Pathogenicity of Rhynchosporium secalis isolates from Norway on 30 cultivars of barley

The pathogenic variability of the barley scald fungus, Rhynchosporium secalis , in central Norway was examined in 1994. The climate in this region is usually cold and wet during the growing season of spring barley. Leaf blotch is prevalent and causes significant yield losses. Forty-two isolates of the fungus, from naturally infected spring barley in four counties, were differentiated into 32 pathotypes by the standard differential set for R. secalis . All pathotypes were complex and had virulence for nine to 22 differentials. The cultivar Osiris was resistant to all isolates tested. The cultivars C.I.8162, Hudson, Atlas 46 and C.I.3515 were resistant to the majority of the isolates. Several differentials with various resistance genes were susceptible to up to 100% of the isolates. Isolates were derived from local cultivars with no known resistance genes, suggesting that R. secalis populations in central Norway are characterized by a high degree of seemingly unnecessary pathogenicity. Because of the great variability and complexity of the pathotypes, traditional breeding methods using single major genes are not likely to be effective in central Norway.     

Effect of barley yellow dwarf virus on susceptibility of barley cultivars to net blotch (Pyrenophora teres) and leaf blotch (Rhynchosporium secalis)

The effect of barley yellow dwarf virus (BYDV) on the development of net blotch (Pyrenophora eves) and leaf blotch (Rhynchosporium secalis) was examined on seven barley cuitivars. Seedlings were infected with BYDV at the two-leaf stage (G.S. 12). Their susceptibility to three isolates of p teres and isolates of two races (U.K. 1, U.K. 2) of R. secalis was examined at the four-leaf stsge (G.S. 14) and when plants were more mature (G.S. 33/38). At G.S. 14 numbers of lesions produced by P. teres and R. secalis were reduced, on average, by 37 and 72% respectively, and at G.S. 33/38 by 61 and 74%.     

Relationship between leaf emergence and optimum spray timing for leaf blotch (Rhynchosporium secalis) control on winter barley

For wheat, the optimum time to apply fungicide to control disease on a given leaf layer is usually at, or shortly after, full leaf emergence. Data from field experiments on barley were used to investigate whether the same relationship was applicable to control of leaf blotch on barley. Replicated plots of winter barley were sown in the autumns of 1991, 1992 and 1993 at sites in southwest England with high risk of Rhynchosporium secalis infection. Single fungicide treatments at four doses (0·25, 0·5, 0·75 or 1·0 times the label rate) were applied at one of eight different spray times, starting in mid-March in each year, with intervals of 10–11 days between spray timings. Disease was assessed every 10–11 days and area under the disease progress curve (AUDPC) values were used to construct fungicide dose by spray time response surfaces for each of the upper four leaves, for each year. Spray timings shortly before leaf emergence were found to minimize the AUDPC for each year and leaf layer, and also the effective dose (the dose required to achieve a specified level of control), similar to wheat. Fungicide treatments on barley were effective for a longer period before leaf emergence than afterwards, probably because treatments before emergence of the target leaf reduced inoculum production on leaves below. This partly explains why fungicides tend to be applied earlier in the growth of barley compared with wheat.     

Resistance, epidemiology and sustainable management of Rhynchosporium secalis populations on barley

Rhynchosporium secalis is one of the most destructive pathogens of barley worldwide, causing yield decreases of up to 40% and reduced grain quality. Rhynchosporium is a polycyclic disease. Primary inoculum includes conidia produced on crop debris, infected seeds and possibly ascospores, although these have not yet been identified. Secondary disease spread is primarily by splash dispersal of conidia produced on infected leaves, which may be symptomless early in the growing season. Host resistance to R. secalis is mediated by both 'major' or host-specific genes (complete resistance) and 'minor' genes of smaller, generally additive effects (partial resistance). Crop growth stage and plant or canopy architecture can modify the expression of resistance. Resistance genes are distributed unevenly across the barley genome, with most being clustered on the short arms of chromosomes 1H, 3H, 6H and 7H, or in the centromeric region or on the long arm of chromosome 3H. Strategies used to manage rhynchosporium epidemics include cultivar resistance and fungicides, and also cultural practices such as crop rotation, cultivar mixtures and manipulation of sowing date, sowing rate or fertiliser rate. However, the high genetic variability of R. secalis can result in rapid adaptation of pathogen populations to render some of these control strategies ineffective when they are used alone. Sustainable control of rhynchosporium needs to integrate major-gene-mediated resistance, partial resistance and other strategies such as customized fungicide programmes, species or cultivar rotation, resistance gene deployment, clean seed and cultivar mixtures.     

Quantification of seedborne infection by Rhynchosporium secalis in barley using competitive PCR

The potential of the competitive polymerase chain reaction (PCR) assay for quantification of seedborne infection by Rhynchosporium secalis in barley was examined using a primer set (RS1 and RS3) derived from the internal transcribed spacer (ITS) regions of ribosomal RNA genes of this pathogen. Introduction of a heterologous internal control, which competes for the same primer set in the conventional PCR assay, allowed for detection and quantification of R. secalis fungal biomass. In order to generate a standard calibration curve, DNA prepared from infected seeds with different levels of R. secalis infection was subjected to competitive PCR assay. The resulting PCR product ratio for each PCR reaction ( R. secalis -amplified DNA/internal control template-amplified DNA) increased proportionally with increasing levels of infected seed DNA in the reaction mixture. Naturally infected seed lots collected from 1995 to 1999 were used to demonstrate the potential of the competitive PCR assay as an alternative seed health testing method. The results from this competitive PCR assay were compared with those from conventional visual disease assessment and an agar plate assay. Although relatively good correlation between visual disease assessment and the competitive PCR was found in the case of artificially mixed seed samples, there was poor correlation in the experiments using naturally infected seed samples.     

Virulence spectrum to barley (Hordeum vulgare L.) in isolates of Rhynchosporium secalis from Syria

Journal of Plant Diseases and Protection - Scald caused by Rhynchosporium secalis, is an economically important disease found worldwide. Information on the variability of virulence in R. secalis is...     

The role of seeds and airborne inoculum in the initiation of leaf blotch (Rhynchosporium secalis) epidemics in winter barley

Both airborne spores of Rhynchosporium secalis and seed infection have been implied as major sources of primary inoculum for barley leaf blotch (scald) epidemics in fields without previous history of barley cropping. However, little is known about their relative importance in the onset of disease. Results from both quantitative real‐time PCR and visual assessments indicated that seed infection was the main source of inoculum in the field trial conducted in this study. Glasshouse studies established that the pathogen can be transmitted from infected seeds into roots, shoots and leaves without causing symptoms. Plants in the field trial remained symptomless for approximately four months before symptoms were observed in the crop. Covering the crop during part of the growing season was shown to prevent pathogen growth, despite the use of infected seed, indicating that changes in the physiological condition of the plant and/or environmental conditions may trigger disease development. However, once the disease appeared in the field it quickly became uniform throughout the cropping area. Only small amounts of R. secalis DNA were measured in 24 h spore‐trap tape samples using PCR. Inoculum levels equivalent to spore concentrations between 30 and 60 spores per m³ of air were only detected on three occasions during the growing season. The temporal pattern and level of detection of R. secalis DNA in spore tape samples indicated that airborne inoculum was limited and most likely represented rain‐splashed conidia rather than putative ascospores.     

Pyrenophora teres growth and severity of net blotch on barley under salt stress

European Journal of Plant Pathology - Pyrenophora teres causes one of the most destructive barley diseases in semi-arid production areas worldwide. With the burgeoning issue of soil salinization,...     

The effect of component number on Rhynchosporium secalis infection and yield in mixtures of winter barley cultivars

Mixtures of winter barley cultivars containing up to six components were grown over three years with and without fungicide treatment. Yield increases were recorded for mixtures compared with the mean of their monoculture components and there was a significant trend towards greater benefit from increased number of components. These benefits were partially attributable to a corresponding increase in control of Rhynchosporium secalis as component number increased. The potential for exploitation of mixtures in cereals for control of splash-dispersed pathogens is discussed.     

Effects of Rhynchosporium secalis at specific growth stages on barley grown in a controlled environment

Under controlled environmental conditions, inoculation of barley at five growth stages with Rhynchosporium secalis reduced grain yield by up to 30%. Greater yield losses resulted from inoculations carried out after tillering than from those during or before tillering. Most of the yield loss was due to a reduction in the number of heads per plant. The number of grains per head was reduced only when plants were inoculated at mid- or late-tillering growth stages. The mean weight of individual grains was not reduced significantly by any treatment. Inoculation at stem elongation also resulted in less leaf area and water use, and delayed anthesis. Inoculation affected root dry weight more than shoot dry weight and all inoculations reduced root dry weight at the boot growth stage     

The occurrence of carbendazim resistance in Rhynchosporium secalis on winter barley in England and Wales in 1992 and 1993

In 1992, samples of Rhynchosporium secalis from 19 winter barley crops in England and Wales were examined for carbendazim resistance. Of the 120 isolates obtained, 14·2% were resistant to carbendazim. A larger survey was carried out in England and Wales in 1993 when samples from 74 crops were examined. On this occasion 16·6% of the 639 isolates obtained were resistant to carbendazim, and resistant isolates were detected in 46% of crops.     

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.     

Colonization of tomato seedlings by bioluminescent Clavibacter michiganensis subsp. michiganensis under different humidity regimes

Tomato bacterial canker, caused by Clavibacter michiganensis subsp. michiganensis, is transmitted by infected or infested seed and mechanically from plant to plant. Wounds occurring during seedling production and crop maintenance facilitate the dissemination of the pathogen. However, the effects of environmental factors on C. michiganensis subsp. michiganensis translocation and growth as an endophyte have not been fully elucidated. A virulent, stable, constitutively bioluminescent C. michiganensis subsp. michiganensis strain BL-Cmm 17 coupled with an in vivo imaging system allowed visualization of the C. michiganensis subsp. michiganensis colonization process in tomato seedlings in real time. The dynamics of bacterial infection in seedlings through wounds were compared under low (45%) and high (83%) relative humidity. Bacteria multiplied rapidly in cotyledon petioles remaining after clip inoculation and moved in the stem toward both root and shoot. Luminescent signals were also observed in tomato seedling roots over time, and root development was reduced in inoculated plants maintained under both humidity regimes. Wilting was more severe in seedlings under high-humidity regimes. A strong positive correlation between light intensity and bacterial population in planta suggests that bioluminescent C. michiganensis subsp. michiganensis strains will be useful in evaluating the efficacy of bactericides and host resistance.     

Influence of barley variety mixtures on severity of scald and net blotch and on yield *

Two susceptible barley cultivars and two moderately resistant advanced breeding lines were grown as pure stands and as the 11 possible equi-proportional mixtures over three field seasons, in the presence of scald and net blotch. Plots were either inoculated with infested straw, or non-inoculated and sprayed with fungicide. On average, the variety mixtures restricted leaf disease development by 12%. However, mixtures of either or both susceptible cultivar/s with the moderately resistant line 1861018 restricted disease severity by 20-32%. Generally, the mixtures did not increase yield relative to the mean of the pure stands. However, in the year of highest disease severity, yield was increased by 7% owing to mixing in the inoculated plots. In addition, one mixture consistently increased yield (mean of 4%) and one mixture had consistently lower yields (mean of 6%)) over years and inoculation treatments. Thus, careful evaluation is required in this environment and with these genotypes in order to identify mixtures with positive effects on disease control and yield.     

Potato brown rot incidence and severity under different management and amendment regimes in different soil types

Ralstonia solanacearum race 3 biovar 2, the causative agent of potato brown rot (bacterial wilt), is an economically important disease in tropical, subtropical and temperate regions of the world. In view of previous reports on suppression of the disease by organic amendments, and the expansion of organic agriculture, it was timely to compare the effects of organic and conventional management and various amendments on brown rot development in different soils (type: sand or clay; origin: Egypt or the Netherlands). Brown rot infection was only slightly reduced in organically compared to conventionally managed sandy soils from Egypt, but organic management significantly increased disease incidence and pathogen survival in Dutch sandy and clay soils, which correlated with high DOC contents in the organic Dutch soils. There was no correlation between disease incidence or severity and bacterial diversity in the potato rhizosphere in differently managed soils (as determined by 16S DGGE). NPK fertilization reduced bacterial wilt in conventional Egyptian soils but not in Dutch soils. Cow manure amendment significantly reduced disease incidence in organic Dutch sandy soils, but did not affect the bacterial population. However, cow manure did reduce densities of R. solanacearum in Egyptian sandy soils, most probably by microbial competition as a clear shift in populations was detected with DGGE in these and Dutch sandy soils after manure amendment. Amendment with compost did not have a suppressive effect in any soil type. The absence of a disease suppressive effect of mineral and organic fertilization in Dutch clay soils may be related to the already high availability of inorganic and organic nutrients in these soils. This study shows that the mechanism of disease suppression of soil-borne plant pathogens may vary strongly according to the soil type, especially if quite different types of soil are used.     

Production of avocado trees infected with Phytophthora cinnamomi under different management regimes

Avocado root rot is the most important disease of this fruit crop worldwide. This pathology may be caused by several biotic and abiotic agents, with the oomycete Phytophthora cinnamomi being the pathogen more frequently associated with poor phytosanitary conditions. There are disease control methods available that can reduce disease severity and allow plants to recover; however, they are not consistently and promptly applied. In addition, only chemical products are used by farmers as the preferred management method. This research aimed to evaluate different root rot management strategies in a commercial orchard. Data suggest that individual control methods are not as effective as when they are applied in combination, as in the T8 treatment (metalaxyl + mancozeb applied in drench; injection of potassium phosphite to each plant stem; potassium silicate applied in drench; addition of a layer of organic mulch and incorporation of 10 kg of composted substrate, both applied to the ground around the base of each tree). Using this strategy, the area under the disease progress curve for the avocado root rot was reduced by up to 68.6%, and the extra‐quality avocado fruit class increased by as much as 44% compared to the diseased control plants (T0) (P < 0.01). With the combined treatment T8, farm income showed a 9.5‐fold increase, probably due to an increase in the percentage of viable roots by up to 9.4‐fold, which would have improved nutrient and water uptake.     

Contrasted responses of Botrytis cinerea isolates developing on tomato plants grown under different nitrogen nutrition regimes

The nutritional status of a plant is known to influence its susceptibility to pathogens. In the case of Botrytis cinerea, the role of nitrogen fertilization of various host plants on disease development appears to be variable. This study was carried out to characterize possible variability associated with isolates and inoculum density of B. cinerea in its ability to infect leaf‐pruning wounds and to develop stem lesions on tomato plants, as affected by the nitrogen input. Six isolates differing in their aggressiveness to tomato were compared. They all had similar reaction patterns in vitro in response to differential nitrogen levels. In tests on plants grown with contrasted regimes of nitrate fertilization, overall disease severity was lower for all isolates on plants with higher nitrogen inputs, regardless of inoculum concentration. However, differences among isolates were observed in the effect of plant nitrogen nutrition on infection and on lesion expansion. Disease onset was delayed on all plants with higher nitrogen inputs, but the response was greater for isolates with lower aggressiveness on tomato. The highest contrast among isolates was observed with the colonization of stems. The daily rate of stem lesion expansion decreased with increasing nitrogen fertilization levels for the more aggressive isolates, while it increased for the less aggressive isolate. Hypotheses to explain these results are discussed in light of the possible physiological effects of nitrogen fertilization on nutrient availability for the pathogen in the host tissue and of possible production of defence metabolites by the plant.