Extended leaf phenology presents an opportunity for herbicidal control of invasive forest shrubs

For many of the shrub species invading temperate deciduous forests, extended leaf phenology contributes substantially to annual carbon gains, helping to make possible rapid growth and spread. We carried out a pair of proof‐of‐concept studies to evaluate the susceptibility of such shrubs to foliar herbicide treatment during the period of delayed senescence, i.e. well after it is typically attempted. We first evaluated leaf‐level physiology in four species that lose leaves late in autumn; photosynthetic rates were comparable (80–111%) to those reported for the same species in summer. While preliminary, this finding provides a strong indication that diverse shrub taxa remain susceptible to control into late autumn. In addition, we conducted a field trial involving one of these species (Lonicera maackii) to directly evaluate the effectiveness of foliar treatment in November; applications included glyphosate and two concentrations of aminocyclopyrachlor plus metsulfuron methyl. Treatments killed (72%) or severely injured (14%) target plants, inducing greater cambial damage in plants that retained greater fractions of their canopy or had smaller canopy widths; there were no statistically significant differences among application types. This study demonstrated that late autumn can be a viable period in which to treat weedy species that delay senescence strongly, such as the many invasive Lonicera species in North America. Given that climate change and urbanisation are further delaying senescence in invasive plant populations, our study serves as a call for further investigation into what promises to be an increasingly viable opportunity for weed control in deciduous forests.     

Fates of Setaria faberi and Abutilon theophrasti seeds in three crop rotation systems

Weed seeds in and on the soil are the primary cause of weed infestations in arable fields. Previous studies have documented reductions in weed seedbanks due to cropping system diversification through extended rotation sequences, but the impacts of different rotation systems on additions to and losses from weed seedbanks remain poorly understood. We conducted an experiment in Iowa, USA, to determine the fates of Setaria faberi and Abutilon theophrasti seeds in 2‐, 3‐ and 4‐year crop rotation systems when seed additions to the soil seedbank were restricted to a single pulse at the initiation of the study. Over the course of the experiment, seedlings were removed as they emerged and prevented from producing new seeds. After 41 months, seed population densities dropped >85% for S. faberi and >65% for A. theophrasti, but differences between rotation systems in the magnitude of seedbank reductions were not detected. Most of the reductions in seedbank densities took place from autumn through early spring in the first 5 months following seed deposition, before seedling emergence occurred, suggesting that seed predation and/or seed decay was important. For S. faberi, total cumulative seedling emergence and total seed mortality did not differ between rotation systems. In contrast, for A. theophrasti, seedling emergence was 71% lower and seed mortality was 83% greater in the 3‐ and 4‐year rotation systems than in the 2‐year system. Results of this study indicate that for certain weed species, such as A. theophrasti, crop rotation systems can strongly affect life‐history processes associated with soil seedbanks.     

Germination ecology of seeds of the annual weeds Capsella bursa-pastoris and Descurainia sophia originating from high northern latitudes

Low temperatures may inhibit dormancy break in seeds of winter annuals, therefore it was hypothesized that seeds of Capsella bursa‐pastoris and Descurainia sophia that mature at high latitudes in late summer–early autumn would not germinate until they had been exposed to high summer temperatures. Consequently, germination would be delayed until the second autumn. Most freshly matured seeds of both species collected in August and September in southern Sweden were dormant. After 3 weeks of burial at simulated August (20/10°C) and September (15/6°C) temperatures, 28 and 27%, respectively, of the C. bursa‐pastoris and 56 and 59%, respectively, of the D. sophia seeds germinated in light at 15/6°C. In contrast, in germination phenology studies conducted in Sweden, only a few seeds of either species germinated during the first autumn following dispersal. However, there was a peak of germination of both species the following spring, demonstrating that dormancy was lost during exposure to the low habitat temperatures between late summer and early autumn and spring. Nearly 100% of the seeds of both species subjected to simulated annual seasonal temperature changes were viable after 30.5 months of burial. In the burial study, exhumed seeds of C. bursa‐pastoris were capable of germinating to 98–100% in light at the simulated spring–autumn temperature regime (15/6°C) in both spring and autumn, while those of D. sophia did so only in autumn. In early spring, however, seeds of D. sophia germinated to 17–50% at 15/6°C. Thus, most seeds of these two annual weeds that mature in late summer do not germinate in the first autumn, but they may do so the following spring or in some subsequent autumn or spring.     

Phenological dynamics of the invasive plant Acacia longifolia in Portugal

Phenological shifts may play an important role in the success of invasive species. In the coastal regions of continental Portugal, Acacia longifolia is one of the most widespread invasive plants, but there are significant gaps in our understanding and interpretation of its phenology. This study is the first to investigate the phenological variation of A. longifolia and its association with climate in populations representing a temperature and precipitation gradient in mainland Portugal. The results highlighted significant variations in the vegetative (leaf initiation) and reproductive phenophases (bud formation, flowering and fruiting) among populations. Overall, leafing was observed throughout the year, bud formation occurred between summer and spring, anthesis between late winter and the beginning of spring, and fruiting started in late winter. Most of the reproductive phenophases varied across elevation or latitudinal gradients, with populations that were subjected to more stressful climatic conditions advancing their flowering and fruiting phenophases. The occurrence of each phenophase was strongly associated with the climatic conditions in the previous 5–12 months, suggesting that plants receive their phenological cues well in advance of their phenological response. Among climatic factors, temperature and irradiance were the most significant predictors of the phenological cycle of A. longifolia, but precipitation also exerted a greater influence on budding and flowering of the species. The phenological response of A. longifolia in a variety of sites represents an important step towards understanding how its phenology may react to the projected climate change in the Mediterranean region.     

The Epidemiology of Purple Leaf Blotch on Leeks in Victoria, Australia

The incidence of purple leaf blotch disease was investigated on seven successive commercial leek crops grown at Cranbourne, Victoria between 1996 and 1997. First symptoms occurred on older leaves, 54–69 days after transplanting. Lesions with typical symptoms were colonised by either Alternaria porri (6%), Stemphylium vesicarium (42%) or mixtures of both pathogens (52%). Purple leaf blotch was caused by a disease complex and was endemic at nobreak Cranbourne due to the continuous cropping of leeks. Disease incidence in all monitored crops increased as plants matured (123–158 days after transplanting) until harvest but never exceeded 11% due to fortnightly applications of mancozeb. Disease levels showed no significant correlation with weekly temperature, precipitation, relative humidity or leaf wetness duration. Disease levels were significantly (P < 0.05) higher on autumn/winter (May/June) 1997 crops when 38 periods of leaf wetness 8 h because of dew and low temperatures (10–13 °C). The weekly rate of increase of disease incidence was significantly (P < 0.01) correlated with days after transplantation. nobreak Concentrations of airborne A. porri and S. vesicarium conidia within leek crops showed a diurnal periodicity and maximum numbers were trapped between 11:00 and 15:00 h. The concentration of airborne S. vesicarium conidia was three to six times the concentration of airborne A. porri conidia. Conidia were more abundant during spring/summer (September–February). Ascospores of Pleospora allii were found during May–September. The greater concentrations of airborne S. vesicarium conidia suggest that it may be the dominant pathogen in the purple leaf blight complex. Fungicide sprays were unnecessary until 8–10 weeks after transplanting, and regular protectant sprays curtailed but did not eradicate purple leaf blight. The results indicated that predictive models, based on temperature and the frequency of leaf wetness periods 8 h, will assist in reducing fungicide inputs as plants mature and, in southern Victoria, fungicide applications on leeks should be timed for autumn/winter when infection periods occur.     

The ecological life cycle of Thlaspi perfoliatum and a comparison with published studies on Thlaspi arvense

Thlaspi perfoliatum L. is an introduced winter annual that grows in waste places, but it is not a troublesome agricultural weed. Seeds are dormant at maturity in spring, afterripen during summer and germinate in autumn. Non-dormant seeds that fail to germinate in autumn are induced into secondary dormancy by low winter temperatures and cannot germinate the following spring. These seeds afterripen during summer and germinate in autumn if conditions are suitable. Vernalization is not an absolute requirement for flowering. However, it shortens the time to flowering, and vernalized plants produce many more flowers and seeds than non-vernalized plants. Results from studies on seed germination and flowering of T. perfoliatum are compared to published studies on the life cycle of Thlaspi arvense L., an introduced species that is a serious agricultural weed which behaves both as a winter and a summer annual.     

Histological characterization of the early-stage infection events of Setosphaeria turcica in maize

Northern corn leaf blight (NCLB) caused by Setosphaeria turcica is a major foliar disease of maize. The early-stage infection events of this pathogen on maize leaves are unclear. We investigated the optimum temperature for conidial germination and appressorium formation, and characterized penetration and growth of S. turcica in maize leaf sheath and onion epidermis cells, including use of histological staining to assess plant cell viability. The results showed that the optimum temperature for conidial germination and appressorium formation was 20°C. On the maize leaf sheath, the appressoria were formed by germinated conidia, and penetration on the epidermal cells occurred at 8 h postinoculation (hpi). Round vesicles developed beneath the appressoria. Between 16 and 24 hpi, the branched invasive hyphae invaded three to five adjacent cells at most infection sites. The invasive hyphae tended to move along the cell wall and crossed from one cell to another. In the onion epidermis cells, the appressoria formed at 8 hpi, and in most cases the epidermal cells were penetrated through the juncture of the cell walls. At 16–24 hpi, the primary hyphal terminus swelled to a vesicle. The maize leaf sheath cells died at 8 hpi, whereas the onion cells did not. Our findings documented in detail the penetration and invasive hyphal growth in maize leaf sheath and onion epidermis, as well as viability of plant cells, at the early stages of infection, and provide a foundation for elucidating the underlying mechanism of S. turcica–maize interactions.     

Seasonal emergence of weeds in cultivated soil in New Zealand

Patterns of seedling emergence of ten weed species from soil cultivated at intervals of approximately one month are described. All species examined showed clearly defined emergence patterns. Polygonum persicaria L. seedlings emerged in spring and early summer. Seedlings of Chenopodium album agg., Portulaca oleracea L. and Solanum nigrum L. emerged in late spring, summer and early autumn.Plantago major L., Rumex spp. (mostly R. obtusifolius L.) and Trifolium spp. (mostly T. repens L.) emerged mostly in spring and early summer. Coronopus didymus (L.) Sm. and Juncus bufonius L. tended to emerge in both autumn and spring.Veronica persica Poir. emerged in spring, summer and autumn. Close similarity between emergence patterns in New Zealand and northern Europe, despite differences in rainfall and temperature, suggests that annual dormancy:nondormancy cycles of seeds buried in the soil are largely responsible for seedling emergence patterns. In New Zealand, seedling emergence tended to be more spread out than in Europe, although seasonal patterns were still distinct. Further work on dormancy cycles in these species would be useful, as would a comparison of the factors inducing and breaking dormancy of comparable seed populations in northern and southern Europe and New Zealand. When seeds were left buried for several years before being encouraged to germinate, seedling emergence patterns tended to be of smaller amplitude, although the overall patterns were still very similar.     

Ophraella communa,the ragweed leaf beetle,has successfully landed in Europe: fortunate coincidence or threat?

We report the occurrence of the North American ragweed leaf beetle Ophraella communa in Europe. During our surveys to monitor populations of the invasive alien plant Ambrosia artemisiifolia in Europe, we found the beetle south of the Alps, in more than 130 sites in southern Switzerland (Ticino) and northern Italy (Lombardia, Piemonte and Emilia‐Romagna). At sites where O. communa was present, up to 100% of the plants were attacked with damage levels high enough to completely defoliate and prevent flowering and seed set of most ragweed plants. That in its first year of discovery, O. communa was already found over a large area of c. 20 000 km² and in all habitat types occupied by A. artemisiifolia reflects its great dispersal potential and wide habitat suitability. This oligophagous beetle is a successful biological control agent against A. artemisiifolia in China, but despite extensive host specificity tests, the risk of attack and the level of damage of sunflower under field conditions remain unclear. The recently launched COST Action on ‘Sustainable management of Ambrosia artemisiifolia in Europe (SMARTER)’ offers an ideal framework to respond quickly to the recent establishment of O. communa in Europe and to collect data that can help determine whether this event should be considered a troublesome introduction or whether it is likely to become the first case of a successful biological control of an invasive weed in continental Europe.     

表面活性剂、喷液量及杂草叶角对普杀特喷后附着量的影响

表面活性剂、喷液量及杂草叶角对普杀特在非敏感杂草马唐叶片上的喷后附着量有明显的独立影响和交互影响,其中表面活性剂的影响最大。供试的6种非离子型表面活性剂均因显著降低了普杀特药液的表面张力从而显著提高了其在马唐叶片上的喷后附着量,其中ScOil和Silwet—L77对其提高幅度最大;普杀特在马唐叶片上的喷后附着量随着喷液量的减少逐步增加,但在0—90°范围内随着马唐叶角的减小而减少,其由于喷液量的增大或马唐叶角的减小而引起的下降量,可通过加入非离子型表面活性剂而得到补偿,加入非离子型表面活性剂配合下午微液量(90L/ha)喷施,可望最大限度地增加普杀特在其非敏感杂草叶片上的喷后附着量,进而提高其药效,降低其使用成本和减少其在土壤中的残留。     

Relating plant and pathogen development to optimise fungicide control of phoma stem canker (<Emphasis Type="Italic">Leptosphaeria maculans</Emphasis>) on winter oilseed rape (<Emphasis Type="Italic">Brassica</Emphasis><Emphasis Type="Italic">napus</Emphasis>)

In winter oilseed rape experiments at Rothamsted in 2000/01 to 2002/03 growing seasons, the severity of phoma stem canker epidemics in summer depended on the timing of phoma leaf spot epidemics in the previous autumn, and hence on the timing of Leptosphaeria maculans ascospore release. The first major release of L. maculans ascospores was earlier in 2000 (26 September) and 2001 (18 September) than in 2002 (21 October). Consequently, the autumn phoma leaf spot epidemic was also earlier in 2000 and 2001 than in 2002. The resulting stem canker epidemics were severe by harvest (July) in 2001 and 2002 but not in 2003. No correlation was found between the severity or duration of phoma leaf spotting (lesion days or lesion °C-days) and the subsequent severity of phoma stem canker epidemics. Rates of leaf production and loss were similar in the three growing seasons. Out of ca. 25 leaves produced on plants during each season, leaf numbers 10–14 generally remained on plants for the longest. Treatment with flusilazole + carbendazim in autumn decreased the severity of phoma leaf spotting for several weeks after treatment, decreased the severity of stem canker the following summer and increased yield significantly in 2001 and 2002 but not in 2003. The most effective timings for flusilazole + carbendazim application were when leaves 7–11 were present on most plants and at least 10% of plants were affected by phoma leaf spot. Two half-dose applications of fungicide reduced phoma stem canker and increased yield more than a single full dose application when phoma leaf spot epidemics were early (<800 °C-days after sowing).     

Morphological variation of Solanum elaeagnifolium in south‐eastern Australia

Solanum elaeagnifolium (silverleaf nightshade) is an invasive perennial weed in Australia, with aerial growth commencing in spring from either the perennial root system or the soil seedbank, with senescence occurring in autumn. A total of 642 S. elaeagnifolium individuals were collected at flowering from 92 locations in south‐eastern Australia to study morphological variation and its implications for management. Large morphological variation was found between individuals from different locations. Leaf length, width and area ranged from 1.44 to 10.6 cm, 0.39 to 4.09 cm and 0.41 to 25.8 cm² respectively. Plants from higher rainfall regions were significantly taller and had larger leaves, suggesting a possible correlation between rainfall and morphology. Scanning electron microscopy comparison of leaf surfaces showed lower trichome and stomatal densities on the adaxial surface (67.0 ± 3.3 trichomes mm^?2 and 603.4 ± 29.2 stomata mm^?2 respectively) than on the abaxial surface (131.9 ± 7.2 trichomes mm^?2 and 813.7 ± 30.5 stomata mm^?2 respectively). The morphological plasticity of S. elaeagnifolium highlighted in this study could probably contribute to its adaptability and partly explain its establishment and continuing expansion in Australia.     

Sprouting capacity from intact root systems of Cirsium arvense and Sonchus arvensis decrease in autumn

Perennial weeds are often controlled by mechanical means, which aim at stimulating axillary and adventitious buds to sprout. This happens when the apical dominance of the main shoot is removed by defoliation or when the underground system is fragmented. By repeating the measures, the result is a depletion of storage compounds, which weakens the plants and reduces their capacity to grow and reproduce. However, timing is critical. Earlier research has indicated that emergence from fragments of Sonchus arvensis cease during a period in autumn, while the seasonal pattern of sprouting in Cirsium arvense appears to be inconsistent. We studied the emergence pattern of defoliated plants with undisturbed root systems, from late summer to early spring. Potted plants grown outdoors were exhumed at regular intervals, put under forcing conditions for 4 weeks, after which shoots above and below soil level were counted and weighed together with the remaining root systems. In both species, the number and weight of emerged shoots decreased during a period in the autumn. In C. arvense, underground shoots were constantly produced during the same period, while fewer underground shoots were present in S. arvensis. For the latter species, apical dominance does not fully explain the effect; thus, endodormancy might be involved. Root weight increased until withering and did not explain the lack of emergence. Our results suggest an impaired sprouting capacity of undisturbed root systems of C. arvense and S. arvensis during September–October, which has implications for the timing and method of control of these species.     

Temperature at the early stages of Clavibacter michiganensis subsp. michiganensis infection affects bacterial canker development and virulence gene expression

Clavibacter michiganensis subsp. michiganensis (Cmm), the causal agent of bacterial canker and wilt, causes severe economic losses in tomato net‐houses and greenhouses worldwide. In this study, seedlings which were transplanted and inoculated monthly over 2 years wilted and died earlier in the spring (21–24°C) and autumn (18–23°C) than in the winter (15–18°C) and summer (28–31°C): T₅₀ (the time taken for 50% of the plants to wilt or die) was 2 and 3–4 months after inoculation, respectively. A highly significant correlation was found between the average temperatures during the first month after inoculation and T₅₀; the shortest T₅₀ mortality (70 days) was observed for an average temperature of 26°C. Expression of virulence genes (pat‐1, celA, chpC and ppaA) by Cmm was higher in plants inoculated in the spring than in those inoculated in the summer. In another set of experiments, seedlings were inoculated and maintained in controlled‐environment growth chambers for 2 weeks. Subsequently, they were transplanted and maintained in commercial‐type greenhouses for 4–5 months. The temperatures prevailing in the first 48 h after inoculation were found to affect Cmm population size and virulence gene expression and to have season‐long effects on bacterial canker development.     

Control of Elymus repens by rhizome fragmentation and repeated mowing in a newly established white clover sward

Control of perennial weeds, such as Elymus repens, generally requires herbicides or intensive tillage. Alternative methods, such as mowing and competition from subsidiary crops, provide less efficient control. Fragmenting the rhizomes, with minimal soil disturbance and damage to the main crop, could potentially increase the efficacy and consistency of such control methods. This study's aim was to investigate whether fragmenting the rhizomes and mowing enhance the control of E. repens in a white clover sward. Six field experiments were conducted in 2012 and 2013 in Uppsala, Sweden, and Ås, Norway. The effect of cutting slits in the soil using a flat spade in a 10 × 10 cm or 20 × 20 cm grid and the effect of repeated mowing were investigated. Treatments were performed either during summer in a spring‐sown white clover sward (three experiments) or during autumn, post‐cereal harvest, in an under‐sown white clover sward (three experiments). When performed in autumn, rhizome fragmentation and mowing reduced E. repens shoot biomass, but not rhizome biomass or shoot number. In contrast, when performed in early summer, rhizome fragmentation also reduced the E. repens rhizome biomass by up to 60%, and repeated mowing reduced it by up to 95%. The combination of the two factors appeared to be additive. Seasonal differences in treatment effects may be due to rhizomes having fewer stored resources in spring than in early autumn. We conclude that rhizome fragmentation in a growing white clover sward could reduce the amount of E. repens rhizomes and that repeated mowing is an effective control method, but that great seasonal variation exists.     

Biology,distribution and impacts of silverleaf nightshade (Solanum elaeagnifolium Cav.)*

Silverleaf nightshade, Solanum elaeagnifolium, is listed as a noxious weed in its native range (Americas) and as an invasive alien plant in many countries across the world. Its local pattern of distribution in the EPPO region indicates that it is still in an establishment phase. S. elaeagnifolium invasiveness could be related to several biological traits: copious production of sexual and asexual propagules, its facility for long‐distance dispersion, its ability to endure considerable drought and to dominate shallow‐rooted vegetation, especially during summer dry periods, and its capacity to suppress more valuable species because it is unpalatable and toxic to cattle. Invasion of the Mediterranean semi‐arid region by S. elaeagnifolium was mainly facilitated by irrigation and animal production. Low moisture stress may favour this invasive species and sheep are an important pathway for its long‐distance dispersal. Once established, it is one of the most difficult weeds to eradicate because of its network of creeping horizontal and deep vertical roots. Its phenotypic plasticity enables it to adopt a rosette‐like growth pattern to escape control by repeated slashing. Thus, S. elaeagnifolium monitoring in the EPPO region is vital in order to contain established populations and prevent invasion.     

Seasonal variation in the emergence of annual weeds — an introductory investigation in Sweden

Seeds of thirteen annual weed species were collected in early autumn. Within a few weeks they were incorporated in soil. They were stored either in the field or in climate chambers where seasonal variations in field soil temperature were simulated. Seedling emergence under various conditions was determined at different times, within a period of 1 or 2 years after the incorporation of seeds in soil. Some material was transferred from the storage places at different times of year and placed in cabinets with standardized temperature regimes for 3-week emergence tests. These showed increased emergence during late autumn and winter and a maximum at some time in winter in both summer and winter annuals. Some winter annuals showed a second maximum in late summer. Counts of seedlings in the storage places, in unstirred soil, showed maximum emergence in spring for both winter and summer annuals — or in summer for Spergula arvensis and, possibly, Capsella bursa-pastoris. In certain treatments in the field, the soil was stirred once, on different dates, simulating tillage at different limes of year. The emergence following stirring of the soil was greatest in spring for the summer annuals, except for Spergula arvensis, which showed more emergence from early to mid summer. Several winter annuals showed an emergence peak after stirring in late summer or early autumn. Variations saisonnières dans la levèe des mauvaises herbes annuelles-étude préliminaire en Suède     

Identification and characterization of Xanthomonas albilineans causing sugarcane leaf scald in China using multilocus sequence analysis

Xanthomonas albilineans is the causal agent of leaf scald, a disease that can cause considerable damage to sugarcane industries. This study analysed the phylogenetic relationship of 14 samples of X. albilineans from China and 13 reference strains retrieved from the GenBank database by multilocus sequence analysis (MLSA). To reach this goal, five housekeeping genes of X. albilineans were amplified from diseased leaves and sequenced: gyrB, abc, rpoD, atpD and glnA. Based on the concatenated sequence of these genes (4473 nt), the 14 samples of X. albilineans from China had 99.9–100% sequence identity with one another and with five strains of the pathogen from the French West Indies and the USA (Florida). The 27 samples or strains of X. albilineans were distributed in two distinct clades in the MLSA-based phylogenetic tree. Clade 1 was formed by four strains of the pathogen from Fiji, Papua New Guinea and the USA. All the other strains from worldwide locations, including the 14 samples from China, were grouped in clade 2. This latter clade included all strains of the pathogen that were associated with outbreaks of leaf scald that have occurred over the last two decades, especially in the Caribbean islands and the USA. The very low diversity of X. albilineans in four Chinese provinces suggests recent spread of a single strain (from genetic group PFGE-B) of the leaf scald pathogen within China.     

Olive scab: a review1

Olive scab or leaf spot, caused by the fungus Spilocaea oleagina, is widespread in the Mediterranean region. Losses arise mostly from defoliation of severely infected trees, with consequently reduced yield. Symptoms are mainly confined to leaves and appear as dark brown, circular, zonate spots surrounded by yellow haloes (‘peacocks eye’). S. oleagina shows a typical subcuticular growth, forming flat colonies within the cutinized layer of the thick epidermal cell wall. This habit has been associated with a defence reaction of the host involving mobilization and breakdown of the phenolic glucoside oleuropin and inhibition of pectolytic enzymes produced by the pathogen. The disease is particularly severe in densely planted groves of susceptible olive cultivars and in nurseries. Infections may occur throughout the year, except during hot and dry summers, when favourable temperatures (opt. 16–21°C) and rains occur. Conidia, formed at the apex of short ampulliform conidiophores, are usually carried by rain droplets, but recent data show that humid air currents and insects also contribute to limited aerial dissemination. Usually, the incubation period is about 2 weeks; however, if infection is followed by a hot season, it may last several weeks. Spots already formed in spring may stop growing in summer and resume their growth and sporulation in autumn. Chemical control schedules include fungicide (especially copper) treatments during the main infection seasons (spring and autumn).     

Monitoring of plant and airborne inoculum of Sclerotinia sclerotiorum in spring oilseed rape using real‐time PCR

Sclerotinia stem rot of spring oilseed rape (Brassica napus) is caused by Sclerotinia sclerotiorum. In Sweden, the disease leads to severe crop damage that varies from year to year. A real‐time PCR assay was developed and used to determine the incidence of S. sclerotiorum DNA on petals and leaves of spring oilseed rape as well as in air samples, with the aim of finding tools to improve precision in disease risk assessment. Five field experiments were conducted from 2008 to 2010 to detect and study pathogen development. Assessments of stem rot showed significant differences between experimental sites. The real‐time PCR assay proved fast and sensitive and the relationship between percentage of infected petals determined using a conventional agar test and the PCR assay was linear (R² > 0·76). There were significant differences in S. sclerotiorum incidence at different stages of flowering. The incidence of S. sclerotiorum DNA on the leaves varied (0–100%), with significantly higher incidence on leaves at lower levels. In one field experiment, S. sclerotiorum DNA was not detected on petals during flowering, whereas the pathogen was detected on leaves, with a corresponding stem rot incidence of 7%. The amount of S. sclerotiorum DNA in sampled air revealed that spore release did not coincide with flowering on that experimental site. Thus, using a real‐time PCR assay to determine the incidence of S. sclerotiorum on oilseed rape leaves, rather than on petals, could potentially improve disease risk assessment.