Severity Of Stem Canker Research Articles

Effects of fungicide on growth of Leptosphaeria maculans and L. biglobosa in relation to development of phoma stem canker on oilseed rape (Brassica napus)

Controlled‐environment and field experiments were done to investigate effects of the fungicide Punch C (flusilazole plus carbendazim) on growth of Leptosphaeria maculans and L. biglobosa in oilseed rape. In controlled‐environment experiments, for plants inoculated with L. maculans, fungicide treatment decreased lesion size and amount of L. maculans DNA in leaves; for plants inoculated with L. biglobosa, fungicide did not affect lesion size or amount of pathogen DNA. When release of ascospores was monitored using a Burkard spore sampler, the timing and pattern of ascospore release differed between the four seasons. In 2006/2007, the majority of ascospores released were L. maculans, whilst in 2007/2008 the majority were L. biglobosa; in both seasons L. maculans ascospores were released before L. biglobosa ascospores. In field experiments in 2002/2003 and 2003/2004, fungicide treatment decreased severity of stem canker on cv. Apex, but gave no significant yield response. In 2006/2007 and 2007/2008, fungicide treatment decreased phoma leaf spot incidence in autumn and stem canker severity at harvest, and increased yield. Fungicide treatment decreased stem canker severity more on cv. Courage, with a good yield response, than on cv. Canberra. In 2002/2003 and 2003/2004, fungicide treatment decreased the frequency of spread of L. maculans into stem pith tissues and in 2006/2007 fungicide decreased the amount of L. maculans DNA in stem tissues (measured by quantitative PCR). These results are used to suggest how effects of fungicides on interactions between L. maculans and L. biglobosa might affect severity of phoma stem canker and yield response.

Adaptation to increasing severity of phoma stem canker on winter oilseed rape in the UK under climate change

SUMMARYVarious adaptation strategies are available that will minimize or negate predicted climate change-related increases in yield loss from phoma stem canker in UK winter oilseed rape (OSR) production. A number of forecasts for OSR yield, national production and subsequent economic values are presented, providing estimates of impacts on both yield and value for different levels of adaptation. Under future climate change scenarios, there will be increasing pressure to maintain yields at current levels. Losses can be minimized in the short term (up to the 2020s) with a ‘low’-adaptation strategy, which essentially requires some farmer-led changes towards best management practices. However, the predicted impacts of climate change can be negated and, in most cases, improved upon, with ‘high’-adaptation strategies. This requires increased funding from both the public and private sectors and more directed efforts at adaptation from the producer. Most literature on adaptation to climate change has had a conceptual focus with little quantification of impacts. It is argued that quantifying the impacts of adaptation is essential to provide clearer information to guide policy and industry approaches to future climate change risk.

Effects of the nematicide 1,3-dichloropropene on weed populations and stem canker disease severity in potatoes

The soil fumigant 1,3-dichloropropene (1,3-D) has been used in the UK for the control of potato cyst nematodes (PCN), Globodera pallida (Stone) and Globodera rostochiensis (Wollenweber), but its potential herbicidal activity has not been extensively investigated in this country. Field and glasshouse studies were therefore conducted to evaluate the potential of 1,3-D for the control of weeds in potatoes, and observations were made on the severity of potato stem canker, caused by the fungus Rhizoctonia solani Kühn [teleomorph: Thanatephorus cucumeris (Frank) Donk]. Autumn application of 1,3-D at 211.5 L active substance (a.s.) ha −1 significantly suppressed the number of germinating weeds and the percentage of weed ground cover by 83% and 79%, respectively, relative to controls. There were also species-specific significant decreases (field pansy, Viola arvensis, in particular) in the number of weed seeds germinating in field soil in the glasshouse post-1,3-D treatment. The effect of 1,3-D declined in time and single (autumn or spring) or combined application produced a slight, but not significant, reduction in the number of weeds germinated on potato ridges relative to those recorded in untreated soil. The severity of stem canker on potato plants was not significantly reduced by 1,3-D but both mean number and weight of stems per plant were significantly increased compared with plants from untreated plots. These studies demonstrated that 1,3-D, in addition to giving PCN control, has efficacy against weeds; implications are the potential for reduced herbicide input in the crop rotation with accompanying economic and environmental benefits.

Defoliation of Brassica napus increases severity of blackleg caused by Leptosphaeria maculans: implications for dual-purpose cropping

Canola (Brassica napus) crops for grazing and grain (dual-purpose) production provide an economic break-crop alternative for dual-purpose cereals in Australian mixed farming systems. Infection by Leptosphaeria maculans is the most prevalent disease in Australian canola crops with airborne inoculum released throughout the autumn and winter when crops are grazed. Glasshouse and field experiments were conducted to investigate the effect of mechanical defoliation (simulated grazing) on disease severity at plant maturity. In glasshouse experiments, stem canker severity increased from 4% to 24% in severely defoliated plants, but light defoliation had no effect compared with undefoliated control plants. Disease severity was increased with defoliation in all field experiments. Defoliation increased crown canker severity from 22.6% to 39.3% at Wagga Wagga and from 3.0% to 7.1% at Canberra and lodging from 9.6% to 11.9% at Naracoorte in the same set of cultivars assessed at each site. The increase in disease severity with defoliation was less in canola lines with moderate to high levels of stem canker resistance. Plants defoliated before stem elongation tended to develop less disease than those defoliated during the reproductive phase of plant growth. These findings suggest that the impact of grazing on L. maculans infection of canola crops can be minimised by sowing cultivars with a high level of stem canker resistance and grazing during the vegetative stage of plant growth prior to stem elongation. Further research is required to determine whether these management strategies are applicable in canola crops defoliated by grazing animals.

SIPPOM-WOSR: A Simulator for Integrated Pathogen POpulation Management of phoma stem canker on Winter OilSeed Rape: II. Sensitivity analysis

SIPPOM, a simulator for integrated pathogen population management, has been developed to assess and rank Integrated crop management (ICM) strategies, at the regional scale. The input variables are weather data, soil characteristics, the description of cropping systems (crop sequence and winter oilseed rape crop management) and their spatial distribution, plus the initial size and genetic structure of pathogen populations. Here, we use SIPPOM to simulate phoma stem canker severity, the genetic structure of the pathogen populations, and the yield loss caused by the disease. Sensitivity analysis is conducted to quantify how strongly state variables (sub-model output variables) respond to variations in parameters. The results indicate which parameters need to be more accurately estimated, and it elucidates the steadiness of the rankings of contrasting control strategies under various weather conditions when parameters were varied. Due to the complexity of SIPPOM, the scope of this work was limited to a sensitivity analysis of each sub-model independently. Three values of each parameter were tested under various environmental conditions and crop management according to their expected or known effects on disease and yield. Qualitatively speaking, variations in input variables and parameters provided sub-model output variables that behaved as expected by experts. Parameters with the greatest effect on state variables and that need to be estimated more accurately are for instance those related to pseudothecia maturation and disease severity index estimates. Improvements are foreseen ( e.g., the calculation of both the number of phoma leaf spots and the severity disease index). Because the ranking of the simulated control strategies remained steady, despite large variations in the simulated variables linked with variations in parameters, the sensitivity analysis shows that the model, as it stands, can be used to compare and rank ICM strategies with respect to their effectiveness. Possibilities of a sensitivity analysis of the overall model are discussed.

First report of Holocryphia eucalypti on Eucalyptus grandis in Uganda

In recent years, Uganda has embarked on extensive establishment of plantations of fast growing eucalypts to meet local demand for construction timber. In 2007, stem cankers were observed on a small number of five year-old Eucalyptus grandis trees in a clonal trial in Kifu, near Kampala in Uganda. The cankers comprised shallow fissures with orange fruiting bodies, and a sharply defined necrosis at the canker edge. The cankers were similar to those caused by the fungus Holocryphia eucalypti on eucalypts in South Africa. The morphological characteristics of the conidia (aseptate, hyaline, cylindrical characteristics) and the orange stromata also matched published descriptions of H. eucalypti. Spore masses from orange fruiting bodies on the bark of several trees were aseptically transferred to malt extract agar plates. Using protocols from Gryzenhout et al. (2006), DNA sequences of the internally transcribed spacer regions ITS1 and ITS2 and 5.8S rRNA were obtained. The Ugandan isolate sequences (GenBank GQ292540) gave 100% bootstrap values when compared with known isolates of H. eucalypti. Greenhouse inoculation studies on one E. grandis clone under quarantine conditions in South Africa confirmed that H. eucalypti from Uganda was pathogenic. Stem inoculations with H. eucalypti on young trees (∼5 mm diam) produced lesions within three weeks of inoculation, while trees with control inoculations using sterile agar discs produced no lesions. The fungus was re-isolated from the lesions but not the control plants. Previously, H. eucalypti was known only from Australia and South Africa (Gryzenhout et al., 2006). In Australia, H. eucalypti is associated with more severe stem cankers and is considered an important pathogen of stressed trees especially (Wardlaw, 1999). Symptoms in Uganda were similar to that in South Africa, with no tree mortality observed and only the formation of superficial bark cankers. The implications of this disease for Uganda are unclear and further monitoring of its status is needed.

Effect of crop management on epidemics of phomopsis stem canker ( Diaporthe helianthi) for susceptible and tolerant sunflower cultivars

Phomopsis stem canker ( Diaporthe helianthi) is a worldwide fungal disease which is responsible for high yield losses in sunflower crop in the main regions of production. Field data on the influence of crop management on the incidence and severity of stem canker were reported by Debaeke et al. (2003) but a more thorough study was required to analyse step-by-step the spread of the fungus within the canopy from spore deposition on leaves to stem injury. In a 2-year study (2000–2001), the effects of crop management (plant density, N fertilization, and irrigation) and genotypic tolerance (susceptible vs tolerant cultivars) on the epidemics of Diaporthe helianthi were monitored under conditions of reinforced inoculum. The incidence and severity of leaf and stem symptoms were closely related to canopy development (leaf area index) and microclimatic conditions (relative humidity) resulting from different crop management options. Increasing plant density resulted in a greater proportion of girdling stem lesions, detrimental to yield, because of earlier infection under dense canopies. The number of girdling lesions per plant was maximal with high N fertilization but more leaves were infected without N fertilization. The field data were used to evaluate satisfactorily the epidemiological Asphodel model regarding the main periods of leaf infection. The relative humidity within the canopy (observed), the number of leaf infection events (simulated by Asphodel) and the final proportion of stem lesions (observed) were positively related which clearly demonstrated the key role of crop management in the development of stem canker in sunflower.

Quantitative resistance increases the durability of qualitative resistance to Leptosphaeria maculans in Brassica napus

It has frequently been hypothesized that quantitative resistance increases the durability of qualitative (R-gene mediated) resistance but supporting experimental evidence is rare. To test this hypothesis, near-isogenic lines with/without the R-gene Rlm6 introduced into two Brassica napus cultivars differing in quantitative resistance to Leptosphaeria maculans were used in a 5-yr field experiment. Recurrent selection of natural fungal populations was done annually on each of the four plant genotypes, using crop residues from each genotype to inoculate separately the four series of field trials for five consecutive cropping seasons. Severity of phoma stem canker was measured on each genotype and frequencies of avirulence alleles in L. maculans populations were estimated. Recurrent selection of virulent isolates by Rlm6 in a susceptible background rendered the resistance ineffective by the third cropping season. By contrast, the resistance was still effective after 5 yr of selection by the genotype combining this gene with quantitative resistance. No significant variation in the performance of quantitative resistance alone was noted over the course of the experiment. We conclude that quantitative resistance can increase the durability of Rlm6. We recommend combining quantitative resistance with R-gene mediated resistance to enhance disease control and crop production.

Geographic variation in severity of phoma stem canker and Leptosphaeria maculans/ L. biglobosa populations on UK winter oilseed rape (Brassica napus)

Phoma stem canker, caused by Leptosphaeria maculans and L. biglobosa, is the most important disease of oilseed rape in Europe. Differences between L. maculans and L. biglobosa in their life-cycles enable the two species to co-exist on oilseed rape crops over a cropping season. This review considers the factors affecting geographic variation in the severity of phoma stem canker epidemics and in the structure of the population of the pathogens in the UK, where the most severe epidemics occur in the south of England and cankers do not develop in Scotland. It is concluded that this variation is directly related to differences in climate, since weather-based models show that stem canker severity increases with increasing winter/spring temperature and temperatures are greater in the south of the UK. It may also be related to differences in pathogen populations, since surveys showed that the proportion of the more damaging L. maculans in stem cankers was greatest in southern England, with most L. biglobosa in northern England. Regional variation in agronomic practices such as cultivar choice and fungicide use may also indirectly influence phoma stem canker severity. Differences in cultivar choice result in differences in L. maculans race structure, which may influence the severity of epidemics. Differences in fungicide use may also influence pathogen populations, since L. maculans and L. biglobosa differ in their sensitivities to different azole fungicides. These factors are discussed in relation to strategies for sustainable production of oilseed rape by adaptation to threats posed by climate change.

Phoma stem canker (Leptosphaeria maculans/L. biglobosa) of oilseed rape (Brassica napus): is the G2 Disease Index a good indicator of the distribution of observed canker severities?

Several disease indices (DI) have been proposed to assess the severity of phoma stem canker at late growth stages. The DI commonly used in France (called G2 DI) consists of integrating the distribution of the proportions of plants observed within six canker severity classes into a single index. This index uses arbitrarily chosen coefficients associated to each class of canker severity. A given value of this DI can therefore be obtained with different distributions among the canker severity classes. Consequently, it is important to assess to what extent the G2 DI is representative of the distribution of plants in canker severity classes. We show in this study that the G2 DI is a good indicator of the distribution of the observed canker severities, using a dataset from different years, sites and cultivars; this illustrates the robustness of the G2 DI.

Tolerance of several Novi Sad sunflower hybrids to phomopsis stem canker under natural infection

Sunflower diseases have a major impact on sunflower yield and quality of seed. When conditions during vegetation are favorable, the intensity of damage can be very high. Therefore, surveys of economically important diseases are necessary. A survey of incidences and severity of phomopsis stem canker was performed on 9 locations during 2007 and 2008. A total of 8 hybrids per location was examined. Lodging of plants as a symptom of stem canker disease was not detected. The mean disease incidence was less than 5%, depending on hybrid, location and year. Generally, higher deasease incidence was recorded in the first year of survey than in the second. The hybrids Velja, NS-H-111 and Baca demonstrated an elevated level of resistance. Despite significant differences in disease severity that was found among the hybrids tested, all of them were highly resistant to phomopsis stem canker.

Biological and chemical control and their combined use to control different stages of the Rhizoctonia disease complex on potato through the growing season

Abstract Rhizoctonia solani causes stem canker and black scurf diseases on potato and negatively affects the yield in all potato‐growing areas. While seed‐borne infection can be efficiently controlled by dressing with fungicides, few means of effective control are available against soil‐borne infection. In this study, commercially available antagonistic fungi and bacteria, and the combination of antagonistic Trichoderma harzianum and seed dressing with flutolanil, were tested for their efficacy in the control of soil‐borne infection of R. solani in the field. Combined use of flutolanil and T. harzianum was found feasible because even the highest tested concentration of flutolanil [13.0 μg active ingredient (a.i.) mL−1] had little effect on the growth of T. harzianuminvitro, whereas over 100‐fold lower concentrations (0.1 μg a.i. mL−1) were sufficient to strongly inhibit the growth of R. solani (EC50 0.045 ± 0.0068 μg a.i. mL−1). The variables under focus in plants inoculated with R. solani were the relative stem lesion index; sprout/stem number; stolon number, weight and incidence of symptoms on stolons; total yield and the yield of marketable sized tubers, and incidence of black scurf on the marketable‐sized tubers. Flutolanil and its combined application with T. harzianum reduced the damage to sprouts and severity of stem canker at the early stages of growth (up to 30 days postplanting). Towards the end of the growing season, T. harzianum was required to reduce disease severity. When applied in‐furrow alone or in combination with flutolanil‐dressed seed potatoes, T. harzianum increased the proportion of marketable‐sized tubers in yield from 35% to 60% and decreased the incidence of black scurf on progeny tubers from 31% to 11%, which was not achieved using flutolanil alone. The number and weight of stolons and the yield of plants remained lower in the inoculated plants than un‐inoculated control plants regardless of the method of control used. The other two antagonists tested, Streptomyces griseoviridis and Gliocladium catenulatum, showed no consistent control of R. solani. Taken together, the results suggest that combining the application of the antagonist T. harzianum with seed dressing with flutolanil may provide the best protection of the potato crop against damage caused by R. solani throughout the growing season.

Variabilidade de isolados de Myrothecium roridum provenientes de meloeiro cultivado no Estado do Rio Grande do Norte

The variability of 53 isolates of Myrothecium roridum obtained from melon (Cucumis melo) fields in the State of Rio Grande do Norte (Brazil) was estimated based on: severity of Myrothecium stem canker in the cultivars AF-682 and Orange Flesh, the pathogen's physiology (mycelium growth rate - MGR - and sporulation - SP) and its sensitivity to the fungicide fluazinam. All isolates were pathogenic to AF-682 and Orange Flesh; however, better discrimination of pathogen variability was achieved in the latter. Variation among isolates was high when assessing all variables, except for MGR and sensitivity to fungicides (all sensitive). There was no correlation between SEV and any other variables. The variation among isolates within municipalities (60%) was higher than the variation among municipalities. There is evidence of high genetic variability in the populations of M. roridum, but no association between the assessed variables and locations was detected.

A method for screening soybean seedlings for resistance to northern stem canker caused by Diaporthe phaseolorum var. caulivora

Five soybean varieties, Williams, Harosoy, AC Hime, Coles and Leo were inoculated 2 and 4 wk after planting with Diaporthe phaseolorum var. caulivora to determine if differences in resistance to stem canker could be detected in the early stages of plant development. Seedling stems were severed above the unifoliolate leaves and inoculum was applied to the cut surface. Stem lesion lengths were measured at 1, 2 and 3 wk following inoculation. The mean lesion length on seedlings inoculated 2 wk after planting (27.2 mm) was significantly less than seedlings inoculated 4 wk after planting (44.5 mm). Based on mean lesion length, Williams was the most susceptible variety followed by Harosoy. AC Hime, Coles and Leo were equally resistant. Stem canker development measured at 2 and 3 wk following inoculation was negatively correlated with stem moisture at the time of inoculation suggesting that stem moisture is important in the development and severity of stem canker. Key words: Diaporthe phaseolorum var. caulivora, Glycine max, inoculation method, northern stem canker, resistance, screening method

Resistance to Leptosphaeria maculans (phoma stem canker) in Brassica napus (oilseed rape) induced by L. biglobosa and chemical defence activators in field and controlled environments

Effects of pretreatment of Brassica napus leaves with ascospores of Leptosphaeria biglobosa or chemical defence activators [acibenzolar‐S‐methyl (ASM) or menadione sodium bisulphite (MSB)] on infection by ascospores of Leptosphaeria maculans (phoma stem canker) and development of disease were studied in controlled‐environment (phoma leaf spot) and field (phoma leaf spot and stem canker) experiments. In controlled‐environment experiments, pretreatment of oilseed rape leaves (cv. Madrigal) with L. biglobosa, ASM or MSB delayed the appearance of L. maculans phoma leaf spot lesions. These pretreatments also decreased the phoma leaf spot lesion area in both pretreated leaves (local effect) and untreated leaves (systemic effect). In winter oilseed rape field experiments in the 2002/03 and 2003/04 growing seasons, pretreatment with L. biglobosa or ASM in October/November decreased not only the number of phoma leaf spot lesions per leaf caused by L. maculans in autumn/winter, but also the severity of phoma stem canker in the subsequent spring/summer. Effects were greater in 2002/03 (when natural L. maculans ascospore release began in September 2002) than in 2003/04 (when ascospore release began in December following a period of dry weather in August/September 2003). These results suggest that pretreatment with biological or chemical defence activators can induce local and systemic resistance to L. maculans, with both short‐term effects on the development of phoma leaf spotting and long‐term effects on the development of stem canker 8 months later.

Effects of Different 3-Year Cropping Systems on Soil Microbial Communities and Rhizoctonia Diseases of Potato

ABSTRACT Eight different 3-year cropping systems, consisting of soybean-canola, soybean-barley, sweet corn-canola, sweet corn-soybean, green bean-sweet corn, canola-sweet corn, barley-clover, and continuous potato (non-rotation control) followed by potato as the third crop in all systems, were established in replicated field plots with two rotation entry points in Presque Isle, ME, in 1998. Cropping system effects on soil microbial community characteristics based on culturable soil microbial populations, single carbon source substrate utilization (SU) profiles, and whole-soil fatty acid methyl ester (FAME) profiles were evaluated in association with the development of soilborne diseases of potato in the 2000 and 2001 field seasons. Soil populations of culturable bacteria and overall microbial activity were highest following barley, canola, and sweet corn crops, and lowest following continuous potato. The SU profiles derived from BIOLOG ECO plates indicated higher substrate richness and diversity and greater utilization of certain carbohydrates, carboxylic acids, and amino acids associated with barley, canola, and some sweet corn rotations, indicating distinct differences in functional attributes of microbial communities among cropping systems. Soil FAME profiles also demonstrated distinct differences among cropping systems in their relative composition of fatty acid types and classes, representing structural attributes of microbial communities. Fatty acids most responsible for differentiation among cropping systems included 12:0, 16:1 omega5c, 16:1 omega7c, 18:1 omega9c, and 18:2omega6c. Based on FAME biomarkers, barley rotations resulted in higher fungi-to-bacteria ratios, sweet corn resulted in greater mycorrhizae populations, and continuous potato produced the lowest amounts of these and other biomarker traits. Incidence and severity of stem and stolon canker and black scurf of potato, caused by Rhizoctonia solani, were reduced for most rotations relative to the continuous potato control. Potato crops following canola, barley, or sweet corn provided the lowest levels of Rhizoctonia disease and best tuber quality, whereas potato crops following clover or soybean resulted in disease problems in some years. Both rotation crop and cropping sequence were important in shaping the microbial characteristics, soilborne disease, and tuber qualities. Several microbial parameters, including microbial populations and SU and FAME profile characteristics, were correlated with potato disease or yield measurements in one or both harvest years. In this study, we have demonstrated distinctive effects of specific rotation crops and cropping sequences on microbial communities and have begun to relate the implications of these changes to crop health and productivity.

Efficacy of several potential biocontrol organisms against Rhizoctonia solani on potato

Twenty-eight potential biocontrol organisms were tested for efficacy against Rhizoctonia solani on potato in a series of greenhouse trials. Organisms tested consisted of field isolates of Paenibacillus polymyxa, Pseudomonas fluorescens, Penicillium sp., Trichoderma sp., and Rhizoctonia zeae; known biocontrol isolates including Laetisaria arvalis, Verticillium biguttatum, Cladorrhinum foecundissimum, and Stilbella aciculosa; and commercial products of Bacillus subtilis (Kodiak), Trichoderma virens (SoilGard), and T. harzianum (RootShield). Different formulations and rates of several fungal isolates and the efficacy of combinations of effective antagonists were also investigated. None of the treatments, including a chemical control (azoxystrobin), effectively controlled stem canker and black scurf in all trials. However, B. subtilis GB03, R. zeae LRNE17E, S. aciculosa 112-B, and the chemical control were most effective in reducing stem canker severity (40–49% reduction) relative to the infested controls over all trials. L. arvalis ZH-1, R. zeae LRNE17E, and the chemical control reduced black scurf severity 54–60% relative to the infested control. Other treatments also significantly reduced stem canker and black scurf, however they were slightly less effective. Biocontrol amendment rate was not correlated with disease control, although the higher rates usually provided the best control. One combination of biocontrol organisms, B. subtilis and T. virens, demonstrated somewhat better control of stem canker than each organism alone, suggesting that this approach may provide improved biocontrol efficacy.

Cultural practices and genetic resistance as factors affecting soybean stem canker and plant yield in the Cerrado

Field experiments were conducted in the 1995-96 soybean (Glycine max) growing season to evaluate the effects of cultural practices and host genetic resistance on the intensity of soybean stem canker, caused by Diaporthe phaseolorum f.sp. meridionalis (Dpm). Experiments were conducted in a commercial field severely infected in the previous (1994-95) season. In one study, minimum tillage (MT) and no-tillage (NT) cropping systems were investigated for their effects on disease development and on plant yields in cvs. FT-Cristalina (susceptible) and FT-Seriema (moderately resistant). Another study evaluated the effects of plant densities (8, 15, 21 and 36 plants/m) on disease development in cvs. FT-Cristalina, FT-101 (moderately resistant) and FT-104 (resistant). Disease incidence and severity were consistently lower in NT than in MT, and plant yields were increased by 23% and 14% in the NT system for the susceptible and moderately resistant cultivars, respectively, compared to the yields in the MT system. The Gompertz and Logistic models described well the disease progress curves in all situations. For both susceptible and moderately resistant cultivars, disease severity increased proportionately to the increase in plant densities. At the end of the season, 100% of the plants of cv. FT-Cristalina were infected by Dpm, at all plant densities. Disease levels on cv. FT-101 were intermediate while only very low disease levels were recorded on cv. FT-104. There was a consistent negative correlation between stem canker severity and yield. Some practices demonstrated potential for direct application in disease control, and could be combined considering their additive effects.

Epidemiology and management of Leptosphaeria maculans (phoma stem canker) on oilseed rape in Australia, Canada and Europe

Phoma stem canker (blackleg), caused by Leptosphaeria maculans, is an important disease on oilseed rape (canola, rapeseed, Brassica napus, Brassica juncea, Brassica rapa) causing seedling death, lodging or early senescence in Australia, Canada and Europe, but not in China. The two forms of L. maculans (A group and B group) that occur on oilseed rape are now considered to be separate species. The epidemiology and severity of phoma stem canker differs between continents due to differences in the pathogen population structure, oilseed rape species and cultivars grown, climate and agricultural practices. Epidemics are most severe in Australia, where only the A group occurs, and can be damaging in Canada and western Europe, where both A and B groups occur, although their proportions vary within regions and throughout the year. Epidemics are slight in China, where the A group has not been found. Dry climates (Australia, western Canada) lengthen the persistence of infected debris and may synchronize the release of airborne ascospores (after rain) with seedling emergence. L. maculans spreads from cotyledon and leaf infections down petioles to reach the stem, with infections on cotyledons and leaves early in the season producing the most damaging stem cankers at the stem base (crown). Development of both crown cankers and phoma stem lesions higher up stems is most rapid in regions with high temperatures from flowering to harvest, such as Australia and Canada. Breeding for resistance (genetic, disease escape or tolerance), stubble management, crop rotation and fungicide seed treatments are important strategies for control of phoma stem canker in all areas. Fungicide spray treatments are justified only in regions such as western Europe where high yields are obtained, and accurate forecasts of epidemic severity are needed to optimize their use.

Transgenic canola lines expressing pea defense gene DRR206 have resistance to aggressive blackleg isolates and to Rhizoctonia solani

We have previously demonstrated that transgenic Brassica napus plants expressing pea DRR206 constitutively are resistant to the hemibiotrophic blackleg fungus, Leptosphaeria maculans isolate PG2. The present work seeks to determine whether DRR206 is effective against a wider range of fungi. Transgenic plants expressing DRR206 exhibit decreased severity of stem canker in adult plants inoculated with aggressive L. maculans isolates PG3 and PG4. Decreased seedling mortality with the biotrophic root pathogen Rhizoctonia solani is also seen. Finally, leaves of DRR206 transgenic plants inoculated with the necrotroph Sclerotinia sclerotiorum show smaller lesions at 48 h after inoculation, leading to a delay, but not a prevention, of disease development. These results demonstrate the effectiveness of DRR206 against several fungal species with three distinct modes of pathogenicity. Although its precise function remains to be determined, a recent report shows that pea DRR206 shares strong amino acid sequence similarity with `dirigent proteins' which couple monolignol radicals to form the lignan (+) pinoresinol.