the control treatment. In soybean, the highest disease suppression was achieved with this combined substrate too. Mycelial growth in sunflower root exudates from uninoculated plants was higher than that in sunflower plants inoculated with S. sclerotiorum. The substrate did not influence mycelial growth in root exudates. Soybean root exudates did not show this effect of the pathogen. This study proves that the addition of organic matter inputs such as biochar and compost can improve plant growth and can also have favorable effects against S. sclerotiorum infection. In addition, it is demonstrated that both the pathogen and host plant are instrumental in determining the mode of action of the supplemented organic material. The addition of the same organic material can lead to different responses in certain pathogens, such as S. sclerotiorum, depending on the host plant.

An Effective Method of Ribes spp. Inoculation with Blackcurrant Reversion Virus under In Vitro Conditions.

Blackcurrant reversion virus (BRV) is the most destructive currant-infecting and mite-transmitted pathogen from the genus Nepovirus. In this work, BRV transmission in the system Ribes ex vitro–Ribes in vitro was applied for the first time. Triple infection of BRV identified in blackcurrant cv. Gojai was used for phylogenetic analysis and inoculation assay. Transmission of BRV was successful due to its stability in the inoculum for up to 8 days at 4 °C; all BRV isolates were infectious. Our suggested inoculation method through roots was applied in six Ribes spp. genotypes with 100.0% reliability, and the expression levels of defence-related gene PR1 to biotic stress was observed. The prevalence of the virus in microshoots after 2–14 days post-inoculation (dpi) was established by PCR. In resistant genotypes, the BRV was identified up to 8 dpi; meanwhile, infection remained constant in susceptible genotypes. We established that BRV transmission under controlled conditions depends on the inoculum quality, post-inoculation cultivation temperature, and host-plant susceptibility to pathogen. This in vitro inoculation method opens possibilities to reveal the resistance mechanisms or response pathways to BRV and can be used for the selection of resistant Ribes spp. in breeding programs.

Chemo-Ecological Insights into the Use of the Non-Host Plant Vegetable Black-Jack to Protect Two Susceptible Solanaceous Crops from Root-Knot Nematode Parasitism.

Plant parasitic nematodes (PPNs) develop through three major stages in their life cycle: hatching, infection, and reproduction. Interruption of any of these stages can affect their growth and survival. We used screenhouse pot experiments, laboratory in vitro hatching and mortality assays, and chemical analysis to test the hypothesis that the non-host Asteraceae plant vegetable black-jack (Bidens pilosa) suppresses infection of the PPN Meloidogyne incognita in two susceptible Solanaceae host plants, tomato (Solanum lycopersicum) and black nightshade (S. nigrum). In intercrop and drip pot experiments, B. pilosa significantly reduced the number of galls and egg masses in root-knot nematode (RKN)-susceptible host plants by 3-9-fold compared to controls. Chemical analysis of the most bioactive fraction from the root exudates of B. pilosa identified several classes of compounds, including vitamins, a dicarboxylic acid, amino acids, aromatic acids, and a flavonoid. In in vitro assays, the vitamins and aromatic acids elicited the highest inhibition in egg hatching, whereas ascorbic acid (vitamin) and 2-hydroxybenzoic acid (aromatic acid) elicited strong nematicidal activity against M. incognita, with LC50/48 h values of 12 and 300 ng/μL, respectively. Our results provide insights into how certain non-host plants can be used as companion crops to disrupt PPN infestation.

Real-Time Feeding Behavior Monitoring by Electrical Penetration Graph Rapidly Reveals Host Plant Susceptibility to Crapemyrtle Bark Scale (Hemiptera: Eriococcidae).

Simple SummaryCrapemyrtle bark scale (CMBS; Acanthococcus lagerstroemiae), an invasive polyphagous sap feeder in the United States, has spread across 16 U.S. states in less than two decades, posing potential risks to the Green Industry. Confirming the host range is crucial for effective pest management of invasive insects. However, host range confirmation relying on greenhouse or field trials is often inefficient and time-consuming. In this study, we used the electrical penetration graph (EPG) to monitor the stylet penetration of CMBS in plant tissue in real-time. An R programming-based application was developed to better characterize the insect EPG waveforms recorded by EPG. By analyzing EPG-based EPG parameters, we demonstrated that CMBS has difficulty accessing the phloem tissue (salivation and ingestion) of a resistant plant compared to a susceptible plant. Importantly, we hereby present CMBS typical feeding behaviors on susceptible and non-susceptible plants comparatively, which provides direct evidence for revealing unknown hosts rapidly.Host range confirmation of invasive hemipterans relies on the evaluation of plant susceptibility though greenhouse or field trials, which are inefficient and time-consuming. When the green industry faces the fast-spreading threat of invasive pests such as crapemyrtle bark scale (Acanthococcus lagerstroemiae), it is imperative to timely identify potential host plants and evaluate plant resistance/susceptibility to pest infestation. In this study, we developed an alternative technology to complement the conventional host confirmation methods. We used electrical penetration graph (EPG) based technology to monitor the A. lagerstroemiae stylet-tip position when it was probing in different plant tissues in real-time. The frequency and relative amplitude of insect EPG waveforms were extracted by an R programming-based software written to generate eleven EPG parameters for comparative analysis between plant species. The results demonstrated that the occurrences of phloem phase and xylem phase offered conclusive evidence for host plant evaluation. Furthermore, parameters including the percentage of insects capable of accessing phloem tissue, time duration spent on initiating phloem phase and ingesting phloem sap, provided insight into why host plant susceptibility differs among similar plant species. In summary, this study developed a novel real-time diagnostic tool for quick A. lagerstroemiae host confirmation, which laid the essential foundation for effective pest management.

Impact of Calonectria Diseases on Ornamental Horticulture: Diagnosis and Control Strategies.

Diseases caused by fungi in the genus Calonectria pose a significant threat to the ornamental horticulture industries in Europe and the United States. Calonectria spp. are particularly challenging pathogens to manage in ornamental production systems and the urban landscape for multiple reasons. A high level of species diversity and poorly resolved taxonomy in the genus makes proper pathogen identification and disease diagnosis a challenge, though recent molecular phylogenetic studies have made significant advances in species delimitation. From a disease management perspective, Calonectria spp. produce long-lived survival structures (microsclerotia) that contaminate nursery production systems and can survive multiple years in the absence of a susceptible plant host. Latent infection of plant material is poorly understood but likely contributes to long-distance dissemination of these fungal pathogens, including the clonal Calonectria spp. responsible for the global emergence of boxwood blight. Breeding for disease resistance represents a sustainable strategy for managing Calonectria diseases but is challenging due to the perennial nature of many ornamental plants and high levels of susceptibility in commercial cultivars. Ultimately, long-term sustainable management of Calonectria diseases will require an improved understanding of pathogen biology as well as integration of multiple disease management strategies.

Lipid-rich endo-metabolites from a vertically transmitted fungal endophyte Penicillium sp. PM031 attenuate virulence factors of phytopathogenic Ralstonia solanacearum

The bacterial wilt caused by Ralstonia solanacearum is a destructive plant disease globally. Since a completely non-biological control measure could be a matter of environmental concern, investigations of developing eco-friendly strategies are required to control this phytopathogen. Attenuation of the bacterial virulence in addition to destroying the pathogen may be an alternative and overarching approach to control this disease. In this study, we have explored the potentiality of a vertically transmitted endophytic fungus Penicillium sp. PM031 isolated from stem of in vitro grown, wilt susceptible tomato cultivar to control this phytopathogen. The endophytic fungus was unable to inhibit the bacterial growth during direct confrontation in co-culture system; rather its growth and extracellular secretion were affected by the bacterium. Interestingly, the PM031-derived endo-metabolites, containing ~80% of lipid molecules, showed the dose-dependent growth inhibitory effect against R. solanacearum. Metabolite treatment with a concentration of 2500 and 5000 μg/ml significantly inhibited the bacterial growth 24.72% and 64.31%, respectively. Higher concentrations of endo-metabolite treatment exhibited antibacterial activity by rupturing cellular membranes. Furthermore, the endo-metabolites negatively influence the virulence factors necessary in early phases of bacterial infection, such as motility and biofilm formation. Our study highlights even if an endophytic fungus associated with the susceptible host plant cannot tackle R. solanacearum directly, its lipid-rich metabolites have potential to attenuate the virulence of phytopathogen. We believe this study can be a stepping stone to develop suitable formulations to control the bacterial wilt in a sustainable way, which will reduce excessive uses of synthetic bactericides.

Identification of risk areas for Orobanche cumana and Phelipanche aegyptiaca in China, based on the major host plant and CMIP6 climate scenarios.

Parasitic broomrape of the genus Orobanche poses a formidable threat to producing many crops in Europe, Africa, and Asia. Orobanche cumana and Phelipanche aegyptiaca are two of China's most destructive root parasitic plants, causing extreme sunflower, tomato, melon, and tobacco damage. However, the potentially suitable areas of O. cumana and P. aegyptiaca in China have not been predicted, and little is known about the important environmental factors that affect their extension. Due to their invasiveness and economic importance, studying how climate change and host plants may affect broomrapes’ distribution is necessary. In the study, we first predicted the potentially suitable areas of the invasive weeds (O. cumana and P. aegyptiaca) and their susceptible host plants (Helianthus annuus and Solanum lycopersicon) using MaxEnt. Then, the risk zones and distribution shifts of two broomrapes under different climate conditions were identified by incorporating the distribution of their susceptible host plants. The results highlighted that the potential middle‐ and high‐risk zones for O. cumana and P. aegyptiaca amounted to 197.88 × 104 km2 and 12.90 × 104 km2, respectively. Notably, Xinjiang and Inner Mongolia were the highest‐risk areas within the distribution and establishment of O. cumana and P. aegyptiaca. Elevation and topsoil pH were the decisive factors for shaping O. cumana distribution; precipitation seasonality and annual precipitation were the dominant bioclimatic variables limiting the spread of P. aegyptiaca. The potentially suitable areas and risk zones of O. cumana would decrease significantly, and those of P. aegyptiaca would fluctuate slightly under future climate change scenarios. Overall, our study suggested that the two broomrapes’ risk zones will significantly northward to higher latitudes. The results will provide suggestions for preventing O. cumana and P. aegyptiaca.

Recent Advances in Alternaria Phytotoxins: A Review of Their Occurrence, Structure, Bioactivity, and Biosynthesis.

Alternaria is a ubiquitous fungal genus in many ecosystems, consisting of species and strains that can be saprophytic, endophytic, or pathogenic to plants or animals, including humans. Alternaria species can produce a variety of secondary metabolites (SMs), especially low molecular weight toxins. Based on the characteristics of host plant susceptibility or resistance to the toxin, Alternaria phytotoxins are classified into host-selective toxins (HSTs) and non-host-selective toxins (NHSTs). These Alternaria toxins exhibit a variety of biological activities such as phytotoxic, cytotoxic, and antimicrobial properties. Generally, HSTs are toxic to host plants and can cause severe economic losses. Some NHSTs such as alternariol, altenariol methyl-ether, and altertoxins also show high cytotoxic and mutagenic activities in the exposed human or other vertebrate species. Thus, Alternaria toxins are meaningful for drug and pesticide development. For example, AAL-toxin, maculosin, tentoxin, and tenuazonic acid have potential to be developed as bioherbicides due to their excellent herbicidal activity. Like altersolanol A, bostrycin, and brefeldin A, they exhibit anticancer activity, and ATX V shows high activity to inhibit the HIV-1 virus. This review focuses on the classification, chemical structure, occurrence, bioactivity, and biosynthesis of the major Alternaria phytotoxins, including 30 HSTs and 50 NHSTs discovered to date.

Olive Fungal Epiphytic Communities Are Affected by Their Maturation Stage.

The phyllosphere comprises the aerial parts of plants and is colonized by a great diversity of microorganisms, either growing inside (as endophytes) or on the surface (as epiphytes) of plant tissues. The factors that structure the diversity of epiphytes and the importance of these microorganisms for host plant protection have been less studied when compared to the case of endophytes. In this work, the epiphytic fungal communities from fruits of the olive tree (olives) in different maturation stages (green and semi-ripened), obtained from different olive orchard managements (integrated and organic production) and from distinct cultivars displaying different susceptibilities to olive anthracnose (Cobrançosa and Madural), are compared by using a metabarcoding approach. We discuss whether such differences in host resistance against anthracnose depend on both the fungal taxa or fungal community composition. A total of 1565 amplicon sequence variants (ASVs) were obtained, mainly belonging to the Ascomycota phylum and Saccharomycetes class. Although significant differences on epiphytic fungal richness were observed among olives obtained in different production systems and maturation stages, these factors in addition to host cultivar did not influence the composition of the epiphytes. Despite these results, a co-inertia analysis showed that Aureobasidium spp. and Sporocadaceae spp. were positively associated with the green olives of the cv. Madural produced under integrated production, while Saccharomycetales spp. (Kluyveromyces, Candida, Kazachstania and Saccharomyces) were positively associated with the semi-ripened olives of the cv. Cobrançosa obtained from organic production. The discriminant power of these fungi, some of them recognized as biocontrol agents, suggest that they might be important in conferring differences on host plant susceptibility to anthracnose.

Development of Azalea Lace Bug, Stephanitis pyrioides, on Susceptible and Resistant Rhododendron species in Western Washington.

The invasive azalea lace bug, Stephanitis pyrioides (Scott) (Tingidae: Hemiptera), is an important pest of Rhododendron (L.) (Ericales: Ericaceae). Feeding by nymphs and adults removes chlorophyll, reduces rates of photosynthesis and transpiration, and causes leaf stippling, which reduces the aesthetic value of infested plants. Rhododendron spp. are a major component of landscapes in the Pacific Northwest. Previous studies on the seasonality of S. pyrioides in North America are largely from the southeastern United States, which could have limited applicability in the Pacific Northwest. To quantify S. pyrioides seasonality in western Washington, we sampled ~200 leaves from 18 Rhododendron plants 1-2 times per wk from April to October over 2 yr, and microscopically counted the number of eggs, early instars, late instars, and adults. We developed a degree-day model for first generation S. pyrioides, which we used to estimate that S. pyrioides undergoes two full and a partial third generation in western Washington. Our model estimates 5 and 50% early instar occurrence, after hatching from overwintering eggs, at 69 and 171 accumulated degree-days from 1 January, respectively, when using a base threshold of 10.2°, which can be used to optimize the timing of management decisions. We also observed faster development and adult emergence when S. pyrioides nymphs feed on susceptible host plants relative to more resistant host plants, which may influence the timing of management decisions and potentially increase the probability of a full third generation. This research enhances our knowledge of an emerging invasive species in the Pacific Northwest.

Impact of Temperature and Rainfall Change on Epidemics Caused by Plant Viruses: A Review

Plant virus diseases are one of the limiting factors to crop productivity by diminishing the quantity, quality and responsible for significant economic losses worldwide. The Epidemic of plant virus diseases is the result of interactions between virus, host plant, vector, and environmental factors. Changes in host plants and insect vector dynamics that result from temperature and rainfall change could have an influence on the spread of plant viruses. The rising of temperature and heat stress increase the susceptibility of host plants to virus infection and accelerates the fitness of viruses to cause disease. The increasing temperature also changes insect vector population dynamics by accelerating insect phenology, causing earlier and prolonged colonization because it makes appropriate environmental conditions for the insect vectors. Insect populations of most virus vectors build up more rapidly in areas with high temperatures and high relative humidity and decline at low temperature. In addition, the rising temperatures can increase the efficiency of virus transmission from infected to healthy plants by insect vectors. An increasing frequency of heavy rainfall events is likely to slow the virus prevalence and incidence by washing insect vectors, thus reducing vector density. Flooding within annual crop growing period enhances the subsequent growth of weed and volunteer crop plant which act as reservoirs of insect vectors and the viruses, and its occurrence outside growing seasons increases subsequent growth of such reservoirs. Keywords : Host plant; Vectors; Virus epidemics DOI: 10.7176/JEES/12-1-01 Publication date: January 31 st 2022

Secretome analysis identified extracellular superoxide dismutase and catalase of Macrophomina phaseolina.

Macrophomina phaseolina, a necrotrophic fungal pathogen is known to cause charcoal rot disease in food crops, pulse crops, oil crops and cotton and fibre crops. Necrotrophic fungi survive on dead plant tissue. It is well known that reactive oxygen species (ROS) are produced by the host plant during plant-pathogen interaction. However, it is still unclear how M. phaseolina can overcome the ROS-induced cellular damage. To mimic the invasion of M. phaseolina inside the plant cell wall, we developed solid substrate fermentation where M. phaseolina spore suspension was inoculated on a wheat bran bed and incubated for vegetative growth. To analyse the secretome of M. phaseolina after different day interval, its secretory material was collected and concentrated. Both superoxide dismutase (SOD) and catalase were detected in the secretome by zymogram. The presence of SOD and catalase was further confirmed by liquid chromatography based mass spectrometry. The physicochemical properties of M. phaseolina catalase in terms of stability towards pH, temperature, metal ions and chaotropic agent and inhibitors indicated its fitness at different environmental conditions. Apart from the production of catalase in SSF, the studies on this particular microorganism may also have significance in necrotrophic fungal pathogen and their susceptible host plant interaction.

The Raf-like kinase Raf36 negatively regulates plant resistance against the oomycete pathogen Phytophthora parasitica by targeting MKK2.

Oomycetes represent a unique group of plant pathogens that are phylogenetically distant from true fungi and cause significant crop losses and environmental damage. Understanding of the genetic basis of host plant susceptibility facilitates the development of novel disease resistance strategies. In this study, we report the identification of an Arabidopsis thaliana T‐DNA mutant with enhanced resistance to Phytophthora parasitica with an insertion in the Raf‐like mitogen‐activated protein kinase kinase kinase gene Raf36. We generated additional raf36 mutants by CRISPR/Cas9 technology as well as Raf36 complementation and overexpression transformants, with consistent results of infection assays showing that Raf36 mediates Arabidopsis susceptibility to P. parasitica. Using a virus‐induced gene silencing assay, we silenced Raf36 homologous genes in Nicotiana benthamiana and demonstrated by infection assays the conserved immune function of Raf36. Mutagenesis analyses indicated that the kinase activity of Raf36 is important for its immune function and interaction with MKK2, a MAPK kinase. By generating and analysing mkk2 mutants and MKK2 complementation and overexpression transformants, we found that MKK2 is a positive immune regulator in the response to P. parasitica infection. Furthermore, infection assay on mkk2 raf36 double mutant plants indicated that MKK2 is required for the raf36‐conferred resistance to P. parasitica. Taken together, we identified a Raf‐like kinase Raf36 as a novel plant susceptibility factor that functions upstream of MKK2 and directly targets it to negatively regulate plant resistance to P. parasitica.

The polyphagous shot hole borer beetle: Current status of a perfect invader in South Africa

The polyphagous shot hole borer (PSHB) beetle is a recent invader in South Africa. Together with its fungal symbiont, Fusarium euwallaceae, it can rapidly kill highly susceptible host plants. Its impact is most profound in urban areas, but it has also been found infesting important forestry, agricultural crop and native species. Since its first detection in 2012, PSHB has spread to all but one province in the country. The beetle–fungus complex has several biological traits that enhance its anthropogenically mediated dispersal, establishment and survival in novel environments – factors that have likely facilitated its rapid spread across the country. We review the history of the PSHB invasion in South Africa, its taxonomic status and the reasons for its rapid spread. We highlight its potential impact and challenges for its management. Finally, we provide an updated distribution map and list of confirmed host plants in South Africa. Of the 130 plant species identified as hosts, 48 of these (19 indigenous and 29 introduced) are reproductive hosts able to maintain breeding PSHB populations. These reproductive hosts may succumb to beetle infestations and act as ‘pest-amplifiers’. The economic impact on urban forests, plantation forestry and agricultural crops may be severe, but the ecological impact of PSHB invasion in native ecosystems should not be underestimated.Significance: We provide an updated host list and distribution map for South Africa of the globally significant tree pest, the polyphagous shot hole borer (PSHB, Euwallacea fornicatus). The South African PSHB invasion represents the largest outbreak of this pest in its global invaded range. PSHB was confirmed to infest 130 plant species in urban, agricultural, and native ecosystems in South Africa, including 44 previously unreported hosts. Impact in South Africa is in its infancy but will likely be substantial to local economies and ecosystems. Mitigation has proven difficult, but numerous research projects have been initiated throughout the country.