Pathogen Propagules Research Articles

Detection of Podosphaera macularis in Air Samples by Quantitative PCR.

Detection and quantification of pathogen propagules in the air or other environmental samples is facilitated by culture-independent assays. We developed a quantitative PCR assay for the hop powdery mildew fungus, Podosphaera macularis, for detection of the organism from air samples. The assay uses primers and a TaqMan probe designed to target species-specific sequences in the 28S large subunit of the nuclear ribosomal DNA. Analytical sensitivity was not affected by the presence of an exogenous internal control or potential PCR inhibitors associated with DNA extracted from soil. The level of quantification of the assay was between 200 and 350 conidia when DNA was extracted from a fixed number of conidia. The assay amplified all isolates of P. macularis tested and had minimal cross-reactivity with other Podosphaera species when assayed with biologically relevant quantities of DNA. Standard curves generated independently in two other laboratories indicated that assay sensitivity was qualitatively similar and reproducible. All laboratories successfully detected eight unknown isolates of P. macularis and correctly discriminated Pseudoperonospora humuli and a water control. The usefulness of the assay for air sampling for late-season inoculum of P. macularis was demonstrated in field studies in 2019 and 2020. In both years, airborne populations of P. macularis in hop yards were detected consistently and increased during bloom and cone development.

Invasibility of a North American soil ecosystem to amphibian-killing fungal pathogens.

North American salamanders are threatened by intercontinental spread of chytridiomycosis, a deadly disease caused by the fungal pathogen Batrachochytrium salamandrivorans (Bsal). To predict potential dispersal of Bsal spores to salamander habitats, we evaluated the capacity of soil microbial communities to resist invasion. We determined the degree of habitat invasibility using soils from five locations throughout the Great Smoky Mountains National Park, a region with a high abundance of susceptible hosts. Our experimental design consisted of replicate soil microcosms exposed to different propagule pressures of the non-native pathogen, Bsal, and an introduced but endemic pathogen, B. dendrobatidis (Bd). To compare growth and competitive interactions, we used quantitative PCR, live/dead cell viability assays, and full-length 16S rRNA sequencing. We found that soil microcosms with intact bacterial communities inhibited both Bsal and Bd growth, but inhibitory capacity diminished with increased propagule pressure. Bsal showed greater persistence than Bd. Linear discriminant analysis (LDA) identified the family Burkolderiaceae as increasing in relative abundance with the decline of both pathogens. Although our findings provide evidence of environmental filtering in soils, such barriers weakened in response to pathogen type and propagule pressure, showing that habitats vary their invasibility based on properties of their local microbial communities.

Iron necessity for chlamydospore germination in Fusarium oxysporum f. sp. cubense TR4.

Fusarium wilt disease of banana, caused by the notorious soil-borne pathogen Fusarium oxysporum f. sp. cubense Tropical Race 4 (Foc TR4), is extremely difficult to manage. Manipulation of soil pH or application of synthetic iron chelators can suppress the disease through iron starvation, which inhibits the germination of pathogen propagules called chlamydospores. However, the effect of iron starvation on chlamydospore germination is largely unknown. In this study, scanning electron microscopy was used to assemble the developmental sequence of chlamydospore germination and to assess the effect of iron starvation and pH in vitro. Germination occurs in three distinct phenotypic transitions (swelling, polarized growth, outgrowth). Outgrowth, characterized by formation of a single protrusion (germ tube), occurred at 2 to 3h, and a maximum value of 69.3% to 76.7% outgrowth was observed at 8 to 10h after germination induction. Germination exhibited plasticity with pH as over 60% of the chlamydospores formed a germ tube between pH 3 and pH 11. Iron-starved chlamydospores exhibited polarized-growth arrest, characterized by the inability to form a germ tube. Gene expression analysis of rnr1 and rnr2, which encode the iron-dependent enzyme ribonucleotide reductase, showed that rnr2 was upregulated (p < 0.0001) in iron-starved chlamydospores compared to the control. Collectively, these findings suggest that iron andextracellular pH arecrucial for chlamydospore germination in Foc TR4. Moreover, inhibition of germination by iron starvation may be linked to a different mechanism, rather than repression of the function of ribonucleotide reductase, the enzyme that controls growth by regulation of DNA synthesis.

Overcoming Immune Deficiency with Allogrooming

Allogrooming appears to be essential in many social animals for protection from routine exposure to parasites. In social insects, it appears to be critical for the removal of pathogenic propagules from the cuticle before they can start an infectious cycle. For subterranean termites, this includes fungal spores commonly encountered in the soil, such as Metarhizium conidia, that can quickly germinate and penetrate the cuticle. We investigated whether there is a difference in reliance on social and innate immunity in two closely related subterranean termites for protection from fatal infections by two locally encountered Metarhizium species. Our results indicate that relatively weak innate immunity in one termite species is compensated by more sustained allogrooming. This includes enhanced allogrooming in response to concentrations of conidia that reflect more routine contamination of the cuticle as well as to heavy cuticular contamination that elicits a networked emergency response.

Omnivorous Notoxus trinotatus Pic (Coleoptera: Anthicidae) is a newly recognized vector of northern leaf blight in maize.

The adaptations of omnivorous insects to food are manifested in a multifaceted manner, and the availability of food resources directly determines insect feeding tendencies, which contribute to a complex insect-food relationship and impact insect functionality in the environment. Stable isotope analysis was applied to test the feeding preference and further define the functional role of omnivorous beetles in cropland. Our results confirmed that as an omnivorous beetle, the fungivorous nature of Notoxus trinotatus accounted for a prominent proportion food selection at the adult stage, and more importantly, this dietary feature contributed to the dispersal of the northern corn leaf blight in maize (NLB) during the feeding trials. In addition to the preference for fungi, water supplementation was an essential element extending adult longevity, which directly prolonged the contact time of adults with pathogenic fungi in agricultural fields. Consistent with the herbivorous characteristics of beetles, before the emergence of NLB fungal pathogens, corn tissues served as the main food, which provided the beetles with more opportunities to transmit fungal pathogen propagules. We conclude that the role of N. trinotatus in carrying NLB pathogen is due to its feeding on this plant mycopathogen, and an increased abundance of beetles carrying the pathogen may increase the rate of NLB disease infestation. More focus should be concentrated on the functions of fungivorous beetles, not only as pathogen-transmitting pests, but also as an element among the balanced biotic factors in farmland.

Insight into melatonin-mediated response and signaling in the regulation of plant defense under biotic stress.

Melatonin plays a crucial role in the mitigation of plant biotic stress through induced defense responses and pathogen attenuation. Utilizing the current knowledge of signaling and associated mechanism of this phytoprotectant will be invaluable in sustainable plant disease management. Biotic stress in plants involves complex regulatory networks of various sensory and signaling molecules. In this context, the polyfunctional, ubiquitous-signaling molecule melatonin has shown a regulatory role in biotic stress mitigation in plants. The present review conceptualized the current knowledge concerning the melatonin-mediated activation of the defense signaling network that leads to the resistant or tolerant phenotype of the infected plants. Fundamentals of signaling networks involved in melatonin-induced reactive oxygen species (ROS) or reactive nitrogen species (RNS) scavenging through enzymatic and non-enzymatic antioxidants have also been discussed. Increasing evidence has suggested that melatonin acts upstream of mitogen-activated proteinase kinases in activation of defense-related genes and heat shock proteins that provide immunity against pathogen attack. Besides, the direct application of melatonin on virulent fungi and bacteria showed disrupted spore morphology, destabilization of cell ultrastructure, reduced biofilm formation, and enhanced mortality that led to attenuate disease symptoms on melatonin-treated plants. The transcriptome analysis has revealed the down-regulation of pathogenicity genes, metabolism-related genes, and up-regulation of fungicide susceptibility genes in melatonin-treated pathogens. The activation of melatonin-mediated systemic acquired resistance (SAR) through cross-talk with salicylic acid (SA), jasmonic acid (JA) has been essential for viral disease management. The high endogenous melatonin concentration has also been correlated with the up-regulation of genes involved in pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). The present review highlights the versatile functions of melatonin towards direct inhibition of pathogen propagule along with active participation in mediating oxidative burst and simulating PTI, ETI and SAR responses. The hormonal cross-talk involving melatonin mediated biotic stress tolerance through defense signaling network suggests its suitability in a sustainable plant protection system.

Biological control of soil-borne phytopathogenic fungi through onion waste composting: implications for circular economy perspective

The production of onion waste derived mainly from bulbs affected by fungal diseases, during onion classification and storage presents an important agro-environmental issue in onion production regions. Composting is an environmentally friendly strategy to recycle agricultural waste and produce organic fertilizers. Modifications of the microbial community in soil can affect the ability of pathogen propagules to survive, germinate and infect plant roots. Hence, the main objective of this work was to exploring the mechanisms involved on the presence of three soil-borne phytopathogenic fungi during the composting process of onion waste under the hypothesis if that the resulting compost effectively prevents or minimizes the dispersion of phytopathogenic fungi. To this end, three composting piles of 60 tonnes each were built by layering onion waste affected by phytopathogenic fungi and cow dung at 1:1 ratio. Temperature, moisture, pH, electrical conductivity (EC) and Aspergillus niger, Penicillium sp. and Fusarium sp. growth were monitored for 100 days. During the first 28 days of composting, the presence of phytopathogenic fungi increased significantly showing thereafter a downward trend. Final estimations of fungal populations densities indicated a predominance of A. niger and an effective reduction in the abundance of Fusarium sp. This pilot-scale work demonstrates the feasibility of composting onion waste contaminated with phytopathogenic fungi and highlights the positive environmental impact associated with this practice. Therefore, the composting of onion waste and cow dung is a feasible and sustainable procedure to recycle onion waste and to promote circular economy in onion production regions.

STUDIES ON THE CONTROL OF SOME PLANT PATHOGENIC SOIL-BORNE FUNGI

Six Fusarium oxysporum f.sp. lycopersici (FOL) were obtained from tomato plants showed wilt disease symptoms and three isolates of Macrophomina phaseolina fungus were observed. Isolate (2) of FOL and isolate (3) of M. phaseolina were chosen for study, where they produce more propagules. However, five isolates of Trichoderma spp. were obtained from the rhizosphere of healthy tomato plants. The present study proved that using plant extracts, plant oils, biocontrol agents and fungicides; individually, reduced the growth and sporulation of the two pathogens; under laboratory conditions. The application of such treatments to the infested soil decreased the population of the pathogen propagules, significantly. Such individual treatments decreased the disease incidence with either pathogens and improved tomato growth parameters; significantly.

Effect of different soil amendments and bio agents on development of dry root rot (DRR) diseases of chickpea caused by Rhizoctonia bataticola (M.phaseolina)

Chickpea (Cicer arietinum L.) is one of the important pulse crops of India. Occurrence of dry root rot (DRR) caused by Rhizoctonia bataticola (Taub.) Butler is emerging as a serious biotic constraint in recent years for successful and profitable cultivation of chickpea. The efficacy of various soil amendments and bio agents viz wheat straw @ 20g/kg of soil, bajra straw @ 20g/kg, mung bean straw @ 20g/kg, mustard cake @ 5g/kg, ground nut cake @ 5g/kg and farm yard manure @ 20g/kg of soil as mixture with soil and Trichoderma viride@ 4gm/kg seed, T. harzianum@4gm.kg seed, T. virens0.5 gm/ke seed & Bacillus subtilis@ 6gm/kg as seed treatment respectively were evaluated against Rhizoctonia bataticola causing root rot in Chickpea. Study of DRRin chickpea done under both pot and field conditionduring years 2018-2019 and 2019-2020. Soil amendment with mustard cake amendment was found most effective in reducing the disease incidence (33.33 & 38.33%) during both years (2018-2019 & 2019-2020) of study followed by wheat straw, bajra straw and FYM. Among the bio agents Trichoderma viride (24.44 & 26.66%) was found most effective in minimizing DRR incidence and mean pathogen propagules g-1 soil (1.16 x 103) during both years of study.

Integration of Steam with Allyl-isothiocyanate for Soil Disinfestation

Steam has long been used to disinfest greenhouse soils. However, there is increasing interest in expanding the use of steam for in-field soil disinfestation as an alternative to chemical fumigants. Previous studies demonstrated that allyl-isothiocyanate (AITC) reduced viability of weed seeds and plant pathogen propagules, but AITC has a low vapor pressure and is relatively immobile in soil. Heat has been used in the past to enhance the mobility of soil fumigants such as methyl bromide (i.e., “hot gassing”). The effect of steam heat on the mobility of AITC is unknown. The objective of this study was to investigate the potential synergistic effect of steam plus ATIC against weed seeds and a plant pathogen. AITC alone did not reduce the viability of the four weed species and the number of Verticillium dahliae microsclerotia. The steam + AITC treatment reduced the viability of V. dahliae at 12.5 and 18 cm distances by 82% and 88%, respectively, and knotweed and nettle seeds at 70 cm from injection point by 75% and 86%, respectively, from the center of microplots compared with steam alone. The results suggest that AITC and steam have a complementary effect on soilborne pests because steam increases the mobility of AITC.

The effect of aggregation of pathogen and biocontrol microbe propagules on biocontrol potential: a simple modelling study

Effective use of biocontrol agents (BCAs) is a potentially important component of sustainable agriculture. The ecological processes determining the success of biocontrol are complex, which may partly explain the limited success of biocontrol against plant diseases in field crops. Understanding the ecological characteristics of BCAs in addition to biocontrol mechanisms and direct biocontrol efficacy, including their survival and dispersal under heterogeneous conditions, is critically important to improve biocontrol efficacy. In this simulation study, we focused on the effects of the spatial aggregation of initial pathogen and biocontrol propagules (inocula) under spatially homogeneous and heterogeneous conditions on biocontrol potential. The simulation showed that, as expected, increasing the biocontrol propagule density led to increased biocontrol potential. Under a given inoculum density, increasing spatial aggregation of BCAs is not only likely to reduce biocontrol potential but also to increase variabilities in biocontrol outcomes. A spatially random distribution of BCA propagules is most likely to result in best biocontrol outcomes. Spatial aggregation of BCAs is more important than spatial heterogeneity in influencing biocontrol potential. Thus, the present simulation study illustrates the importance of ensuring a close-to-homogeneous distribution of BCA propagules for maximising biocontrol potential. By the same reasoning, a faster host growth rate will reduce biocontrol potential if BCA cannot keep up with host growth in terms of coverage: increasing BCA aggregation essentially leads to decreased effective coverage.

Detection of Geosmithia morbida on Numerous Insect Species in Four Eastern States

Thousand cankers disease is caused by the coalescence of numerous Geosmithia morbida cankers on branches and stems of Juglans species, leading to branch dieback and eventual tree death. The fungus sporulates in galleries of the walnut twig beetle (Pityophthorus juglandis), allowing for acquisition of pathogen propagules and its subsequent transmission to other branches or trees following adult emergence. Recently, G. morbida has been isolated from Xylosandrus crassiusculus and Xyleborinus saxesenii collected in Ohio and Stenomimus pallidus collected in Indiana. These beetles are known to colonize diseased Juglans nigra in these states. In this study, an operational trap survey for ambrosia beetles, bark beetles, and other weevils was conducted in four eastern states, and captured beetles were assayed to detect G. morbida using both culture and PCR-based methods. A new primer pair (GmF3/GmR13), based on the β-tubulin region, was designed for G. morbida DNA detection. The pathogen was detected on 18 insect species using molecular methods, and live cultures were isolated from two species. This is the first report of the pathogen in Illinois and Minnesota.

Nonviable biomass of biocontrol agent Papiliotrema flavescens OH 182.9 3C enhances growth of Fusarium graminearum and counteracts viable biomass reduction of Fusarium head blight

Microbially-based plant disease control products have achieved commercial market success, but the efficacy of biocontrol products is sometimes deemed inconsistent. Declines in product viability before or after application results in a mixture of non-viable and viable antagonist cells being present on infection courts. We hypothesized that these non-viable cells could provide a nutritive benefit to plant pathogen propagules and thereby reduce the efficacy and consistency of performance of a biocontrol product. We tested this hypothesis on the pathogen Fusarium graminearum, an important causal agent of Fusarium head blight (FHB) of wheat, and a yeast antagonist Papiliotrema flavescens (formerly Cryptococcus flavescens) strain OH 182.9 3C. When F. graminearum was grown in a liquid basal salts medium without carbon or nitrogen but supplemented with increasing levels of non-viable cells of strain OH 182.9 3C, the amount of pathogen biomass produced increased (P ≤ 0.05) with each sequentially higher level of non-viable cell amendment tested. Total conidial germination of F. graminearum on membranes was initially greatest in the control and least when conidia were combined with live cells of antagonist OH 182.9 3C. Though the presence of dead cells initially resulted in reduced conidial germination, the treatment increased bipolar germination of conidia after 6 h compared to the control (89% and 45%, respectively) (P ≤ 0.05, FPLSD). Treatment with a combination of dead and live cells supported more bipolar germination than live cells alone (51% and 29%, respectively). After 12 h, dead cells had a similar stimulating effect on the branching of germ tubes and negated the reduction in branching seen when treating with live cells alone. Scanning electron microscopy indicated that the presence of non-viable cells of P. flavescens OH 182.9 3C modified the ectotrophic growth of F. graminearum on wheat lemma tissues. In greenhouse experiments, live cells of P. flavescens tended to reduce FHB incidence, severity and the deoxynivalenol content of spring wheat grain compared to the control but treatment with live cells amended with dead cells increased all of these disease parameters compared to the live cells only treatment (P ≤ 0.05, FPLSD). Results indicate that non-viable antagonist cells present in a biocontrol cell treatment can reduce disease control efficacy or even cause the biocontrol treatment to support a net adverse effect on plant health.

Tree pathogens and their insect-mediated transport: Implications for oak tree die-off in a natural park area

Successful infection by a tree pathogen depends both on species-specific traits, related to its infection biology, and on external factors, i.e. the biotic and abiotic components of the ecosystem. Among species-specific traits, virulence, biomass (propagule) production and dispersal ability all contribute to pathogen establishment and spread. Among the external factors, two crucial roles are played by an environment conducive to disease and by vector occurrence. We investigated the endophytic fungi and xylophagous beetles associated with declining oaks in a natural stand of central Italy, particularly focusing on the fungi isolated from insects’ body. This paper extends and reinterprets the outcome of a previous study, which looked into the whole mycota (pathogenic/neutral/beneficial) found on these oaks. A rich assemblage of fungi was found on declining trees on which a number of wood-boring beetles (Buprestidae and Cerambycidae) also occurred. In the laboratory two-thirds of the insects proved to convey propagules of prominent fungal pathogens (Botryosphaeriaceae, Pestalotiopsidaceae, Plectosphaerellaceae, Pleosporaceae); this confirms pathogen pervasiveness in oak stands is enhanced by their association with xylophagous insects. The outcome of this research will benefit forest manager and other stakeholder efforts to manage stands and species invasions, also in the light of climate change.

Thermal sensitivity of Calonectria henricotiae and Calonectria pseudonaviculata conidia and microsclerotia

ABSTRACTKnowledge of the thermal sensitivity of conidia and microsclerotia is useful for developing plant disease management approaches that deploy heat to inactivate infectious vegetative propagules of fungal pathogens. For boxwood blight disease, heat treatment of cuttings that harbor conidia and microsclerotia would provide a useful management tool for suppressing the pathogenic activity of Calonectria pseudonaviculata (present in the United States) and C. henricotiae (a quarantine pathogen not present in the United States). In this study, we investigated the thermal sensitivity of conidia and microsclerotia of the boxwood blight pathogens C. henricotiae and C. pseudonaviculata treated in water at 45, 47.5, 50, 52.5, and 55 C. For conidia, as time of exposure increased at each temperature, the proportion of germinated conidia decreased. The predicted time required to inactivate 90% of C. pseudonaviculata conidia (LD90) decreased as water temperature increased from 45 to 55 C and ranged from 35.4 to 5.6 min, respectively. Inactivation of conidia was dependent on isolate, species of Calonectria, and length of exposure at each temperature tested. Microsclerotia of C. henricotiae and C. pseudonaviculata displayed reduced germination with increasing exposure and higher temperatures of hot water. Microsclerotia of C. henricotiae were significantly more resistant to heat treatment than C. pseudonaviculata at 47.5 and 50 C, whereas microsclerotia of both species were rapidly killed at 55 C.

Metarhizium brunneum and Beauveria bassiana release secondary metabolites with antagonistic activity against Verticillium dahliae and Phytophthora megasperma olive pathogens

The antifungal activity of the entomopathogenic fungi isolates Metarhizium brunneum EAMb 09/01-Su and Beauveria bassiana EABb 09/16-Su against the olive pathogens Verticillium dahliae and Phytophthora megasperma was evaluated. The crude extract and its partial purified fractions obtained from both isolates showed complete inhibition of the mycelial growth of V. dahliae. However, the same fractions obtained from B. bassiana EABb 09/16-Su were less effective against the mycelial growth of P. megasperma. Our results showed that M. brunneum EAMb 09/01-Su had the highest fungicidal activity, inhibiting the mycelial growth of both pathogens. Both fractions IV and VI from M. brunneum EAMb 09/01-Su were evaluated by the progress of the Verticillium wilt of the plant and showed clear antagonism against V. dahliae, reducing the severity of the symptoms between 38.9 and 58.9%. The active metabolite obtained from M. brunneum not only reduced the germination of V. dahliae microsclerotia but also inhibited hyphal formation, decreased the number of reproductive structures, and changed the conformation of the microsclerotial colony, preventing the formation of new microsclerotia. The evaluation of temperature, UV light and pH on the antifungal activity of the dialyzed fraction from M. brunneum showed that the antifungal activity was stable at 60 °C (2 h) and 120 °C (20 min) and to UV light radiation. However, the exposition to the dialyzed fraction at pH 7.5 and 8.5 affected the antifungal activity. The results of this study clearly showed that the entomopathogenic fungus M. brunneum EAMb 09/01-Su produces antifungal compounds that reduce the number of the pathogen propagules in the soil and the severity of Verticillium wilt. Therefore, this resource should be further studied as an alternative for the integrated management of Verticillium wilt and Phytophthora root rot, which is the most severe disease in all traditional olive-growing regions worldwide, and as a reliable and sustainable strategy for a reduced reliance on pesticides.

Ecology of invasive forest pathogens

Invasive forest pathogens are a major threat to forests worldwide, causing increasing damage. The knowledge of both the specific traits underlying the capacity of a pathogen to become invasive, and the attributes predisposing an environment to invasion are to be thoroughly understood in order to deal with forest invasions. This paper summarizes the historical knowledge on this subject. Many aspects of the ecological processes underlying alien forest pathogens invasions are still unknown, which raises several scientific issues that need further study. The introduction of invasive forest pathogens to areas where naive hosts are found, is mainly due to global plant trade. Rapid transportation and reduced delivery times increase the chances of survival of pathogen propagules and of their successful establishment in new environments. In forest pathogens, the reproduction mode seems not to be a crucial determinant of invasiveness, as highly destructive pathogens have a variety of reproductive strategies. The most important drivers of forest pathogen invasions appear to be (a) great adaptability to new environmental conditions; (b) efficient dispersal over long and short distances, possibly assisted by the capacity to form novel associations with endemic and/or alien insect vectors; (c) the ability to exchange genetic material or hybridize with resident or alien species. Moreover, these features interact with some key traits of the invaded environment, e.g. environmental variability and biodiversity richness. Host resistance and natural enemies may occur as a result of rapid selection/adaptation after the epidemic phase of invasion.

Fusarium oxysporum and the Fusarium Wilt Syndrome.

The Fusarium oxysporum species complex (FOSC) comprises a multitude of strains that cause vascular wilt diseases of economically important crops throughout the world. Although sexual reproduction is unknown in the FOSC, horizontal gene transfer may contribute to the observed diversity in pathogenic strains. Development of disease in a susceptible crop requires F. oxysporum to advance through a series of transitions, beginning with spore germination and culminating with establishment of a systemic infection. In principle, each transition presents an opportunity to influence the risk of disease. This includes modifications of the microbial community in soil, which can affect the ability of pathogen propagules to survive, germinate, and infect plant roots. In addition, many host attributes, including the composition of root exudates, the structure of the root cortex, and the capacity to recognize and respond quickly to invasive growth of a pathogen, can impede development of F. oxysporum.

Estimation of viable propagules of black-foot disease pathogens in grapevine cultivated soils and their relation to production systems and soil properties

The study aimed to assess comparatively the accuracy and efficiency of three culture media protocols for estimating black-foot disease pathogens populations in soils and to examine how shifts in the abundance and composition of black-foot pathogens correspond to changes in specific soil properties. Firstly, culture media were compared by evaluating the mycelial growth of selected black-foot pathogens and by estimating the population of Dactylonectria torresensis from artificially infested soils. Secondly, the most efficient culture medium was selected for estimating the viable propagules of black-foot disease pathogens in eight naturally infested soils. An analysis of the soil physicochemical properties was conducted. Data were statistically analyzed in order to explore possible relationships between the studied variables. Glucose-Faba Bean Rose Bengal Agar (GFBRBA) was selected as the most efficient culture medium. All naturally infested soils tested positive for the presence of black-foot pathogens. D. torresensis was the most frequently isolated species, followed by Dactylonectria alcacerensis and Ilyonectria liriodendri. A positive relationship between calcium carbonate and the Colony-Forming Units (CFUs) level of black-foot pathogens in soil was obtained. In this study, we provide an early, specific, and accurate detection of viable propagules of black-foot pathogens in soil, which is critical to understand the ecology of these fungi and to design effective management strategies.

Effect of Plant Age and Leaf Position on Susceptibility to Wheat Stripe Rust.

In addition to pathogen propagule dispersal, disease spread requires successful infection of host tissue. In plant disease epidemiology, susceptibility of host tissue is often assumed to be constant. This assumption ignores changes in host phenology due to developmental stage. To examine this assumption, 3-, 4-, and 5-week-old wheat plants were inoculated with equal quantities of urediniospores of Puccinia striiformis f. sp. tritici, the causal agent of wheat stripe rust (WSR). Disease severity on each leaf was assessed and fit by mixed-effect linear model as a function of leaf position and plant age. Younger plants had significantly greater disease severity than older plants, with mean severities of 50.4, 30.1, and 12.9% on plants that were 3, 4, and 5 weeks old, respectively, at time of inoculation. This effect was greater on leaves higher on the plant. Within same-aged plants, younger leaves had significantly greater disease severity than older leaves, with mean severities of 40.2, 34.8, and 17.7% on the uppermost, second, and third leaf, respectively. These results suggest that the vertical distribution of WSR lesions in agricultural fields could be driven by differences in host susceptibility more so than propagule dispersal.