Cultivar Mixtures Research Articles

Characterisation and Classification of Italian Virgin Olive Oils by Near- and Mid-Infrared Spectroscopy

Virgin olive oil quality is the result of complex interactions between olive variety, environment and cultivar practice. Evaluation of its quality is based on chemical and sensory analyses (ECC Regulation) that are time-consuming, expensive and destructive of the sample. Spectroscopic techniques present significant advantages in terms of speed and cost of analysis per sample. Italian extra virgin olive oils from Lombardy, Tuscany and Calabria were analysed by conventional analytical and spectroscopic methods. The sample set was composed of 60 single-cultivar (Casaliva, Leccino and Frantoio) extra virgin olive oils (monovarietal extra virgin olive oils) and 59 extra virgin olive oils produced from a mixture of cultivars from each geographical area (industrial extra virgin olive oils). Free acid content, peroxide value and spectrophotometric indices ( K232, K270 and Δ K) were measured. Olive oils were also analysed by near infrared (NIR) and mid-infrared (MIR) spectroscopy in transmission and attenuated total reflectance, respectively, to classify oils on the basis of their geographical origin. Principal component analysis was applied both to chemical and spectral data as an exploratory technique. Classification methods studied were linear discriminant analysis, partial least squares discriminant analysis and soft independent modelling of class analogy (SIMCA). Both FT-NIR and FT-IR allowed sample classification of oils on the basis of geographical origin. NIR spectroscopy was able to classify better the industrial extra virgin olive oils producing a correct classification of about 90% of the samples, while the MIR technique was able to classify both monovarietal and industrial olive oils, allowing a higher correct classification of samples (>95%). SIMCA applied to MIR spectra classified about 70% of samples correctly on the basis of geographical origin.

Mixing feed barley cultivars did not decrease leaf disease or increase grain yield

Yield reductions of up to45%have been recorded in Western Australia when leaf disease has not been controlled using fungicides in two-row spring barley (Hordeum vulgare L.). Several overseas studies have shown that mixtures of barley cultivars can reduce the reliance on fungicides, decrease disease incidence and increase grain yield relative to pure stands. Four barley cultivars (cvv. Barque, Dash, Keel and Molloy) were mixed in 2-way, 3-way and 4-way mixtures and grown at eight sites along the 450-mm rainfall isohyet. At each site the performance of the 11 possible mixtures was compared against the same cultivars whengrown as pure stands (1-way) for disease severity, grain yield and grain quality. The sites were subject to natural disease infection, with one or two leaf diseases becoming established at each site. The diseases present were: net form net blotch (caused by Pyrenophora teres f. teres) or spot form net blotch (Pyrenophora teres f. maculata) and/or powdery mildew (Erysiphe graminis f. sp. hordei). Three disease susceptible controls (cvv. Baudin, Doolup and Vlamingh) were sown at each site to rate the disease risk. At sites with a low ( 20%) was there a reduction in disease severity due to mixing cultivars. No evidence was found to support the general view that mixtures themselves, or an increased number of components in the mixture, increase grain yield and grain yield stability. There were, however, cultivar combinations that improved crop performance at a site relative to one of the pure stand components, but not both. This mixture effect at a site was both cultivar and pathogen dependent. Impacts of mixtures on grain quality were variable depending on quality parameter, site and pathogen present, but were generally small.

Production and persistence of annual pasture legumes at five saline sites in southern Australia

Herbage production and persistence of 42 annual pasture legumes from 33 species were measured at five sites across southern Australia. Medicago polymorpha L. was highly productive on soils, particularly those not prone to waterlogging, with soil surface (0–10 cm soil depth) salinity [estimated by electrical conductivity (ECe)] levels in summer of up to 36 dS/m, whereas M. truncatula Gaertn. was productive on ECe levels of at least 11 dS/m. Trifolium michelianum Savi and T. resupinatum L. were highly productive on soils subject to waterlogging, but only where 0–10 cm summer ECe levels were less than 8 dS/m. No commercial species were adapted to highly saline (0–10 cm summer ECe levels >8 dS/m), waterlogged sites. However, Melilotus siculus (Turra) Vitman ex B.D. Jacks. has the potential to fulfil this role, provided an appropriate Rhizobium strain can be selected. Mixtures of species and cultivars should be sown to account for high spatial variability for salinity and waterlogging. Traits for annual legume success in saline landscapes include salinity and waterlogging tolerance in germinating seedlings and mature plants, early flowering, hardseededness and delayed softening of hard seeds. Establishment of regenerating seedlings is associated with the timing of hardseed softening in relation to rainfall events capable of leaching topsoil salts. It is proposed that salinity measurements to determine annual legume suitability for winter-dominant rainfall areas are made in summer or early autumn, when at their highest levels. Transects along salinity and waterlogging gradients are suggested as an alternative method to traditional plots for genotype evaluation.

Herbage accumulation, botanical composition, and nutritive value of five pasture types for dairy production in southern Australia

Four pasture treatments were compared with a perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) mixture, the commonly sown pasture type, at 3 sites in south-western Victoria, to determine the extent to which total annual herbage accumulation and seasonal growth pattern could be manipulated in non-irrigated dairy systems. The pasture treatments were: (1) short-term winter-active (STW), based on Italian ryegrass (Lolium multiflorum); (2) long-term winter-active (LTW), based on Mediterranean tall fescue (Festuca arundinaceae) or cool-season active perennial ryegrass; (3) long-term summer-active (LTS), based on Continental tall fescue; and (4) perennial ryegrass pasture receiving moderately high nitrogen (N) inputs in winter and spring (total of 210 kg N/ha.year) to increase herbage accumulation during the ryegrass growing season (RHN). The perennial ryegrass and white clover mixture served as the control treatment. The 3 sites were Heytesbury (heavy clay soil), Terang (duplex soil), and Naringal (light soil). All pasture treatments were grazed by dairy cows. Herbage accumulation rate, botanical composition, and nutritive value of all pasture treatments were measured for 3 years. The winter-active pasture types had little effect on the overall seasonal distribution of forage supply or total annual yield compared with the control treatment. By contrast, the long-term summer-active pasture type significantly increased herbage accumulation during summer compared with all other pasture types (mean of an additional 1.3 t DM/ha during summer compared with the control treatment across all sites and years). However, this came at the cost of lower winter production (mean 0.8 t DM/ha). The LTS pasture also produced less dry matter than all other treatments in the first year after sowing at 2 of the 3 sites, reflecting the slower establishment of tall fescue. Ryegrass receiving high N inputs grew more total herbage on an annual basis than LTS. Mean total annual herbage accumulation (kg DM/ha.year) over the 3 years and 3 sites was in the order: RHN (14400) > LTS (13760) = Control (13170) > STW (12450) = LTW (12170) (P < 0.01). The results demonstrated that improvements over the industry ‘standard’ perennial ryegrass/white clover pasture management system in total herbage accumulation and the seasonal pattern of pasture growth are possible using existing pasture technology (mixtures of different species and cultivars, and N fertiliser inputs). However, significant interactions among sites, years and pasture treatments for several variables reinforced the need to understand limitations to plant growth on a site-by-site basis, to select plants that match the environmental conditions, and to then manage them appropriately to reach their growth potential.

The Beginning of a New Invasive Plant: A History of the Ornamental Callery Pear in the United States

The Callery pear (Pyrus calleryana Dcne. [Rosales: Rosaceae]), an ornamental tree from China, has begun appearing in disturbed areas throughout the United States. To understand the relatively recent spread of this species into natural areas, we review its horticultural history, the traits promoting its invasiveness, and its current invasive status. Cultivated varieties (cultivars) of this species sold in the United States originate from different areas in China and represent genotypes that have been planted in high densities in residential and commercial areas in the introduced range. The species cannot self-pollinate because of a self-incompatibility system, but recent fruit set is due to crossing between different cultivars or between the scion and rootstock of cultivated individuals. Consequently, individual cultivars themselves are not invasive, but the combination of cultivars within an area creates a situation in which invasive plants can be produced. Because of the established nature of this species in urban areas, the spread of wild P. calleryana will most likely continue, especially as new cultivars continue to be introduced into the mixture of cultivars already present.

Effects of cultivar mixtures on scab control in apple orchards

The effects of two mixtures of resistant and susceptible apple cultivars on the development of scab caused byVenturia inequaliswere observed in an experimental orchard over four years, initially for two years without fungicides against scab, and subsequently for two years with a moderate fungicide schedule. The row‐by‐row and within‐row mixtures included a susceptible cultivar and a resistant cultivar in equal proportions. Without fungicides, the results showed a significant reduction of disease incidence over both years (7·3 to 21·3%), and severity in the second year (35·4%) in the within‐row mixtures, compared to the monoculture of the susceptible cultivar. The best results were obtained when the within‐row mixture was associated with moderate fungicide treatments; in this case the reduction in disease incidence reached 75·1% on leaves and 69·7% on fruits during the growth phase. The characteristics of theVenturia inaequalis/Malus × domesticapathosystem and the results obtained in this experiment suggest a moderate but not negligible ability of cultivar mixtures for reducing epidemics of the disease.

Weed, disease and grain yield effects of cultivar mixtures in organically managed spring wheat

Two heritage wheat cultivars, Red Fife and Marquis, along with two modern wheat cultivars, 5602HR and AC Barrie, were assessed in organically managed monocultures and mixtures to discern if mixtures provided a benefit to grain yield and disease and weed suppression in Manitoba over 3 site-years. 5602HR was the highest yielding sole cultivar, while Marquis and AC Barrie were the lowest yielding sole cultivars. Red Fife yielded similar to the modern cultivar 5602HR in several cases. Cultivar mixtures did not provide a yield advantage but did stabilize yields of mixtures containing disease susceptible cultivars. Key words: Cultivar mixtures, heritage cultivar, yield stability

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.

Effects of two mixtures of perennial ryegrass cultivars with contrasting heading dates, and differing in spring‐grazing frequency and silage harvest date, on characteristics of silage from first‐cut swards

AbstractThis experiment quantified the effects of: (i) heading date of perennial ryegrass, (ii) grazing frequency in spring and (iii) date of silage harvest, on the ensilability of herbages harvested for silage, and on the conservation and estimated nutritive value of the resultant silages. Replicated field plots with two perennial ryegrass mixtures (intermediate‐ and late‐heading cultivars) were subjected to three spring‐grazing regimes (no grazing, grazing in late March and grazing in both late March and late April) and were harvested on four first‐cut harvest dates between 20 May and 21 June. Herbage from each of the four replicates of these 24 treatments was precision‐chopped and ensiled unwilted and with no additive in laboratory silos. Herbage from the sward with the intermediate‐heading cultivar had a higher (P < 0·001) dry‐matter (DM) content and buffering capacity than that from the late‐heading cultivar, whereas water‐soluble carbohydrate concentrations increased (P < 0·001) with more frequent grazing in spring. Later harvesting enhanced herbage ensilability through an increased (P < 0·001) DM content and reduced (P < 0·001) buffering capacity and pH. Fermentation profiles of the silage were not markedly influenced by the cultivar mixture used but grazing in both late March and late April resulted in a more extensive fermentation with the acids produced increasingly dominated by lactic acid. The concentrations of acetic acid, and to a lesser extent, ethanol declined as silage harvest date was delayed. Overall, the relative effects of grass cultivar mixture were smaller than those of spring‐grazing treatment or silage‐harvesting date although on any given harvest date the herbage from the intermediate‐heading cultivar mixture was easier to preserve as silage than herbage from the late‐heading cultivar mixture. Delaying the harvesting of the late‐heading swards by 8 d removed the differences related to growth stage in buffering capacity, pH and DM content.

Adventitious Presence of Herbicide Resistant Wheat in Certified and Farm‐Saved Seed Lots

ABSTRACTVarietal purity in wheat (Triticum aestivum L.) seed production is necessary for agronomic uniformity and to enable potential market segregation. We conducted a survey of certified and farm‐saved seed samples using a nontransgenic imidazolinone‐resistant (IR) wheat cultivar in 2004 and 2005 in eastern Colorado. The objective was to compare varietal purity based on type of seed producer and IR wheat history. Ninety‐two samples of non‐IR varieties were taken from certified and farm‐saved seed growers, who either produced or had never produced IR wheat. Adventitious IR seeds were detected using a seed‐soaking technique in samples from each producer type and each IR production history. Levels of IR seed ranged from 0 to 11.28%. One certified sample and three farm‐saved samples exceeded the 0.1% threshold for off‐types in certified wheat seed. Using a two‐factor analysis, farm‐saved production class and positive IR history increased the estimated proportion of adventitious seed. Based on grower interviews, higher levels of adventitious seed presence were associated with volunteer plants from previous crops of the resistant cultivar and mechanical mixture during harvesting. Production practices for certified seed address these factors and may need to be strengthened if more stringent purity criteria are adopted. This information is important for risk assessment and policy development for potential commercial release of transgenic wheat varieties.

Fitness Characters in Parasexual Recombinants of the Rice Blast Fungus, Pyricularia oryzae

We examined the fitness characters in parasexual recombinants derived from cocultures of two isolates, NAO-02 (race 133.1) and TH77-1 (race 047.0), of the rice blast fungus, Pyricularia oryzae, in liquid yeast extract medium. The cocultured isolates were transferred to oatmeal agar to produce a conidial inoculum. Then, the conidial suspension was sprayed on the japonica rice cultivar Akiyutaka, which has two resistance genes to rice blast: Pik and Piz. Three pathogenic variants were isolated from the typical leaf blast lesions that were subsequently produced. We used randomly amplified polymorphic DNA (RAPD) to examine whether the variants were derived from parasexual recombination between the parents. Eleven RAPDs indicated that the variants possessed genomic DNA from both parents. After inoculating Akiyutaka 7 times in succession, the variants maintained their original pathogenicity. The parasexual recombinants produced more conidia and larger lesions on the host than did parent TH77-1, but fewer conidia and smaller lesions than parent NAO-02. Disease development on plants inoculated with either variant or a parental isolate was also midway between the disease caused by each of the parents in a field containing a single cultivar, but the variant caused the most severe disease in a plot with a mixture of cultivars. These results suggest that parasexual recombination produces variation in fitness.

Delving of sandy surfaced soils reduces frost damage in wheat crops

Delving is a farming practice involving the mixing of a deep clayey subsoil layer with a sandy topsoil. One of the many effects of this practice is to reduce soil albedo and increase water-holding capacity of the topsoil, thus increasing the potential for storage and release of heat and potential attenuation of the effects of radiative frost. At Keith, a frost-prone location of South Australia, we investigated the effect of management practices with putative capacity to reduce frost damage, with emphasis on delving. Three field experiments were established on Brown Sodosols with a water-repellent sand topsoil. In relation to crops in untreated control soil, delving increased wheat yield from 1.9 to 3.1 t/ha in 2003, and from 0.5 to 1.5 t/ha in 2004. This large delving effect contrasted with the minor effects of other treatments including soil rolling, sowing rate, row spacing, and cultivar mixture. Lack of significant interactions between treatments indicated a robust response to delving across a range of management practices. Topsoil and canopy-height minimum temperatures were consistently higher in the delved treatment. The average difference in canopy-height minimum temperature between delved and control treatments was 0.3–0.4°C, with a maximum of 1.6°C in 2003 and 1.2°C in 2004. A single, robust relationship between yield and frost damage fitted the data pooled across treatments and seasons. This, together with the temperature differential between treatments, and significant relationships between minimum canopy-height temperature around flowering and frost damage supported the conclusion that a substantial part of the yield gain attributable to delving was related to reduced frost damage.

Management of nematodes and a stalk borer by increasing within‐field sugarcane cultivar diversity

The aim of this study was to determine whether an increase in plant diversity could modify the structure of the nematode community in favour of less pathogenic species and whether it affected the lepidopteran stalk borer, Eldana saccharina. Four sugarcane cultivars were planted either singly or as a mixture in the furrow. Nematodes were enumerated when the crops were 6 months old for five successive 10 to 13 month old crop cycles. Results showed that the mixture of cultivars had little significant impact on the overall abundance of the plant‐parasitic or free‐living nematodes. However, the nematode community structure within the mixture was slightly different to that observed on any single cultivar planted on its own and there was a higher than expected proportion of Helicotylenchus dihystera during the first three crops. With successive crops and in all treatments, there was an overall trend of increasing proportions of Meloidogyne javanica in the nematode community. However, the way that the change in the species‐balance occurred in the mixed cultivar plots suggested a resistance to the evolving dominance of M. javanica. These results showed that sugarcane cultivars interacted within the rhizosphere when planted in a mixture because the observed nematode community in the mixture differed from that expected. The level of damage caused by E. saccharina was slightly lower in the mixture than expected from the actual cultivar composition.

Digestibility of lupin kernel meals in feeds for the black tiger shrimp, Penaeus monodon

In recent years, new cultivars of lupins have largely replaced the cultivars that were studied in previous research with shrimp. There was a need to establish if the breeding programs had introduced changes in the new lupin cultivars that would affect the nutritional value of the kernel meal for shrimp. We have determined the apparent dry matter digestibility (ADMD), apparent crude protein digestibility (ACPD) and apparent digestibility of energy (ADE) of the yellow lupin Lupinus luteus cv. Wodjil, as well as of six of the new cultivars of Lupinus angustifolius when used in diets for the black tiger shrimp, Penaeus monodon. The L. angustifolius cultivars represent about 80% of Australia's lupin production. We have also determined the apparent digestibility (AD) of the amino acids of five of the new cultivars of L. angustifolius, and of L. luteus, cv. Wodjil. Ytterbium acetate was used as an inert digestibility marker at a concentration of 0.5 g kg − 1 in the diets. During the periods when faeces were collected, the shrimp were fed every 6 h and faeces were collected within 3 h of being passed. Six replicate tanks were assigned to each treatment. The kernel meal from L. luteus cv. Wodjil had the highest ADMD (70.0%) and ADE (79.9%) but its ACPD was mid-range at 93.8%. The ADMD of the L. angustifolius kernel meals varied between 56.5% and 66.3% with the mean (± s.e.m.) of 62.6% (± 0.95%), and the ADE varied between 69.6% and 77.2% (mean ± s.e.m. = 74.0% ± 0.72%), whereas the ACPD varied between 92.7% and 96.8% (mean ± s.e.m. = 94.3% ± 0.48%). The AD of the amino acids was similar to the ACPD value. Though there were significant differences among the ADs of the new cultivars of L. angustifolius, their values are similar to, though slightly lower than the AD reported for the older cultivar, Gungurru. The general consistency of the L. angustifolius AD results suggests that nutritionists and feed formulators can confidently use mean AD values for dry matter, protein and energy for kernel meals comprising of random mixtures of cultivars.

First Report of Phytophthora infestans on Petunia × hybrida in South Africa

Phytophthora infestans is known worldwide as the destructive, late blight pathogen of potatoes and tomatoes. However, erratic reports dating back to 1856 also have shown it to be pathogenic on petunia (Petunia × hybrida), although it has not been regarded as an important pathogen on this host (4). Recently, reports from North America showed that P. infestans is of commercial importance in greenhouse-grown petunias (2), and that late blight-infected petunias may serve as inoculum to tomatoes growing in the same greenhouse (1,2). In the Western Cape Province of South Africa, two petunia samples were received at the Stellenbosch University Plant Disease Clinic in 2005 that showed symptoms resembling P. infestans infections. The two samples were from nurseries where petunias were either grown under shading nets or in a greenhouse. In the greenhouse-grown petunias, the presumptive late blight infections resulted in substantial losses to the grower. Symptoms included gray, slightly sunken leaf lesions with white sporulation mainly on the adaxial side of the leaves. Leaflets of the petunias were incubated in moist chambers, and sporangia sporulating from lesions were identified morphologically as being P. infestans (3). Subsequently, one isolate was cultured onto synthetic media by carefully transferring sporangia from a lesion with the tip of a bended glass rod onto wheat medium (120 g/L of crushed wheat seeds, blended, boiled and filtered through cheesecloth, plus 15 g of sucrose and agar). The identity of the culture was further confirmed through sequence analyses of the internal transcribed spacer regions (GenBank Accession No. DQ479409). The isolated P. infestans strain (STE-U 6134) has been submitted to the Stellenbosch University culture collection. Inoculum for the pathogenicity tests was produced by first flooding 14-day-old cultures with sterile distilled water to obtain a 2 × 104/ml sporangial suspension, followed by zoospore induction at 4°C. A mixture of petunia cultivars (n = 24) were spray inoculated to runoff with the zoospore induced sporangial suspension. Control plants were sprayed with sterile distilled water. Inoculated plants were incubated at 22 to 25°C and high relative humidity (≥93%) within perspex humidity chambers (60 × 30 × 60 cm) lined with a wet sheet of chromatography paper. The experiment was repeated twice. The first late blight symptoms similar to those of the submitted samples appeared 5 to 7 days after inoculation, with some lesions containing profuse white mycelia and sporangiophores typical for P. infestans. The pathogen was reisolated from the leaf lesions, completing Koch's postulate. To our knowledge, this is the first report of P. infestans causing damage on petunias in South Africa. Future studies should be aimed at investigating whether late blight-infected petunias provide an important source of inoculum for potatoes and tomatoes, which are widely grown in the Western Cape Province of South Africa.

The agronomic value of annual plant diversity in crop-weed systems

Enhanced crop or cultivar diversity within annual cropping systems could provide important ecological and agronomic benefits. The agronomic effects of annual plant diversity from mixtures of crop species and barley (Hordeum vulgare L.) cultivar types were compared using richness levels of 1, 2, 5 or 10 randomly selected taxa in a greenhouse experiment. Increasing crop richness increased overall crop dry matter production, production stability (i.e., decreased CV for dry matter production) and weed suppression. These agronomic variables tended to level off after a richness of five to six crop species, suggesting that further increases in crop diversity are redundant. Increasing barley cultivar richness increased crop production in one of two experimental runs, but no effects were observed for weed suppression. Increased light interception related to greater plant canopy height variation in diverse mixtures of species could have contributed to increased productivity in the crop richness experiment. The results of this study suggest that the ecological functions of diversity provide productivity, yield stability and weed suppression benefits for mixtures of crop species, and even perhaps for mixtures of cultivars within a given crop species. Key words: Diversity (crop), suppression (weed), stability (yield), intercropping, cultivars (barley), oat (wild)

Protection of Susceptible Potato Cultivars Against Late Blight in Mixtures Increases with Decreasing Disease Pressure

ABSTRACT Cultivar mixtures can reduce potato late blight severity on susceptible cultivars. While alternating rows of susceptible and resistant cultivars would be more acceptable than random mixtures for commercial use, they increase the genotype unit area, which is an unfavorable factor for mixture efficiency, and have been minimally efficient when disease pressure is high. The effects of disease pressure on the performance of alternating rows of cultivars possessing various types and levels of resistance were investigated in 2000 and 2001 near Quito, Ecuador, where natural pressure of late blight is high. The experiments included the highly susceptible cvs. Cecilia in 2000 and LBr37 in 2001, as well as C114 (moderately resistant) and PAN (highly resistant), planted as pure stands and as the three possible two-way combinations. Different disease pressures were obtained with three spraying schedules of a contact fungicide: nontreated, one spray every second week, and one spray weekly. The area under the disease progress curve (AUDPC) on the susceptible cultivar was 0 to 20% less in mixed than in pure plots when no fungicide was applied, 13 to 26% less with a biweekly application of fungicide, and 32 to 53% less with a weekly application. These values are comparable to those obtained in previous experiments in smaller plots with designs maximizing the distance between susceptible plants. No significant differences in mixture performance were observed according to the resistant cultivar included. Effects on yield were minimal, because of the impact of factors other than late blight. Disease pressure therefore appears as a major factor conditioning the efficiency of potato cultivar mixtures against late blight.

Amount and quality of grass harvested for first‐cut silage for differing spring‐grazing frequencies of two mixtures of perennial ryegrass cultivars with contrasting heading date

AbstractThere are potential advantages and disadvantages associated with grazing spring perennial ryegrass swards designated for first‐cut silage. These may differ for intermediate‐heading (0·50 ear emergence in the second half of May) and late‐heading (0·50 ear emergence in the first half of June) cultivars. The interactions between cultivar type, spring‐grazing frequency, silage‐harvest date and year were examined in an experiment with a randomized complete block (n = 4) design with a factorial arrangement of treatments, conducted in Ireland. The factors were (i) two perennial ryegrass mixtures: intermediate‐ vs. late‐heading cultivars, (ii) three spring‐grazing regimes: no grazing, grazing in mid‐March or grazing in both mid‐March and mid‐April, (iii) four first‐cut silage‐harvest dates that were at c. 10‐d intervals from 19 May and (iv) 2 years (1998 and 1999). The effects of cultivar mixture on herbage mass of the swards in spring were small and not statistically significant. The late‐heading cultivars provided lower amounts of herbage dry matter for harvesting for first‐cut silage but herbage with higher in vitro organic digestibility values compared with intermediate‐heading cultivars. To achieve the same amount of herbage for silage, the late‐heading cultivars needed to be harvested 8 d later than the intermediate‐heading cultivars. Even with this delay in harvest date, the late‐heading cultivars had higher in vitro organic digestibility values than the intermediate‐heading cultivars. The late‐heading cultivars could be harvested up to 30 d later and produce a higher amount of herbage for first‐cut silage with similar digestibility values compared with the intermediate‐heading cultivars.

Assessing Four-Way Mixtures of Winter Wheat Cultivars from the Performances of their Two-Way and Individual Components

Seven four-way bread wheat mixtures were compared to their components (individual cultivars and two-way mixtures) for septoria tritici blotch severity, grain protein content, and grain yield. Four-way mixtures and two-way mixtures performed better than the average of individual pure line components. Disease severity and protein content were most influenced by mixtures, with mixture efficiencies being greater in the four-way than in the two-way mixtures. Performances of four-way mixtures in terms of diseased leaf area reduction, grain yield, 1000-grain weight, and grain protein content were better predicted by performances of two-way mixtures than by those of pure lines. Our results suggest that two-way mixtures should be screened to remove unfavourable cultivar pairs and those binary mixtures that show complementarity should be selected in order to construct four-way mixtures.

Improved Resistance Management for Durable Disease Control: A Case Study of Phoma Stem Canker of Oilseed Rape (Brassica napus)

Specific resistance loci in plants are generally very efficient in controlling development of pathogen populations. However, because of the strong selection pressure exerted, these resistances are often not durable. The probability of a resistance breakdown in a pathosystem depends on the evolutionary potential of the pathogen which is affected by: (i) the type of resistance (monogenic and/or polygenic), (ii) the type of reproduction of the pathogen (sexual and/or asexual), (iii) the capacity of the pathogen for dispersal, (iv) the resistance deployment strategy (pyramiding of specific resistances, mixture of cultivars, spatio-temporal alternation), (v) the size of the pathogen population, which is affected by control methods and environmental conditions. We propose the concept of Integrated Avirulence Management (IAM) to enhance the durability of specific resistances. IAM involves a strategy to limit the selection pressure exerted on pathogen populations and, at the same time, reduce the size of pathogen populations by combining cultural, physical, biological or chemical methods of control. Several breakdowns of resistance specific to Leptosphaeria maculans, the causal agent of phoma stem canker have occurred in Europe and in Australia. This review paper examines control methods to limit the size of L. maculans populations and discusses how this limitation of population size can enhance the durability of specific resistances. It proposes pathways for the development of a spatially explicit model to define IAM strategies. Simulation results are presented to demonstrate the potential uses of such a model for the oilseed rape/L. maculans pathosystem.