Effects Of Plant Pathogens Research Articles

Meta-Analysis Suggests Differing Indirect Effects of Viral, Bacterial, and Fungal Plant Pathogens on the Natural Enemies of Insect Herbivores

Simple SummaryPlants suffer from disease, caused by fungal, bacterial and viral pathogens. Some insect herbivores avoid diseased plants, while others are attracted to them, often depending on how infection changes the quality of the plant. In turn, the herbivores are attacked by enemies, such as insect predators and parasitoids. Few studies have investigated how plant pathogens affect the herbivores’ insect natural enemies. Using a statistical technique called meta-analysis, we examined 216 measured responses from 29 studies, to see if we could find any patterns. The effect on natural enemies depended on the cause of the disease and type of natural enemy. Fungal pathogens had negative effects on the preference and performance of insect natural enemies, mainly through the disease reducing the quality of their insect herbivore prey. Bacterial pathogens had a positive effect on insect natural enemies, possibly caused by changes in host plant traits. We found no clear effect with viral diseases. We show that the causes of plant disease may have very different effects on ecological interactions between insect herbivores and their enemies, particularly parasitic wasps. This will help us understand how disease can change the patterns of interactions between species in natural and agricultural ecosystems.Indirect effects are ubiquitous in nature, and have received much attention in terrestrial plant–insect herbivore–enemy systems. In such tritrophic systems, changes in plant quality can have consequential effects on the behavior and abundance of insect predators and parasitoids. Plant quality as perceived by insect herbivores may vary for a range of reasons, including because of infection by plant pathogens. However, plant diseases vary in their origin (viral, bacterial or fungal) and as a result may have differing effects on plant physiology. To investigate if the main groups of plant pathogens differ in their indirect effects on higher trophic levels, we performed a meta-analysis using 216 measured responses from 29 primary studies. There was no overall effect of plant pathogens on natural enemy traits as differences between pathogen types masked their effects. Infection by fungal plant pathogens showed indirect negative effects on the performance and preference of natural enemies via both chewing and piercing-sucking insect herbivore feeding guilds. Infection by bacterial plant pathogens had a positive effect on the natural enemies (parasitoids) of chewing herbivores. Infection by viral plant pathogens showed no clear effect, although parasitoid preference may be positively affected by their presence. It is important to note that given the limited volume of studies to date on such systems, this work should be considered exploratory. Plant pathogens are very common in nature, and tritrophic systems provide an elegant means to examine the consequences of indirect interactions in ecology. We suggest that further studies examining how plant pathogens affect higher trophic levels would be of considerable value.

Mycobiome analysis of asymptomatic and symptomatic Norway spruce trees naturally infected by the conifer pathogens Heterobasidion spp.

Plant microbiome plays an important role in maintaining the host fitness. Despite a significant progress in our understanding of the plant microbiome achieved in the recent years, very little is known about the effect of plant pathogens on composition of microbial communities associated with trees. In this study, we analysed the mycobiome of different anatomic parts of asymptomatic and symptomatic Norway spruce trees naturally infected by Heterobasidion spp. We also investigated the primary impact of the disease on the fungal communities, which are associated with Norway spruce trees. Our results demonstrate that the structure of fungal communities residing in the wood differed significantly among symptomatic and asymptomatic Heterobasidion-infected trees. However, no significant differences were found in the other anatomic regions of the trees. The results also show that not only each of individual tree tissues (wood, bark, needles and roots) harbours a unique fungal community, but also that symptomatic trees were more susceptible to co-infection by other wood-degrading fungi compared to the asymptomatic ones.

Tissue Microbiome of Norway Spruce Affected by Heterobasidion-Induced Wood Decay.

Plants live in close association with microbial symbionts, which may affect the host fitness, productivity, and tolerance against biotic and abiotic stressors. The composition of plant microbial communities is influenced by many biotic and abiotic factors, but little is known about the effect of plant pathogens on the structure of these communities. In this study, we investigated the structure of bacterial communities associated with different tissues of asymptomatic and symptomatic (Heterobasidion-rotten) Norway spruce (Picea abies (L.) Karst.) trees. Our results demonstrated that each of the investigated anatomic tissues (root, bark, down stem, upper stem, and needles) harbored a unique bacterial assemblage. However, the health status of the host trees had little effect on the structure of bacterial communities, as the only significant differences among asymptomatic and symptomatic trees were found in the composition of the bacterial communities of needles. Proteobacteria was predominant in all anatomic regions with the highest abundance in needles (86.7%), whereas Actinobacteria showed an opposite trend, being more abundant in the woody tissues than in needles. Additionally, we performed profiling of terpenoid compounds present in spruce xylem and phloem. Total concentrations of monoterpenes and sesquiterpenes were considerably higher in asymptomatic trees. However, we found no significant correlations between terpenoid profiles of spruce trees and the composition of their bacterial communities. Our results provide an insight into the diversity of bacteria associated with Norway spruce tree tissues. At the same time, the health status and terpenoid content of host trees had a limited effect on the composition of bacterial communities in our survey.

Plant pathogen-induced volatiles attract parasitoids to increase parasitism of an insect vector

Interactions between plant pathogens and arthropods have been predominantly studied through the prism of herbivorous arthropods. Currently, little is known about the effect of plant pathogens on the third trophic level. This question is particularly interesting in cases where pathogens manipulate host phenotype to increase vector attraction and presumably increase their own proliferation. Indeed, a predator or a parasitoid of a vector may take advantage of this manipulated phenotype to increase its foraging performance. We explored the case of a bacterial pathogen, Candidatus Liberibacter asiaticus (Las), which modifies the odors released by its host plant (citrus trees) to attract its vector, the psyllid Diaphorina citri. We found that the specialist parasitoid of D. citri, Tamarixia radiata, was attracted more toward Las-infected than uninfected plants. We demonstrated that this attractiveness was due to the release of methyl salicylate. Parasitization of D. citri nymphs on Las-infected plants was higher than on uninfected controls. Also, parasitization was higher on uninfected plants baited with methyl salicylate than on non-baited controls. This is the first report of a parasitoid ‘eavesdropping’ on a plant volatile induced by bacterial pathogen infection, which also increases effectiveness of host seeking behavior of its herbivorous vector.

蜡样芽孢杆菌B3-7在大田小麦根部的定殖动态及其对小麦纹枯病的防治效果

为了阐明蜡样芽孢杆菌B3-7在大田条件下的生态适应性以及对于小麦纹枯病的生防效果，通过利用绿色荧光蛋白编码基因gfp标记生防菌株B3-7，室内比较了GFP标记菌株和原始出发菌株在菌落形态、生长特性，生物薄膜产生以及在小麦根部定殖等方面的特性，结果发现GFP标记菌株和出发菌株在上述特性方面无明显差别。在此基础上，大田条件下测定了GFP标记菌株在小麦根部的定殖动态和对于小麦纹枯病的生防效果。结果发现，GFP标记菌株在小麦根部能够长期定殖，其存在量在小麦分蘖期最大，每克根重达到10⁵ CFU，拔节期后，该细菌数量一直维持在10⁴ CFU之上。同时发现，生防菌株能够有效降低小麦纹枯病的严重度和提高罹病小麦的产量。小麦分蘖期、孕穗期和灌浆期生防菌对于小麦纹枯病的防治效果分别达到60%、34%，34%，小麦成熟后产量提高13%-15%。结果表明，B3-7在大田条件下具有较好的生态适应性和防治小麦纹枯病的能力。;Sharp eyespot of wheat is caused by the fungus <em>Rhizoctonia cerealis</em> and is an important soil-Borne disease of wheat worldwide. At present, no commercially cultivated disease resistant varieties are available to efficiently protect wheat against infection by this pathogen. Control of the plant disease sharp eyespot currently relies on application of pesticides such as Triadimefon and Validamycin; however, pesticides are expensive and pose serious health and environmental hazards. Persistent application of pesticides can cause selection of pesticide resistant pathogenic fungi. Biological control is an environmentally sound and effective means of reducing or mitigating plant diseases and the effects of plant pathogens through the use of natural microorganisms. In the rhizosphere (on the plant root or in close vicinity to the root) bacteria are abundant, most often organized in microcolonies. Bacteria in the rhizosphere, known as rhizobacteria, not only benefit from the nutrients secreted by the plant root but also beneficially influence the plant in a direct or indirect way, resulting in stimulation of plant growth. Among the beneficial microorganisms isolated from the rhizosphere,<em> Bacillus</em>spp. offer several advantages over fluorescent pseudomonads and other Gram-negative bacteria as seed inoculants for protection against root pathogens. <em>Bacillus</em> spp. have a long shelf life because of their ability to form endospores and their unique mechanisms of action as they can produce broad-spectrum antibiotics and compete for ecological niches against plant pathogenic fungi.<br><em>Bacillus cereus</em> B3-7, a biological control strain isolated from the wheat rhizosphere, shows potential for control of sharp eyespot and for efficient yield increase in wheat. To elucidate the ecological adaptability of B3-7 and its efficacy for biological control of sharp eyespot of wheat under field conditions, the green fluorescent protein (<em>gfp</em>) gene was inserted into the B3-7 genome and a GFP-tagged B3-7 strain was constructed. Colony shape, growth rate, biofilm formation and colonization dynamics on wheat roots were analyzed with B3-7 and the GFP-tagged strain in our laboratory. No significant differences were obtained for any of the characteristics tested between the two strains. Colonization ability of the B3-7 GFP-tagged strain on wheat roots was assessed under field conditions. The results indicated that the GFP-tagged strains could persistently colonize the roots of wheat. The number of colonizing bacteria was highest during the tillering period at 10⁵ CFU/g roots. The colonization number decreased to 10⁴ CFU/g roots in the heading period, and persisted through the milking period. Efficacy of B3-7 and the GFP-tagged strain as biological control agents against sharp eyespot of wheat was also evaluated under field conditions. The results revealed that the two strains could reduce disease severity and effectively increase the yield of wheat. The disease control efficacy of the two strains for sharp eyespot of wheat reached 60% to 62%, 34% to 39%, and 34% to 38% during the tillering, heading, and milking periods, respectively. The yield of wheat treated with the biological control strain increased 13% to 15% compared with that of the untreated control. The results indicated that <em>Bacillus cereus</em> B3-7 has the capacity for environmental adaptability and the potential to act as a biological control agent against sharp eyespot of wheat under field conditions.

Effects of plant pathogens on population dynamics and community composition in grassland ecosystems: two case studies

Grassland ecosystems comprise a major portion of the earth’s terrestrial surface, ranging from high-input cultivated monocultures or simple species-mixtures to relatively unmanaged but dynamic systems. Plant pathogens are a component of these systems with their impact dependent on many interacting factors, including grassland species population dynamics and community composition, the topics covered in this paper. Plant pathogens are affected by these interactions and also act reciprocally by modifying their nature. We briefly review these features of disease in grasslands and then introduce the 150-year long-term Park Grass Experiment (PGE) at Rothamsted Research in the UK. We then consider in detail two plant-pathogen systems present in the PGE, Tragopogon pratensis-Puccinia hysterium and Holcus lanata-Puccinia coronata. These two systems have very different life history characteristics: the first, a biennial member of the Asteraceae infected by its host-specific, systemic rust; the second, a perennial grass infected by a host-non-specific rust. We illustrate how observational, experimental and modelling studies can contribute to a better understanding of population dynamics, competitive interactions and evolutionary outcomes. With Tragopogon pratensis-Puccinia hysterium, characterised as an “outbreak” species in the PGE, we show that pathogen-induced mortality is unlikely to be involved in host population regulation; and that the presence of even a short-lived seed-bank can affect the qualitative outcomes of the host-pathogen dynamics. With Holcus lanata-Puccinia coronata, we show how nutrient conditions can affect adaptation in terms of host defence mechanisms, and that co-existence of competing species affected by a common generalist pathogen is unlikely.

Foliar pathogenesis and plant water relations: a review

As the world population grows, there is a pressing need to improve productivity from water use in irrigated and rain-fed agriculture. Foliar diseases have been reported to decrease crop water-use efficiency (WUE) substantially, yet the effects of plant pathogens are seldom considered when methods to improve WUE are debated. We review the effects of foliar pathogens on plant water relations and the consequences for WUE. The effects reported vary between host and pathogen species and between host genotypes. Some general patterns emerge however. Higher fungi and oomycetes cause physical disruption to the cuticle and stomata, and also cause impairment of stomatal closing in the dark. Higher fungi and viruses are associated with impairment of stomatal opening in the light. A number of toxins produced by bacteria and higher fungi have been identified that impair stomatal function. Deleterious effects are not limited to compatible plant-pathogen interactions. Resistant and non-host interactions have been shown to result in stomatal impairment in light and dark conditions. Mitigation of these effects through selection of favourable resistance responses could be an important breeding target in the future. The challenges for researchers are to understand how the effects reported from work under controlled conditions translate to crops in the field, and to elucidate underlying mechanisms.

The effects of plant pathogens on tree recruitment in the Western Amazon under a projected future climate: a dynamical systems analysis

Summary 1. Climate change predictions in the Amazon have largely focused on carbon–water relations, while the impacts of increased air temperature and reduced precipitation on host–pathogen relationships have not been extensively explored. These relationships are known to affect recruitment of many Amazonian plant species. 2. Host–pathogen relationships are well suited to a dynamical analysis of the effects of climate change due to the direct linkages between pathogen behaviour and abiotic factors such as temperature and rainfall. 3. Seedlings of the palm Iriartea deltoidea experience significant mortality due to infection by the fungus Diplodia mutila. This host–pathogen interaction was examined by combining a semi‐analytical model with field data illustrating the temperature sensitivity of D. mutila reproductive rates and I. deltoidea seedling mortality in response to infection. 4. The data–model combination shows that projected climatic shifts in rainfall and temperature for the Amazon region will tend to reduce recruitment by altering pathogen activity and reducing palm fecundity. The magnitude of the reduction is sensitive to the details of the epidemiology of the D. mutila–I. deltoidea host–pathogen system, and ranges from 10% to 56% under plausible scenarios. 5. Although considerable uncertainty remains, the proposed model provides a blueprint for research on one aspect of ecosystem change in future climate models. 6. Synthesis. The study illustrates the potential for ecosystem responses to climate change, which can be investigated through tractable models simple enough to assimilate into climate modelling frameworks. Particular environmental sensitivities in fungal dynamics are identified. The implications of combined plant physiological stress and enhanced pathogenic activity under future climate scenarios are highlighted as critical issues for projecting forest response.

Detection and Measurement of Plant Disease Gradients in Peanut with a Multispectral Radiometer

Percent reflectance values, when used as the independent variable, explained 94.6-96.5% of the variation in peanut rust disease severity along rust gradients. Reflectance of sunlight from peanut canopies at 800 nm provided a rapid and objective measurement of disease intensity and the amount of green leaf area contributing to pod yield. Recording percent reflectance offers a means to quantify the benefits obtained from disease control tactics aimed at protecting and maintaining healthy green leaf area from the effects of plant pathogens

Effects of Plant Pathogens on the Fate of Soil-Applied (14C]Carbofuran in Corn Plants

The effects of common root rot and leaf rust diseases of corn plants on the translocation and metabolism of soil-applied (5 ppm) [14C]carbofuranin corn were investigated. The corn diseases studied were caused by the fungi Giberella zeae (Schw.) Petch or Puccinia sorghi Schw., respectively. Both root rot and leaf rust affected the translocation of [14C]carbofuranfrom soils into the plant tissue. Soil-applied radiocarbon accumulated in the diseased organs of the plants, i.e., in roots of corn with root rot and in leaves of plants with rust, which contained 1.4 and 1.7 times more radiocarbon than roots or leaves from control plants, respectively. The amounts of 14C translocated into leaves were directly related to the transpiration stream, which was increased in both root rot and leaf rust-infected plants. Carbofuran was the major compound recovered from control leaves, amounting to 42% of all organic-soluble radiocarbon present. Similar amounts were isolated as 3-hydroxycarbofuran. Leaves from plants with root rot, however, contained primarily 3-hydroxycarbofuran (50% of organic-soluble 14C) and considerably less carbofuran (24% of organic-soluble 14C). Conversely, leaves with rust contained more carbofuran (47% of organic-soluble 14C) and slightly less 3-hydroxycarbofuran than did control leaves. Based on analyses by gas-liquid chromatography, concentrations of carbofuran in leaves from control plants and from plants with root rot or rust amounted to 8.8, 3.7, and 16.7 ppm, respectively.