Evidence for seed transmission and symptomless growth of Ramularia collo‐cygni in barley (Hordeum vulgare)

Ramularia collo‐cygni (Rcc) is becoming an increasing problem for barley growers across Europe. However, the life cycle of the pathogen is only slowly being elucidated. In this study, Rcc DNA was detected in a number of harvested seed samples from 1999 to 2010, with mean levels peaking in winter barley samples in 2009. A number of experiments were carried out to determine whether the pathogen could move from barley seed to seedlings, and also from seed through the developing plant and into the subsequent generation of seed, both in controlled experiments and in field trials. Results from testing of seed indicated that the fungus is widespread at the end of the growing season in harvested grain samples and can be transmitted to developing plants from infected seed stock. Examination of infected seedlings did not reveal the presence of spores but fungal structures were found within the leaf. The location of the fungus within seed was examined, with Rcc DNA found in both embryo and non‐embryo tissue. The implications for barley production of the pathogen being seedborne are discussed.     

Genetic structure of a Pyrenophora teres f. teres population over time in an Australian barley field as revealed by Diversity Arrays Technology markers

Net form of net blotch caused by Pyrenophora teres f. teres (Ptt) is a major foliar disease of barley (Hordeum vulgare) worldwide. Knowledge of the evolution of Ptt pathogen populations is important for development of durable host-plant resistance. This study was conducted to investigate changes in genetic structure of a Ptt population within a barley field during three cropping years. The susceptible barley cultivar Henley was inoculated with Ptt isolate NB050. Leaf samples were collected during the years 2013–15 and 174 single spore Ptt isolates stored. Genotyping using Diversity Arrays Technology markers identified that 25% of isolates were clones of the inoculated isolate and 75% of isolates were multilocus genotypes (MLGs) differing from the original inoculated genotype. The novel genotypes probably originated from a combination of windborne spores from neighbouring fields, infected seed and sexual recombination in the field. The rapid change in the genotypic composition of the Ptt population in this study suggests adaptive potential of novel genotypes and demonstrates the need for barley breeders to use multiple sources of host-plant resistance to safeguard against resistance being overcome.     

Infection of Rrs1 barley by an incompatible race of the fungus Rhynchosporium secalis expressing the green fluorescent protein

Scald disease of barley, caused by the fungal pathogen Rhynchosporium secalis, is one of the most serious diseases of this crop worldwide. Disease control is achieved in part by deployment of major resistance (Rrs) genes in barley. However, in both susceptible and resistant barley plants, R. secalis is able to complete a symptomless infection cycle. To examine the R. secalis infection cycle, Agrobacterium tumefaciens‐mediated transformation was used to generate R. secalis isolates expressing the green fluorescent protein or DsRed fluorescent protein, and that were virulent on an Rrs2 plant (cv. Atlas), but avirulent on an Rrs1 plant (cv. Atlas 46). Confocal laser scanning microscopy revealed that R. secalis infected the susceptible cultivar and formed an extensive hyphal network that followed the anticlinal cell walls of epidermal cells. In the resistant cultivar, hyphal development was more restricted and random in direction of growth. In contrast to earlier models of R. secalis infection, epidermal collapse was not observed until approximately 10 days post‐inoculation in both cultivars. Sporulation of R. secalis was observed in both susceptible and resistant interactions. Observations made using the GFP‐expressing isolate were complemented and confirmed using a combination of the fluorescent probes 5‐chloromethylfluorescein diacetate and propidium iodide, in the non‐transformed wild‐type isolate. The findings will enable the different Rrs genes to be better characterized in the effect they exert on pathogen growth and may aid in identification of the most effective resistance.     

Studies on inoculum sources of angular leaf spot of beans caused by Phaeoisariopsis griseola in Uganda

Abstract

Angular leaf spot of beans caused by Phaeoisariopsis griseola is a major problem on this crop in Eastern Africa. The sources of inoculum for this disease were investigated. The causal fungus was confirmed as seedborne in all the cultivars tested. The fungus caused seed discolouration but not all infected seeds were discoloured. Seed to seedling transmission was low. The fungus survived in infected crop debris for a maximum of nine and four to six months under indoor and outside conditions respectively. Under soil, the fungus survived for only two months. Infected offseason crops and volunteer plants were present at the time of planting the seasons’ crops and were an obvious source of the inoculum. It is concluded that the seed, crop debris, off‐season crops and volunteer plants are all possible sources of P. griseola infection under the local conditions.     

Is the onset of septoria tritici blotch epidemics related to the local pool of ascospores?

To elucidate the early epidemic stages of septoria tritici blotch, especially the relationship between the onset of epidemics, the local availability of primary inoculum, and the presence of wheat debris, the early disease dynamics and airborne concentration in Zymoseptoria tritici ascospores were concomitantly assessed at a small spatiotemporal scale and over two years, using spore traps coupled with a qPCR assay. One plot, with the crop debris left, provided a local source of primary inoculum, while the other plot, without debris, lacked any. According to the assay's limits of detection, daily spore trap samples were classified as not detectable or not quantifiable, detectable, and quantifiable. The proportions of samples assigned to the different classes and numbers of spores in samples classified as quantifiable were significantly different between years, time periods (from November to March) and spore trap location (field with or without debris). The effect of year on the airborne ascospore concentration was high: 22 daily peaks with more than 230 ascospores m^?3 day^?1 were identified in the autumn of 2012/13, but none in the autumn of 2011/12. The local presence of wheat debris had no obvious effect on the amount of airborne ascospores or on the earliness of the two epidemics, especially in the year with high inoculum pressure (2012/13). These results suggest that the amount of primary airborne inoculum available in a wheat crop is not a limiting factor for the onset of an epidemic.     

The life cycle of <Emphasis Type="Italic">Stemphylium vesicarium</Emphasis>, the causal agent of Welsh onion leaf blight

Symptoms of Welsh onion leaf blight, caused by Stemphylium vesicarium, are divided into two types, i.e., brown oval lesions and yellow mottle lesions. Yellow mottle lesions exert considerable economic damage on Welsh onion in northern Japan. In this study, we investigated the life cycle of the pathogen in terms of seasonal fluctuation of spore dispersal and its relationship with development of disease, formation period of pseudothecia and overwintering of the pathogen based on field surveys, spore trapping and fungal isolation. Conidia were trapped throughout the cropping season except before mid June, when no ascospores were trapped. Brown oval lesions, which contained a large number of conidia, usually occurred in July followed by yellow mottle lesions with an increasing number of conidia trapped. These observations suggest that conidia released from brown oval lesions play an important role as a secondary inoculum source of the disease, leading to the development of yellow mottle lesions. Pseudothecia on leaves were first observed at the end of the cropping season or immediately after harvest (late October). The pathogen overwintered in the form of pseudothecia produced on leaves with or without symptoms. Ascospores failed to be trap in the field during the interval between before and beginning of the cropping season in April–May. However, pot experiments demonstrated that ascospores were released from leaf debris in November and rapidly increased in number after snow melt. From this circumstantial evidence, we hypothesize that ascospores are the primary inoculum source of Welsh onion leaf blight.     

Field studies of anthracnose symptoms and pathogen infection in different phases of the persimmon growing season

Anthracnose is the main disease of persimmon and is caused by Colletotrichum spp. The study of field epidemiology is essential for the development of efficient management of this disease. In this study, we investigated infection by Colletotrichum spp. throughout the persimmon growing season to understand the host–pathogen interactions better. We observed the production of primary inoculum of persimmon anthracnose and described how epidemics progress from secondary infections during the fruit crop season. The field study was carried out in an organic orchard with three susceptible persimmon cultivars, Fuyu, Kakimel and Jiro, for three consecutive seasons. Our results indicate that the pathogen survives in 1-year-old shoots, which are the sources of primary inoculum. Later that growing season, the inoculum reaches flowers and new shoots, developing symptoms and producing the secondary inoculum. Fruit drop was also observed, with or without symptoms of anthracnose, throughout the plant cycle. In some of the symptomless fruit, collected from the plant and from the ground where they had fallen, latent infections of Colletotrichum spp. were detected. Shoots, flowers, immature and ripened fruit remained infected throughout the growing season, producing conidia that could lead to new secondary infections within and among plants. The incidence of anthracnose in fruit at harvest and postharvest proved to be less relevant for disease management. Practices for chemical and cultural control of the disease throughout the persimmon growing season are discussed.     

Quantification of within‐season focal spread of wheat take‐all in relation to pathogen genotype and host spatial distribution

Within season gradient of wheat take‐all was measured in field experiments according to line or random sowing host spatial distribution, and two Gaeumannomyces graminis var. tritici (Ggt) isolates (G1i and G2i), representative of the G1 and G2 genotype groups in terms of aggressiveness. Root disease incidence and severity were assessed at six dates from early March to late June on plants located at regular distances from the inoculum sources. Simple models relating disease intensity at different levels of hierarchy fitted observed data well, and indicated a strong disease aggregation both within and among plants. Disease severity on source plants placed nearby the inoculum source increased over time, ranging from 5 to 46% at the first assessment, and from 55 to 98% at the last assessment, being in general larger for G2i than G1i. In line‐sown plots, disease progressed steadily along the line but did not extend beyond 20 cm, seldom reaching the neighbour line. Disease rarely reached 25 cm in the direct‐seeded crop stands. These results indicate that Ggt intensifies but does not spread to a large extent during a cropping season. Distance from the source, pathogen genotype and assessment date had a significant effect on disease severity according to mixed model analyses, disease spread being larger for G2i than G1i. However, no significant effect of host spatial distribution could be detected. Yield loss within 20 cm of the source plant ranged between 20 and 40%, and was not significantly affected by pathogen genotype or host spatial distribution.     

Are field populations of arbuscular mycorrhizal fungi able to suppress the transmission of seed-borne <Emphasis Type="Italic">Bipolaris sorokiniana</Emphasis> to aerial plant parts?

The development of seed-borne Bipolaris sorokiniana in barley in the presence of arbuscular mycorrhizal fungi was studied. To exploit natural variation in their ability to control disease development, arbuscular mycorrhizal fungi from various Swedish arable soils were multiplied in trap cultures using a mixture of plant species. Six out of eight trap culture soil inocula were able to reduce transmission of B. sorokiniana from seeds to stem bases when grown together with infected barley seed. Based on this result two soil inocula, of different origin, from semi-natural grassland and barley respectively, were chosen for further greenhouse studies. Both soil inocula gave significant reductions in pathogen transmission from seeds to seedlings compared to the untreated control. In addition, treatment with spore inocula, collected from the different trap culture soils, showed disease suppression. Treatment with spores from the pure culture Glomus intraradices gave significant reduction in leaf lesion development. A treatment with the commercial inoculum Vaminoc^® was included and gave some suppression of the pathogen. In conclusion this study has shown that AM soil inocula from trap cultures suppressed the transmission of seed-borne B. sorokiniana in the aerial parts of barley plants.     

Dynamics of cercospora leaf spot disease determined by aerial spore dispersal in artificially inoculated sugar beet fields

Cercospora beticola is one of the most important fungal pathogens of sugar beet, causing cercospora leaf spot (CLS) disease. Due to the decreasing efficacy of various fungicides caused by resistance traits, the development of a sustainable disease management strategy has become more important. Therefore, detailed knowledge about the epidemiology of the pathogen is crucial. Until now, little was known about the spatiotemporal dispersal of C. beticola spores from the primary inoculum source. Rapid detection of C. beticola spores could facilitate a more precise and targeted disease control. Therefore, a TaqMan real-time PCR assay for detection and quantification of C. beticola spores caught with Rotorod spore traps was established. In 2016 and 2017, field trials were conducted to monitor C. beticola aerial spore dispersal and disease development within an inoculated field and in the adjacent noninoculated area. With the established detection method, C. beticola spores were successfully quantified and used as a measure for aerial spore dispersal intensity. The analysis of the spatiotemporal spread of C. beticola spores revealed a delay and decrease of aerial spore dispersal with increasing distance from the inoculated area. Consequently, disease incidence and severity were reduced in a similar manner. These results imply that spore dispersal occurs mainly on a small scale within a field, although long distances can be overcome by C. beticola spores. Moreover, secondary aerial spore dispersal from sporulating leaf spots seems to be the main driver for CLS disease development. These results provide an important basis for further improvement of CLS control strategies.     

Plasmodiophora brassicae resting spore dynamics in clubroot resistant canola (Brassica napus) cropping systems

The soilborne pathogen Plasmodiophora brassicae, causal agent of clubroot of canola (Brassica napus), is difficult to manage due to the longevity of its resting spores, ability to produce large amounts of inoculum, and the lack of effective fungicides. The cropping of clubroot resistant (CR) canola cultivars is one of the few effective strategies for clubroot management. This study evaluated the impact of the cultivation of CR canola on P. brassicae resting spore concentrations in commercial cropping systems in Alberta, Canada. Soil was sampled pre-seeding and post-harvest at multiple georeferenced locations within 17 P. brassicae-infested fields over periods of up to 4 years in length. Resting spore concentrations were measured by quantitative PCR analysis, with a subset of samples also evaluated in greenhouse bioassays with a susceptible host. The cultivation of CR canola in soil with quantifiable levels of P. brassicae DNA resulted in increased inoculum loads. There was a notable lag in the release of inoculum after harvest, and quantifiable P. brassicae inoculum peaked in the year following cultivation of CR canola. Rotations that included a ≥2-year break from P. brassicae hosts resulted in significant declines in soil resting spore concentrations. A strong positive relationship was found between the bioassays and qPCR-based estimates of soil infestation. Results suggest that CR canola should not be used to reduce soil inoculum loads, and crop rotations in P. brassicae infested fields should include breaks of at least 2 years away from B. napus, otherwise the risk of selecting for virulent pathotypes may increase.     

Survival,distribution and genetic variability of inoculum of the strawberry red core pathogen,Phytophthora fragariae var. fragariae,in soil

An experimental field infested with Phytophthora fragariae var. fragariae (Pff) and used for strawberry red core fungicide and cultivar resistance trials until 1981 was surveyed for the presence of inoculum of the pathogen 11 and 12 years later. Alpine strawberries, highly susceptible to all races of Pff, were grown from true seed and planted as a bait crop on a 0·5 m‐spaced grid. Rapid and widespread red core infection was observed, which provided good evidence that oospores had survived in soil for this extended period. Site elevation and the distribution of red core infected plants showed a strong correlation, with a higher frequency of infected and dead plants in the lowest areas of the field. The race designation of 18 recovered isolates were determined and AFLP fingerprint patterns of some of these and their single‐spore derivatives were analysed. The isolates differed little in race type, and the majority were genetically identical at 433 AFLP loci. Races used to inoculate the site in the 1970s were recovered. The fingerprints of the single variant isolate matched that of an isolation made by Hickman in the 1950s, originally used to inoculate the site. Clearly Pff is a very stable and long‐lived pathogen able to retain its genetic integrity and lie dormant in soil for many years, ensuring its survival between epidemiologically favourable conditions which occur erratically.     

Effect of susceptible and resistant canola plants on Plasmodiophora brassicae resting spore populations in the soil

Clubroot, caused by Plasmodiophora brassicae, has become a serious threat to canola (Brassica napus) production in western Canada. Experiments were conducted to assess the effect of growing resistant and susceptible canola genotypes on P. brassicae soil resting spore populations under greenhouse, mini‐plot and field conditions. One crop of susceptible canola contributed 1·4 × 10⁸ spores mL^?1 soil in mini‐plot experiments, and 1 × 10¹⁰ spores g^?1 gall under field conditions. Repeated cropping of susceptible canola resulted in greater gall mass compared to resistant canola lines. It also resulted in reduced plant height, increased clubroot severity in susceptible canola, and increased numbers of resting spores in the soil mix.     

Real-time PCR quantification and spatio-temporal distribution of airborne inoculum of Mycosphaerella graminicola in Belgium

Two kinds of propagules play a role in Mycosphaerella graminicola dissemination: splash-dispersed pycnidiospores and airborne sexual ascospores. A method based on real-time polymerase chain reaction (PCR) assay and using Burkard spore traps was developed to quantify M. graminicola airborne inoculum. The method was tested for its reliability and applied in a spore trap network over a 2-year period in order to investigate the spatio-temporal distribution of airborne inoculum in Belgium. At four experimental sites, airborne inoculum was detected in both years. A seasonal distribution was observed, with the highest mean daily quantities (up to 351.0 cDNA) trapped in July and with clusters detected from September to April. The first year of trapping, a mean daily quantity of 15.7 cDNA of M. graminicola airborne inoculum was also detected in the air above a building in a city where the spatio-temporal distribution showed a similar pattern to that in the field. Mean daily quantities of up to 60.7 cDNA of airborne inoculum were measured during the cereal stem elongation and flowering stages, suggesting that it contributes to the infection of upper leaves later in the season. Most detection, however, tended to occur between flowering and harvest, suggesting significant production of pseudothecia during that period. Variations in mean daily quantities from 1.0 to 48.2 cDNA were observed between sites and between years in the patterns of airborne inoculum. After stem elongation, the quantities detected at a site were positively correlated with the disease pressure in the field. Quantities trapped at beginning of the growing season were also well correlated with the disease level the previous year. Multiple regressions revealed that some factors partly explain the daily variations of airborne inoculum.     

Incidence of Verticillium wilt of artichoke in eastern Spain and role of inoculum sources on crop infection

Surveys of 94 artichoke fields throughout the artichoke production areas of Comunidad Valenciana (eastern Spain) were conducted from 1999 to 2002 to determine the incidence and distribution of Verticillium wilt.Verticillium dahliae was isolated from 80.9% of the sampled fields, and detected in all artichoke-growing areas, with a mean disease incidence of 53.8% infected plants. The disease was found to cause severe damage to cv. ‘Blanca de Tudela’, which is the most important artichoke cultivar grown in Spain, and was also observed on the seed-propagated cv. ‘Imperial Star’. In field trials to study the role of infected planting material and soil inoculum on infection of artichoke plants during the cropping season,V. dahliae was transmitted from infected stumps to the plants, confirming that the use of infected stumps could have greatly contributed to the dissemination of the pathogen. Inoculum density ofV. dahliae in soil had an effect on crop infection, in that a higher number of microsclerotia per gram of soil resulted in a higher percentage of infected plants. In addition, yield of cv. Blanca de Tudela was significantly affected byV. dahliae infection, showing that a higher percentage of infection corresponded with lower yield. http://www.phytoparasitica.org posting July 21, 2005.     

How can research on pathogen population biology suggest disease management strategies? The example of barley scald (Rhynchosporium commune)

Barley scald fungus, Rhynchosporium commune, belongs to a host‐specialized species complex infecting grasses. Coalescent analyses of several genes indicate that R. commune originated c. 2500 years ago, after the domestication of barley. Phylogeographical analyses identified a diversity hotspot in Scandinavia, indicating an origin in northern Europe. After its emergence, R. commune became distributed globally on infected seeds, eventually invading the Fertile Crescent and infecting wild barley progenitors. Analyses of gene flow identified historical routes of migration out of Scandinavia and indicate a high degree of modern gene flow among South Africa, Australia and New Zealand. About 76% of global species diversity is found within barley fields, with all field populations showing a genetic structure consistent with sexual recombination. High levels of regional gene flow suggest wind‐dispersed ascospores and movement of infected seed. Quantitative traits, including pathogen aggressiveness, thermal sensitivity and fungicide resistance, showed high heritability and high levels of diversity within nine globally representative populations. Field experiments provided evidence for a fitness cost associated with complex virulence and a trade‐off between pathogenic and saprophytic fitness. All findings indicate that global R. commune populations have significant potential to evolve rapidly in response to environmental changes, including deployment of resistant cultivars, fungicide applications and global warming. Specific recommendations to improve management of barley scald include: (i) focus resistance breeding efforts in northern Europe, which offers the best location to screen germplasm and may provide useful sources of scald resistance; (ii) improve seed treatment, certification and quarantine programmes to limit long distance pathogen movement; (iii) manage barley stubble to decrease pathogen population size, limit production of sexual inoculum and reduce the pathogen's evolutionary potential.     

Effect of host and non‐host crops on Plasmodiophora brassicae resting spore concentrations and clubroot of canola

Plasmodiophora brassicae, causal agent of clubroot of crucifers, poses a serious threat to Canadian canola production. The effects of fallow (F) periods and bait crops (clubroot‐susceptible canola (B) and perennial ryegrass (R)) on clubroot severity and P. brassicae resting spore populations were evaluated in five sequences: R–B, B–R, R–F, B–F and F–F. Both host and non‐host bait crops reduced clubroot severity in a subsequent crop of a susceptible canola cultivar compared with fallow. Resting spore and P. brassicae DNA concentrations decreased in all treatments, but were lowest for the R–B and B–R bait crop sequences. In addition, two studies were conducted in mini‐plots under field conditions to assess the effect of rotation of susceptible or resistant canola cultivars on clubroot severity and P. brassicae resting spore populations. One study included three crops of susceptible canola compared with a 2‐year break of oat–pea, barley–pea, wheat–wheat or fallow–fallow. The other study assessed three crops of resistant canola, two crops of resistant canola with a 1‐year break, one crop of resistant canola and a 2‐year break, and a 3‐year break with barley followed by a susceptible canola. The rotations that included non‐host crops of barley, pea or oat reduced clubroot severity and resting spore concentrations, and increased yield, compared with continuous cropping of either resistant or susceptible canola. Growing of a susceptible canola cultivar contributed 23–250‐fold greater gall mass compared with resistant cultivars.     

Direct and host‐mediated interactions between Fusarium pathogens and herbivorous arthropods in cereals

Fusarium head blight and fusarium ear rot diseases of cereal crops are significant global problems, causing yield and grain quality losses and accumulation of harmful mycotoxins. Safety limits have been set by the European Commission for several Fusarium‐produced mycotoxins; mitigating the risk of breaching these limits is of great importance to crop producers as part of an integrated approach to disease management. This review examines current knowledge regarding the role of arthropods in disease epidemiology. In the field, diseased host plants are likely to interact with arthropods that may substantially impact the disease by influencing spread or condition of the shared host. For example, disease progress by Fusarium graminearum can be doubled if wheat plants are aphid‐infested. Arthropods have been implicated in disease epidemiology in several cases and the evidence ranges from observed correlations between arthropod infestation and increased disease severity and mycotoxin accumulation, to experimental evidence for arthropod infestation causing heightened pathogen prevalence in hosts. Fusarium pathogens differ in spore production and impact on host volatile chemistry, which influences their suitability for arthropod dispersal. Herbivores may allow secondary fungal infection after wounding a plant or they may alter host susceptibility by inducing changes in plant defence pathways. Post‐harvest, during storage, arthropods may also interact with Fusarium pathogens, with instances of fungivory and altered behaviour by arthropods towards volatile chemicals from infected grain. Host‐mediated indirect pathogen–arthropod interactions are discussed alongside a comprehensive review of evidence for direct interactions where arthropods act as vectors for inoculum.     

Interactions between the root pathogen Rhizoctonia solani AG-8 and acetolactate-synthase-inhibiting herbicides in barley

BACKGROUND: The widespread acceptance of reduced‐tillage farming in cereal cropping systems in the Pacific Northwest of the United States has resulted in increased use of herbicides for weed control. However, soil residual concentrations of widely used imidazalone herbicides limit the cultivation of barley, which is more sensitive than wheat. In addition, increased severity of the root rot disease caused by Rhizoctonia solani is associated with reduction in tillage. Many crops exhibit altered disease responses after application of registered herbicides. In this study, the injury symptoms in barley caused by sublethal rates of two acetolactate synthase (ALS)‐inhibiting herbicides, imazamox and propoxycarbazone‐sodium, were assessed in factorial combinations with a range of inoculum concentrations of the root rot pathogen Rhizoctonia solani AG‐8. RESULTS: Both herbicides and pathogen had negative impacts on plant growth parameters such as root and shoot dry weight, shoot height and first leaf length, and interactions between pathogen and herbicide were detected. CONCLUSIONS: The results suggested that sublethal rates of herbicides and R. solani could alter severity of injury symptoms, possibly owing to the herbicide predisposing the plant to the pathogen. Copyright © 2012 Society of Chemical Industry     

The Importance of Wind Gusts in Distributing Fungal Spores Among Crop Foliage1

New measurements show that in a barley crop wind gusts of more than five times the mean speed occur frequently. Moreover, for the impaction of fungal spores to crop foliage, the known dependence of impaction efficiency on wind speed shows that at low speeds the efficiency increases rapidly as wind speed rises. If it is wind gusts that cause spores to be lifted from plant surfaces and to enter the airstream, then calculations show that in a gusting wind the efficiency of impaction to surfaces will be higher than would be predicted from the mean wind speed. These findings are supported by results from field experiments that investigated the lift-off from plant surfaces and from metal plates, respectively, of spores of Erysiphe graminis (barley mildew) and of lycopodium (club moss). This influence of gusts may thus explain in part why, within crops, spore concentration and deposition change rapidly with distance from spore source.