The occurrence of virulent pathotypes of Leptosphaeria maculans in brassica seed crops in England

Virulent and non-virulent pathotypes of Leptosphaeria maculans were differentiated on the basis of cultural characteristics and virulence to cabbage plants. Surveys of isolates obtained from oilseed rape crops grown in England in 1982 and 1983 showed that virulent pathotypes predominated in some areas whereas in others they were infrequent or absent. Overall 41% of isolates from this crop were of the virulent type. Virulent types usually occurred most frequently in areas with a long history or a high density of oilseed rape. In vegetable and forage brassica seed crops in Essex virulent isolates formed a small proportion of the population, except in one swede crop from which 95% of isolates were virulent. Host specificity was not detected in cross-infection experiments using isolates from different hosts and localities.     

The incidence of Alternaria spp. and Leptosphaeria maculans in commercial brassica seed in the United Kingdom

Tests on samples of oilseed rape seed ( Brassica napus ) sown in the UK between 1981 and 1984 indicated that on average 25% of samples were infected with Alternaria brassicae and 61% with Leptosphaeria maculans , with maximum incidences of infection of 19 and 4.2% respectively. Much infection by Alternaria spp. occurred on vegetable and forage brassica seed produced in the UK between 1979 and 1983. In B. oleracea types A. brassicicola occurred most frequently, affecting 88% of samples and up to 55% of seeds. A. brassicae was detected in 44% of B. oleracea samples and in up to 13% of seeds. Little Alternaria infection occurred in swede or forage rape samples (B. napus ), but A. brassicae affected up to 8–5% of seeds in turnip samples ( B. campestris ). L. maculans occurred in 44% of samples of vegetable and forage brassica seed produced in the UK, with a maximum of 4–6% infected seeds. A. brassicicola was present in 73% of samples of imported B. oleracea seed, affecting up to 25.5% of seeds. A. brassicae was absent from these samples and little L. maculans was detected. Pathogenicity tests on isolates of L. maculans from infected seeds indicated that virulent pathotypes were present in 16 rape seed samples but in only one sample (swede) of vegetable or forage brassica seed. The high incidence of seed infection by these pathogens emphasizes the importance of applying fungicide treatments to all types of brassica seed.     

Detection of airborne inoculum of Leptosphaeria maculans and Pyrenopeziza brassicae in oilseed rape crops by polymerase chain reaction (PCR) assays

The potential use of DNA-based methods for detecting airborne inoculum of Leptosphaeria maculans and Pyrenopeziza brassicae , both damaging pathogens of oilseed rape, was investigated. A method for purifying DNA from spores collected using Hirst-type spore samplers and detecting it using polymerase chain reaction (PCR) assays is described. For both pathogens, the sensitivities of the DNA assays were similar for spore-trap samples and pure spore suspensions. As few as 10 spores of L. maculans or P. brassicae could be detected by PCR and spores of both species could be detected against a background of spores of six other species. The method successfully detected spores of P. brassicae collected using spore traps in oilseed rape crops that were infected with P. brassicae. Leptosphaeria maculans spores were detected using spore traps on open ground close to L. maculans -infected oilseed rape stems. The potential use of PCR detection of airborne inoculum in forecasting the diseases caused by these pathogens is discussed.     

萝卜细菌性黑斑病在高山菜区发生规律观察

由丁香假单胞菌斑点致病变种Pseudomonas syringae pv. maculicola （McCul.） Young et al.引起大面积流行的十字花科黑斑病菌是在湖北长阳发现的一种新的危险性病害,经接种证实该病菌种子可以携带;该病的初次浸染源是由种子携带侵入.十字花科黑斑病在长阳高山蔬菜基地6～9月均可发病,主要危害萝卜叶片;染病萝卜的生物学产量和商品性明显降低.十字花科黑斑病在田间有明显发病中心,发病与品种、气候有关.萝卜品种＂白玉春＂＂春白玉＂发病重.     

Race structure and frequency of avirulence genes in the western Canadian Leptosphaeria maculans pathogen population,the causal agent of blackleg in brassica species

Leptosphaeria maculans is the causal agent of blackleg, a serious disease on canola/rapeseed in western Canada, Australia and Europe. Genetic resistance and extended crop rotation provided effective disease control in western Canada for years but the emergence of new pathogen races has reduced the effectiveness of current management strategies. The objective of this study was to analyse L. maculans isolates derived from canola stubble in commercial fields collected in 2010 and 2011 across western Canada for the presence and frequency of avirulence (Avr) genes. A total of 674 isolates were examined for the presence of Avr alleles AvrLm1, AvrLm2, AvrLm3, AvrLm4, AvrLm6, AvrLm7, AvrLm9, AvrLepR1, AvrLepR2 and AvrLmS using a set of differential host genotypes carrying known resistance genes or PCR amplification of AvrLm1, AvrLm6 and AvrLm4–Lm7. Certain alleles were more prevalent in the pathogen population, with AvrLm6 and AvrLm7 present in >85% of isolates, while AvrLm3, AvrLm9 and AvrLepR2 were present in <10% of isolates. A total of 55 races (different combinations of Avr alleles) were detected, with the two most common ones being AvrLm2–Lm4–Lm6–Lm7 and AvrLm2–Lm4–Lm6–Lm7–LmS. Races carrying as many as seven and as few as one known Avr allele were detected. Selection pressure from the race‐specific resistance genes carried in canola cultivars has probably played a significant role in the current Avr profile, which may have also contributed to the recent increase in blackleg observed in western Canada.     

Variation in host range, systemic infection and epidemiology of Leptosphaeria maculans

The A and B groups (aggressive and non-aggressive) of Leptosphaeria maculans were compared in studies of host range, infection phenotypes and epidemiology. Isolates of both groups infected a wide range of cruciferous hosts including Brassica napus, B. rapa, B. oleracea, B. juncea, B. carinata. B. nigra, Thlaspi arvense and Raphanus sativus. On cotyledons, B-group isolates were generally more aggressive than A-group isolates, causing local lesions and subsequent systemic invasion of the majority of test species. On susceptible stems, A-group isolates caused cortical lesions; B-group isolates, if they reached the stem, usually caused pith lesions with no external symptoms. In a susceptible line of B. napus , CrGC5, systemic infections of the leaf and petiole were similar with both groups, but the B-group rarely entered the stem to form cortical lesions or cankers. At two different field locations, natural epidemics caused by the two groups on oilseed rape leaves were similar in overall pattern, B-group lesions occurred slightly later but the incidence increased more rapidly and reached a maximum slightly earlier than that of the A group. The two groups differed markedly in stem infection patterns. Infection of the cortex near the base of the stem and the development of a typical stem canker was caused entirely by the A group. However the B group often caused considerable damage to the pith. Superficial chlorotic lesions on stems and inflorescences were mainly attributable to the B group. In view of the stem pith infection by the B group without external symptoms, its importance on oilseed rape may have been underestimated previously.     

Epidemiology of dark leaf spot caused by Alternaria brassicicola and A. brassicae in organic seed production of cauliflower

In organic seed production of Brassica vegetables, infections by Alternaria brassicicola and A. brassicae can cause severe losses of yield and seed quality. Four field experiments with or without artificial inoculation with A. brassicicola were conducted in organically managed seed‐production crops of cauliflower cv. Opaal RZ in 2005 and 2006 in the Netherlands. The development of A. brassicicola and A. brassicae on pod tissues and developing seeds was followed and seed quality was assessed. Alternaria brassicicola was externally present on 1·2% of the seeds 14 days after flowering and observed internally within 4 weeks after flowering. In both seasons, seed colonization by the pathogen increased slowly until maturation but sharply increased during maturation. A similar pattern was found for the colonization of pod tissues by A. brassicicola as quantified by TaqMan‐PCR. The incidence of A. brassicicola on mature seeds reached 70–90%. Internal colonization was found for 62–80% of the seeds. External and internal seed colonization by A. brassicae was much lower, with incidences below 3%. The quality of harvested seeds was generally low, with less than 80% of seeds able to germinate. Seed quality was not affected by warm water treatments. It was concluded that A. brassicicola and A. brassicae have the potential to infect pods and seeds soon after flowering. For the production of high quality seeds, producers must prevent such early infections. Therefore, new control measures are needed for use in organic cropping systems.     

The establishment of systemic infection in leaves of oilseed rape by Leptosphaeria maculans

In a series of growth room experiments in which leaves of Brassica napus var. oleifera were inoculated with ascospores or pycnidiospores of Leptosphaeria maculans successful infections progressed through three consecutive phases. Initial establishment in the mesophyll was succeeded by a phase of intercellular exploration, when hyphal proliferation was highly variable and host cell necrosis always ensued, and then by a systemic phase when hyphae were consistently sparse. Host cells associated with the hyphal front were capable of autofluorescence, accumulation of vital stains and plasmolysis, indicating that they were viable and that the pathogen was biotrophic throughout this sequence. During either of the first two phases permanent fungistatic containment, involving the formation of vesicles by disintegration of the hyphae, often occurred. Localization at the first phase was symptomless; at the second it was signified by a lesion with a clearly defined margin.
There was a negative correlation between biotrophic potential and necrotrophic potential of three pathogenic isolates, on both the moderately susceptible cultivar Primor and the resistant cultivar Jet Neuf. As leaves aged, a progressively larger proportion of infections failed to become systemic. With increasing inoculum load, symptomless localization of infection diminished, the phase of necrosis extended, and the probability of irreversible systemic development increased.     

Pathogenic Variability and Prevalence of Avr Genes in Leptosphaeria maculans Populations from Alberta,Canada

Journal of Plant Diseases and Protection - Blackleg (Phoma stem canker), caused by the Leptosphaeria species complex L. maculans and L. biglobosa, is a severe and economically important disease in...     

Strategies to prevent spread of Leptosphaeria maculans (phoma stem canker) onto oilseed rape crops in China; costs and benefits

Field experiments in Europe have shown that Chinese cultivars of winter oilseed rape ( Brassica napus ) are very susceptible to the pathogen Leptosphaeria maculans (cause of phoma stem canker). Climatic and agronomic conditions in China are suitable for L. maculans since the closely related but less damaging pathogen L. biglobosa occurs on the winter and spring oilseed rape crops there. Major gene resistance to L. maculans is not durable; when introduced into commercial oilseed rape cultivars it is rapidly rendered ineffective by changes in the pathogen population. The threat to Chinese oilseed rape production from L. maculans is illustrated by the way in which L. maculans has spread into other areas of the world where previously only L. biglobosa was present, such as Canada and Poland. Models were developed to describe the spread (in space and time) of L. maculans across Alberta province, Canada, based on survey data collected over a 15-year period. These models were used to estimate the potential spread of L. maculans across the Yangtze river oilseed rape growing areas of China and its associated costs. Short-term strategies to prevent occurrence of severe phoma stem canker epidemics in China include training of extension workers to recognise symptoms of the disease and use of PCR-based diagnostics to detect the pathogen on imported seed. Long-term strategies include the introduction of durable resistance to L. maculans into Chinese oilseed rape cultivars as a component of an integrated disease management programme. The costs of such strategies in relation to costs of a phoma stem canker epidemic are discussed.     

The natural occurrence of white clover mosaic virus in sweet pea (Lathyrus odoratus) in south-east England

White clover mosaic virus was isolated from glasshouse-grown sweet pea plants with conspicuous flower 'break' symptoms. Plant growth was severely affected. The virus was transmissible by sap inoculation to sweet pea and other leguminous species. It was found naturally in Trifolium repens growing nearby. No natural vector is known.     

Microsatellite and Minisatellite Analysis of Leptosphaeria maculans in Australia Reveals Regional Genetic Differentiation

ABSTRACT The population genetic structure of the fungal pathogen Leptosphaeria maculans was determined in Australia using six microsatellite and two minisatellite markers. Ascospores were sampled from Brassica napus stubble in disease nurseries and commercial fields in different sites over 2 years. The 13 subpopulations of L. maculans exhibited high gene (H = 0.393 to 0.563) and genotypic diversity, with 357 haplotypes identified among 513 isolates. Although the majority of genetic variation was distributed within subpopulations (85%), 10% occurred between the regions of eastern and Western Australia, and 5% within regions. F(ST) analysis of subpopulation pairs also showed the east-west genetic differentiation, whereas factorial correspondence analysis separated Western Australian subpopulations from eastern ones. Bayesian model-based population structure analyses of multilocus haplotypes inferred three distinct populations, one in Western Australia and an admixture of two in eastern Australia. These two regions are separated by 1,200 km of arid desert that may act as a natural barrier to gene flow, resulting in differentiation by random genetic drift. The genetic differentiation of L. maculans isolates between eastern and Western Australia means that these regions can be treated as different management units, and reinforces the need for widespread disease nurseries in each region to screen breeding lines against a range of genetic and pathogenic populations of L. maculans.     

Survival of Leptosphaeria maculans and associated mycobiota on oilseed rape stubble buried in soil

Leptosphaeria maculans , the causal agent of phoma stem canker on oilseed rape, is an important pathogen in oilseed rape growing regions of the world, including Australia. Survival of L. maculans and associated mycobiota on oilseed rape stubble buried for 13 months in field soil and in sandy soil was studied under South Australian environmental conditions. Stubble weight decreased significantly by the end of the burial period, more so in field (53·7%) than in sandy soil (22%). Pseudothecia did not develop on stubble buried in field soil and few formed when buried in sandy soil. Moist incubation of stubble following retrieval from both media generated pseudothecia; however, pseudothecial development ceased on stubble that had been buried for 10 and 12 months in field and sandy soil, respectively. In total, 20 and 36 genera of fungi were isolated from stubble before and after burial, respectively. Alternaria spp., L. maculans and Stemphylium botryosum were isolated from 81·7, 70 and 60% of stubble pieces before burial, respectively. Isolation frequency of these species decreased significantly throughout the burial period in both media. Conversely, isolation frequency of Stachybotrys chartarum , Fusarium spp. and Trichoderma spp., having pre-burial frequencies of 26·7, 16·7 and 2·5%, respectively, increased over the burial period regardless of soil type. These findings suggest that inoculum production of L. maculans decreases with the increasing burial duration in field soil over 10 months, before ceasing, and this may be due to associated mycobiota.     

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.     

Control of light leaf spot and clubroot in brassica crops using defence elicitors

Plant defence elicitors are compounds that can induce host defence responses against plant pathogens and offer a novel strategy for disease management. Disease control by elicitors can be inconsistent and is often dependent on the crop, the variety and the environment. The use of foliar application of defence elicitors to control light leaf spot (LLS) disease caused by Pyrenopeziza brassicae in the brassica crops winter oilseed rape (WOSR) and Brussel sprouts was evaluated in field trials across multiple years. Elicitor responses in WOSR varied between years. Yield benefits were also inconsistent and did not reflect the level of disease control. Results with Brussel sprouts were more consistent although variation between variety, trial site and year were observed. In particular the salicylic acid analog Acibenzolar-S-Methyl, in the commercial product Bion®, demonstrated good disease control across the field trial sites in the early maturing Brussel sprout variety Cobus. Levels of LLS were consistently reduced when Bion® was alternated within a standard fungicide programme, applied as an individual spray or in combination with other defence elicitors. When applied as a root drench or seed soak Bion® also reduced symptom development of the soil-borne brassica disease clubroot, caused by Plasmodiophora brassicae, in WOSR. These results indicate that defence elicitors such as Bion® can be used as an additional disease management tool alongside host resistance and standard fungicide programmes to protect brassica crops.