Compensating damage effects of seed-borne Fusarium culmorum and Microdochium nivale in winter wheat by increased seeding rates

Fusarium culmorum and Microdochium nivale are considered important seed-borne diseases of wheat in Denmark. Their damage effects consist in reducing seed germination and plant emergence which leads to reduced plant density, panicle numbers/area and yield. The aim of this study is to find out whether and to what degree these damage effects can be compensated by increased seeding rates. A trial was therefore conducted in 2004 in which seed lots of three winter wheat varities (Ritmo, Bill and Boston) having various degrees of infection by F. culmorum and Microdochium nivale were sown at various seeding rates. Seed contamination of the two pathogens was determined by counting the number of discoloured roots in the seed batches (Doyer method). Plant emergence was determined by counting the number of plants/area at seedling stage. A general linear model based on a term for [variety x pathogen] and a term for [variety x pathogen x germinating seeds/m2 x fraction of healthy seeds] explained ca. 70% of the variation in plant emergence. All parameters in the model were highly significant (p £ 0,005) and the R2 values of the individual variety x pathogen combinations ranged from 0.47 (Boston x M. nivale) to 0.85 (Ritmo x F. culmorum). The parameters of the model indicate that Boston was most responsive to an increasing seeding rate and/or healthy seed fraction, followed by Ritmo and Bill. This suggests that recommendations regarding the compensation of damage effects of seed-borne F. spp. and M. nivale by increasing seeding rates should be variety-specific. Supplemental results are expected from detailed analyses of the yield data and from an ongoing field trial. Further work will also include the development and use of molecular diagnostic tools to distinguish between various Fusarium spp. and M. nivale on contaminated seeds

Molecular diagnostic methods can prevent unnecessary rejection of organic seed lots

In some years, large amounts of organic seed lots are discarded due to the presence of high levels of seed-borne diseases. In barley it is especially the presence of the fungal species Pyrenophora teres (barley net blotch) and P. graminea (barley leaf stripe), which causes rejection of seed lots. A real-time PCR method for detection and quantification of P. graminea has been developed

Quantification of leaf stripe, Pyrenophora graminea, in barley seed by real-time PCR

In some years, large amounts of organic seed lots are discarded due to the presence of high levels of seed-borne diseases. In barley it is especially the presence of the fungal species Pyrenophora teres (barley net blotch) and P. graminea (barley leaf stripe), which causes rejection of seed lots. A real-time PCR method for detection and quantification of P. graminea has been developed

Leaf stripe resistance of spring barley cultivars

Results of six years of screening trials clearly indicate that effective resistance against barley leaf stripe is available, also in modern cultivars. Among the spring barley cultivars that are currently most widely grown in Denmark, Cabaret, Troon, Sebastian, Justina and Brazil appear most resistant, but only Brazil combines a favourable resistance performance (= low mean and standard deviation of environment-adjusted leaf stripe incidence) with a high number of observations (= years of testing). However, the cultivars appearing most resistant over the years are relatively old ones such as Vada, Alabama, Odin and particularly Scarlett, which has been resistant in all years in which it was tested. Amongst the new cultivars, Marigold, Native and SW Immer appear to have the most promising resistance properties so far. However, they have only been tested in two years of which only one had reasonably high infection levels. The results furthermore indicate huge year-to-year variation in infection levels, presumably due to variation in environmental conditions related to leaf stripe infection

Vårbygsorters resistens mod stribesyge og nøgenbrand

ORGSEED undersøgelserne viser, at der findes en række vårbygsorter med lovende resistens overfor stribesyge og nøgenbrand. Under dyrkningsforhold, hvor de to sygdomme betragtes som nøgle-problemer, burde sortsvalget derfor tager hensyn til de resistensoplysninger, som ORGSEED projektet har tilvejebragt. Men det skal huskes, at muligheden for at bruge sortsresistens ikke kommer af sig selv. Det kræver stadig årvågenhed med hensyn til at følge skift i sorternes sygdomsreaktion over tid og dette kræver fortløbende resistens-screening og –afprøvning af markedsførte sorter såvel som af forædlingsmaterialet

Simultaneous epidemic development of scald and net blotch on single leaf layers of a spring barley crop

Background and objectives Two pathogens growing on the same leaf compete for the same resources, i.e. space and plant nutrients. This may lead to density dependent disease development. The pathogens may also influence each other directly such that the influence of one on the other is more complex than a simple function of the area of the other pathogen. Different interaction types are, for example, competition, mutualism and exploitation. The importance of such interactions for epidemics of simultaneously occurring pathogens has received little attention. The objective of this study is to investigate the simultaneous epidemic development of Rhynchosporium secalis (causing scald) and Drechslera teres (causing net blotch) on spring barley under field conditions. Materials and methods The field trial was performed with artificial inoculation of R. secalis and D. teres on three spring barley varieties differing in their susceptibility towards the pathogens. The pathogens were inoculated in three combinations: only one was inoculated, they were inoculated together, the second pathogen was inoculated 26 days after the first. A non-inoculated treatment was included. The trial had three replications. Nine plants were harvested from each plot five times during the season. Leaves were dried and disease severity and senescence observed. Only leaves with < 50 % senescence were included in the analysis. Whole-plant disease severity over time was calculated as average of disease severity on leaves weighted by leaf area. Disease development per leaf layer was evaluated by fitting an exponential model to severity data over time for each leaf layer per variety, treatment and replicate. Association between scald and net blotch severity on individual leaves was analysed using Kendall’s tau. Results and discussion Net blotch developed on all leaf layers and reached whole-plant disease severities up to 15%. Scald did not develop on upper leaf layers and whole-plant severity was less than 2%. Disease severity curves at whole-plant level showed no effect of inoculating the other pathogen. The analysis of the growth rate of each disease per leaf layer showed a significant effect of variety and leaf layer within variety but no effect of treatment. However, we observed significant negative associations between the diseases on individual leaves for several combinations of leaf layer and variety. These results show that the individual leaf approach can provide new information and underline the importance of considering interactions between pathogens in the field. Acknowledgement This work was funded by the DARCOF II project BAR-O

Resistant barley varieties may facilitate control of Ramularia leaf spot

Ramularia leaf spot (RLS) is widely present in Denmark nowadays and has the potential to reach severity levels high enough to cause economic damage. The results of multi-location surveys and field trials of several years suggest that varietal resistance can be an efficient means for controlling RLS. The spring barley cultivars most affected by RLS possess mlo-resistance against powdery mildew while the least affected ones do not. Other mildew resistances as well as varietal susceptibility to other foliar diseases were not clearly related to the RLS reaction pattern of neither spring- nor winter barley cultivars. Promising RLS resistance donors may be the spring barleys Power, Helium, Isabella, Nathalie, Cruiser and Isotta and the winter barleys Lonni, followed by Chess, Lomerit, Nobilia, and Carola. However, more work is needed to develop reliable methods for efficient RLS resistance testing and to determine the nature and genetic basis of RLS resistance and its relation to varietal resistances against other diseases

Assessment of disease in low-input cereal cropping systems and variety trials

Disease assessment is the basis for describing disease resistance characteristics of commercial varieties in national variety lists for all crops where disease is considered a problem. It is well known that ‘disease’ in a specific situation depends on a whole range of interacting hosts, pathogens and environmental conditions. However, the biological complexity conflicts with the wish to express disease resistance characteristics of a variety as a single score per disease. This problem will be illustrated by basic dynamics of growth and senescence of both host and pathogen over time. Disease expressed in terms of absolute diseased leaf area, disease relative to total leaf area or green leaf area will be discussed in general. Two particular problems in low-input systems will be discussed, i.e. increased variability in nutritional status in field plots, which may influence the balance between abiotic and biotic stresses (diseases) and interact with disease on individual varieties, as well as the influence of weeds and diverse crops (variety mixtures and composite cross populations). The presence of disease in organic and low-input cropping systems may be underestimated because abiotic stresses, which enhance a general senescence of leaves, may obscure disease assessment, in particular disease on leaves. Weeds may interact by harbouring disease showing similar symptoms as on the target crop, or by ‘diluting’ disease by additional healthy green leaf area of the ‘crop’ like in a variety mixture. Other general topics such as differences between host/pathogen systems, timing of assessment, assessment scale, and interpretation and analysis of data are also discussed

High damage potential of seed-borne spot blotch in organically grown spring barley in Denmark

Spot blotch of barley (Bipolaris sorokiniana, perfect state: Cochliobolus sativus) occurs wherever barley is grown but is normally not considered a major problem in Danish barley production. It is therefore not included in routine disease surveys or official variety testing in Denmark. However, recommended seed contamination thresholds are 30% for spring barley and 15% for winter barley. Several spring barley varieties were grown under organic (no seed dressing, mechanical weeding) and conventional (seed dressing with fungicides, herbicide application) production conditions at three sites in Denmark in 2003. The harvested seeds were analysed for contamination levels of seed-borne B. sorokiniana using a blotter method. The percentage of seeds contaminated with B. sorokiniana ranged from about 5 to 95 % and was highly depending on the site, production system and variety. Contamination levels of organically cultivated plots were about twice as high as those of plots receiving conventional treatments. The germinating ability of seeds was highly affected by the level of B. sorokiniana contamination and declined drastically at contamination levels above 60%. This effect appeared to differ among varieties. The 1000-grain weight was not affected by the level of spot blotch contamination of seeds. The results indicate that the importance of spot blotch may be underrated, especially in organic barley production and that the role of varietal resistance should be investigated. More results are expected from ongoing seed analyses

Predicting spring barley yield from variety-specific yield potential, disease resistance and straw length, and from environment-specific disease loads and weed pressure

Abstract For low-input crop production, well-characterised varieties increase the possibilities of managing diseases and weeds. This analysis aims at developing a framework for analyzing grain yield using external varietal information about disease resistance, weed competitiveness and yield potential and quantifying the impact of susceptibility grouping and straw length scores (as a measure for weed competitiveness) for predicting spring barley grain yield under variable biotic stress levels. The study comprised 52 spring barley varieties and 17 environments, i.e., combinations of location, growing system and year. Individual varieties and their interactions with environments were analysed by factorial regression of grain yield on external variety information combined with observed environmental disease loads and weed pressure. The external information was based on the official Danish VCU testing. The most parsimonious models explained about 50% of the yield variation among varieties including genotypeenvironment interactions. Disease resistance characteristics of varieties, weighted with disease loads of powdery mildew, leaf rust and net blotch, respectively, had a highly significant influence on grain yield. The extend to which increased susceptibility resulted in increased yield losses in environments with high disease loads of the respective diseases was predicted. The effect of externally determined straw length scores, weighted with weed pressure, was weaker although significant for weeds with creeping growth habit. Higher grain yield was thus predicted for taller plants under weed pressure. The results are discussed in relation to the model ramework, impact of the considered traits and use of information from conventional variety testing in organic cropping systems