Potential for using host resistance to reduce production of pseudothecia and ascospores of Leptosphaeria maculans, the blackleg pathogen of Brassica napus

Pseudothecial density of the blackleg fungus Leptosphaeria maculans and discharge of ascospores was measured from stubble of a range of Brassica species, including Brassica napus (canola) cultivars, with a range of blackleg resistance. Since ascospores are the primary inoculum, these parameters reflect inoculum potential for blackleg. Stubble from a representative line of each of B. carinata , B. nigra , Sinapis alba and B. napus cv. Surpass 400 (incorporates blackleg resistance from B. rapa ssp. sylvestris ) had lower pseudothecial density and discharged fewer ascospores than stubble of other B. napus cultivars (Karoo, Oscar, Emblem, Dunkeld and Columbus). These latter B. napus cultivars and a representative B. juncea line had higher pseudothecial densities and discharged higher numbers of ascospores. If this trait of low blackleg inoculum from stubble could be introgressed into commercial canola cultivars, blackleg disease severity could be substantially reduced, resulting in higher and more stable canola yields. However, the trait of reduced ascospore discharge may not be stable, as demonstrated by the B. rapa ssp. sylvestris -derived resistance already being overcome by the blackleg fungus in Australia.     

Blackleg development and tuber yield in relation to numbers of Erwinia carotovora subsp. atroseptica on seed potatoes

The relationship between number of viable cells of Erwinia carotovora subsp. atroseptica on inoculated potato seed tubers and blackleg development was investigated in 2 years for five cultivars grown in the contrasting climates of Scotland and Israel. Blackleg, and to a lesser extent non-emergence, increased with higher numbers of bacteria on the seed tubers at planting. This relationship was also found for several commercial seed stocks of one cultivar naturally contaminated with different numbers of E. carotovora subsp. atroseptica.The threshold number of bacteria necessary for the development of blackleg declined during the growing season and was also higher for the cultivar Pentland Crown in comparison with the others. In general, yield declined linearly with blackleg incidence and there was a 0.8% reduction in yield for every 1 % blackleg at 13 weeks after planting. Yield loss was positively related to the incidence of blackleg late in the season, whereas the relationship between yield loss and the incidence of non-emergence was poor.     

Identification of genomic regions associated with resistance to blackleg (Leptosphaeria maculans) in canola using genome wide association study

Black rot of crucifers is one of the most important diseases of wild rocket (Diplotaxis tenuifolia L. (D.C.)) caused by the seedborne pathogen Xanthomonas campestris pv. campestris. From 2005, it frequently affected this cultivation in the south of Italy, leading to heavy crop losses. In the present work, we aimed to describe the physiological and molecular characteristics of twenty X. campestris pv. campestris strains isolated from plants and seeds. Ten Xanthomonas spp. strains contaminating seeds were identified on the basis of molecular characterization and in vivo pathogenicity on a discriminating host range. Some of seed-borne isolates were ascribed to the species Xanthomonas campestris pv. raphani and X. campestris pv. incanae, indicating the occurrence of non-host pathogenic Xanthomonas on wild rocket seeds. As well as the presence of pathogenic bacteria, even non-pathogenic Xanthomonas spp. strains were detected on the seeds, underlying the importance of identifying them to evaluate the suitability of lots intended for sowing. A phylogeny using 69 Gyrase B (gyrB) sequences retrieved from the literature, was also carried out, highlighting species relatedness. Overall, this study provides a comprehensive framework for Xanthomonas species affecting wild rocket in Southern Italy.

     

Distribution of lyo- and desmo-glycogen in relation to growth and maturity of proglottids of Moniezia benedeni

Lyo- and desmo-glycogen were estimated in relation to growth and maturity of proglottids of a cestode, Moniezia benedeni. Total and lyo-glycogen show an increasing trend from immature to mature region (i.e., from region 1 to 6) and the increased level seen in the 6th region is maintained up to the 9th region with a fall in the 10th (gravid) region. These results indicate that mature proglottids store more carbohydrate than immature and gravid proglottids. Desmo-glycogen showed no variation in different regions of the strobila. The percentage of lyo-glycogen to total carbohydrate reveals that most of the carbohydrate (85-93%) is stored in a labile form.     

Relating epidemic progress from a general disease model to seasonal appearance time of rusts in the United States: implications for soybean rust

ABSTRACT Soybean rust, Phakopsora pachyrhizi, has been considered a threat to the production of the U.S. soybean, Glycine max. During the past decade, this disease gradually spread to Africa, South America, and recently to the United States. Previous soybean rust risk assessments with an assumption of availability of spores early in a season showed that weather conditions (dew and temperature) during a growing season, in general, are suitable for disease development in U.S. soybean-growing regions. Predicting the time of rust appearance in a field is critical to determining the destructive potential of rusts, including soybean rust. In this study, comparative epidemiology was used to assess likely rust incipient time in four locations within the U.S. Soybean Belt from south to north: Baton Rouge, LA; Charlotte, NC; Indianapolis, IN; and Minneapolis, MN. Temperature effects on the infection cycle of five rusts occurring in the Midwest were evaluated using a general disease model. The likely incipient times were examined with the modeling results. Among the rusts studied, early-appearing rusts had suitable conditions for development earlier in a season. However, a lag period of several weeks to more than 3 months was found from the time when conditions are suitable for a rust to develop or when hosts are available to the time when the rust was detected in fields. Length of the lag period differed among the rust species examined. If nature of long-distance dispersal is not significantly different among the rusts, implications of our study to the expected seasonal soybean rust incipience in fields lead to two possible scenarios: (i) average appearance time of soybean rust across the Soybean Belt should be somewhere between appearance times of common corn rust and southern corn rust, and (ii) with late appearance of the disease, late-planted soybean in the south has greater risk.     

Apple scab: numerical optimization of a new thermal time scale and application for modelling ascospore release in southern France

Adequate protection of apple trees during the primary contamination period is a cornerstone for management of apple scab. Correct timing of spring treatments is fundamental and thus, much effort has been devoted to forecasting ascospore release by Venturia inaequalis. Most models rely on degree‐day accumulation starting from a biofix date established yearly on the basis of biological observations. Here, the potential of using a single calendar date as a biofix and new types of time scales were explored, with the help of numerical optimization with field‐collected data. Using data acquired between 1996 and 2008, the daily rate of development for V. inaequalis primary inoculum was assessed by fitting generic time scale functions, a method that requires the smallest number of assumptions about the effect of temperature on the biological phenomenon. An optimal calendar biofix was established for Provence and use of non‐linear functions relating pseudothecial development rate to temperature for accumulating thermal time was compared with the usual linear response in standard degree‐day models. A model was then constructed using four additional years of data for validation. The predictive value of the model was further improved by adjusting the time scale with ‘accelerating rules’ to take into account the positive influence of rainy days on pseudothecial maturation prior to ascospore release. However, ‘halting rules’ inserted in the time scale to account for dry days during the ascospore release period strongly reduced the predictive value of the model for southern France, suggesting the possible occurrence of strains adapted to dry conditions.     

Inheritance of resistance to Bt canola in a field-derived population of Plutella xylostella

Crops expressing Bacillus thuringiensis (Bt) insecticidal Cry proteins are grown on millions of hectares. Recommendations to delay resistance are based on a high expression/refugia strategy that aims to kill resistant heterozygotes and enable some susceptible insects to survive. Leaf-dip bioassays on F1 crosses of Malaysian populations of diamondback moth (Plutella xylostella (L)) showed that CrylAc resistance was not fully recessive. The survival of ca 50% of heterozygotes on Bt canola (Brassica napus L) leaves expressing low concentrations of CrylAc agreed with a non-fully-recessive model for resistance. Extrapolations based on log dose-logit mortality regressions for heterozygotes using leaf-dip bioassays showed that a relatively high level of expression, of ca 2000 ng CrylAc mg(-1) total leaf protein, would be required to give 90% mortality to heterozygotes. If high enough levels of expression of Bt toxin to kill heterozygotes cannot be achieved and maintained under field conditions, the effectiveness of the high-dose/refugia strategy would be reduced.     

Control of blackleg and tuber soft rot of potato caused by Pectobacterium and Dickeya species: a review

This paper briefly reviews research on the causative agents of blackleg and soft rot diseases of potato, namely Pectobacterium and Dickeya species, and the disease syndrome, including epidemiological and aetiological aspects. It critically evaluates control methods used in practice based on the avoidance of the contamination of plants, in particular the use of seed testing programmes and the application of hygienic procedures during crop production. It considers the perspective of breeding and genetic modification to introduce resistance. It also evaluates the application of physical and chemical tuber treatments to reduce inoculum load and examines the possibility of biocontrol using antagonistic bacteria and bacteriophages.     

Development and evaluation of a model for predicting Lolium rigidum emergence in winter cereal crops in the Mediterranean area

Lolium rigidum is an extremely competitive and prevalent grass weed in cereal fields of Mediterranean areas. The proper timing of control measures is a prerequisite to maximising herbicide efficacy, in terms of both improved control and reduced herbicide inputs. The development of models to predict emergence flushes will contribute to this goal. Pooled cumulative emergence data obtained during three seasons from a cereal field were used to develop a Gompertz model. This explained relative seedling emergence from crop sowing onwards as a function of: (i) standard soil thermal time accumulation (TT) with a base temperature of 1.8°C and (ii) soil thermal time accumulation corrected for soil moisture (cTT). For the latter, no thermal time accumulation was computed for days in which the soil water balance within the upper 10‐cm soil layer indicated no water available for plants, because evapotranspiration was greater than rainfall plus the stored water remaining from the previous day. The model was validated with six datasets from four different sites and seasons. Compared with TT, the model based on cTT showed better performance in predicting L. rigidum emergence, particularly in predicting the end of emergence. Complemented with in‐field observations to minimise deviations, the model may be used as a predictive tool to better control this weed in dryland cereal fields of Mediterranean climate areas.     

In vitro evaluation of fungal isolates for their ability to influence leaf rheology,production of pseudothecia,and ascospores of Venturia inaequalis

As part of an ongoing research project on biological control of apple scab, this study presents a novel approach for the in vitro selection of potential antagonists of the saprophytic phase of Venturia inaequalis. A collection of forty-two fungal isolates were tested for their in vitro ability to degrade apple leaf tissue, inhibit pseudothecia, and ascospore production. The inhibition of ascospore production cannot always be linked reliably with leaf degradation or the evaluation of pseudothecia production. Consequently, ascospore production was retained as the most useful screening parameter. Six isolates proved to significantly reduce the ascospore production of Venturia inaequalis. Two were as effective as Athelia bombacina, a previously reported antagonist of pseudothecia formation and inhibited over 98% of the ascospore production. These new organisms are now available for future field tests. Future selections from a large collection of fungal and bacterial saprophytes can now be based on a reliable and simple in vitro screening method.     

Development of a forecasting model for scald,Rhynchosporium secalis,on rye in relation to climatic conditions1

In a large-scale forecasting project financed by the German Ministry of Agriculture, in collaboration with the National Plant Protection Service, data for modelling forecasting systems in field and horticultural crops are obtained and evaluated. Using climatic records from automatic weather stations and weekly disease assessments in the field, a practical model for forecasting Rhynchosporium secalis on rye has been established with a discriminant function in which temperature, leaf wetness and a disease index during the infection period were used as classification variables. This function decides on application of fungicide sprays.     

Distribution of tridemorph and its metabolites in barley plants grown to maturity

The distribution of tridemorph residues in previously treated mature barley plants has been determined using a specific g.l.c. method. Tridemorph residues were found in grain, awns, leaves, rachis and stems, the amount of residue in the grain appearing to be variety-specific.     

Pesticide paddy field model (PADDY) for predicting pesticide concentrations in water and soil in paddy fields

To evaluate the fate of pesticides in paddy fields, the pesticide paddy field model (PADDY) has been developed for predicting pesticide concentrations in paddy fields and the run-off amount of pesticides to the aquatic environment. This model focused particularly on granule formulation because these formulations have been used widely as herbicides on paddy fields in Japan. The behavior of pesticides in paddy fields was assessed by considering the main processes on the basis of a compartment system and the mass-balance equations of pesticides in the compartments were derived from kinetic data. The mathematical model, PADDY, was constructed by numerical solution techniques. A method for measuring the pesticide parameters for this model was also developed. To validate the model, a field experiment was carried out on a paddy field and the concentration changes of pesticides in water and soil were measured. These were in reasonably good agreement with those predicted by PADDY. © 1999 Society of Chemical Industry     

Incidence of apothecia of Tapesia yallundae at setaside sites in England and sensitivity of the ascospore offspring to the fungicides benomyl and prochloraz

A total of 45 field sites in England were surveyed once for the presence of apothecia of Tapesia yallundae from 1992 to 1994. Apothecia were found at 21 locations and were mainly present on less than 3% of stems. However, apothecia were found on 1532% of stems at four sites. Analysis of the growth characteristics of ascospore isolates from seven sites showed that most produced colonies characteristic of the W-type of T. yallundae , with only one site yielding the R-type, Most ascospore isolates were resistant to the fungicide benomyl and effectively all remained sensitive to prochloraz. The results of the study are discussed in relation to the fungicidal control and epidemiology of T. yallundae , and the risk of spread of disease from set-aside sites.     

Methods for detection of the blackleg,ring rot and gangrene pathogens in potato nuclear stock mother tubers and plantlets1

Methods are described for the detection of latent infection by Erwinia carotovora ssp. atroseptica (blackleg and soft rot), Corynebacterium sepedonicum (ring rot) and Phoma exigua vat. foveata (gangrene) which are regarded as the most important tuber-borne pathogens of potato. The methods comprise selective media for E. carotovora ssp. atroseptica and P. exigua var. foveata, indirect fluorescence antibody staining for E. carotovora ssp. atroseptica and C. sepedonicum, and the eggplant test for C. sepedonicum and P. exigua var. foveata. These are used in Northern Ireland to detect pathogen contamination in nuclear stock mother tubers prior to micropropagation. Marked reductions in disease levels of blackleg have been observed in present pre-basic stocks raised by micropropagation compared with those previously propagated from stem cuttings. It was found that Pseudomonas stutzeri, a saprophytic contaminant bacterium, can pass through the micropropagation process in a latent form.     

Epidemiology of Blackleg (Leptosphaeria maculans) of Canola (Brassica napus) in Relation to Maturation of Pseudothecia and Discharge of Ascospores in Western Australia

ABSTRACT The timing of maturation of pseudothecia and discharge of ascospores of the blackleg fungus (Leptosphaeria maculans) is critical in relation to infection early in the cropping season of canola. During 1998 to 2000, development of pseudothecia was investigated on residues of the previous year's canola crop collected from four agroclimatically different locations: Mount Barker (southern high rainfall), Wongan Hills (central medium rainfall), Merredin (central low rainfall), and East Chapman (northern low rainfall) in Western Australia. The pseudothecia matured on residues at different times after harvest in various regions. In general, pseudothecia maturity occurred earlier in the high-rainfall areas than in medium- and low-rainfall areas. An ascospore discharge pattern was investigated from residues of crop from the previous year (6-month-old residues) at three locations-Mount Barker, Wongan Hills, and East Chapman in Western Australia-and from 18-month-old residues that were burnt and raked in the previous year at Mount Barker and East Chapman. Ascospore discharge commenced earlier in high-rainfall (>450 mm) areas (Mount Barker) and late in northern low-rainfall (<325 mm) areas (East Chapman). The major ascospore showers took place during May (late autumn) and June (early winter) at Mount Barker and during July and August (mid- to late winter) at East Chapman. The number of ascospores discharged was extremely low at East Chapman compared with Mount Barker. At both locations, the number of ascospores discharged from 18-month-old residues that were raked and burnt in the previous year were only approximately 10% of those discharged from previous year's residues left undisturbed. The discharge of ascospores on any given day was negatively correlated with accumulated temperatures, maximum temperature, evaporation, minimum and maximum soil temperatures, and solar radiation and was positively correlated with the minimum temperature, rain, and minimum relative humidity. This is the first report describing how pseudothecia mature on residues in different rainfall areas in Western Australia, and it potentially can be used in developing a forecasting system to avoid the synchronization of major ascospore showers with the maximum susceptibility period of canola seedlings.     

Microbiological, immunological and molecular methods suitable for commercial detection and quantification of the blackleg pathogen, Erwinia carotovora subsp. atroseptica, on seed potato tubers: a review

Contamination of seed potato tubers by Erwinia carotovora subsp. atroseptica is widespread with the bacteria usually sited superficially in lenticels and suberized wounds. As seed contamination level is related to blackleg incidence, seed health is best assessed by determining the number of cells of E. c. atroseptica per mL of tuber-peel extract. The relative specificity, sensitivity and ease of use of four recently developed microbiological, immunological and molecular methods to detect and/or quantify tuber contamination are discussed in relation to the testing of commercial seed stock. Sensitivities of all four methods are at or below the threshold level for blackleg development (< 10³ cells mL^-1), but there are differences regarding their specificity and ease of use. Three of them allow enumeration of most live cells of the bacterium, using specific monoclonal and polyclonal antibodies against the predominant serogroup I: (a) immunomagnetic separation of E. c. atroseptica before viable count on a selective-diagnostic growth medium, crystal violet pectate, (b) immunofluorescence staining and counting of colonies in pour-plate medium in tissue culture plates and (c) enrichment of the bacterium in peel-extract dilutions directly in microtitre plates prior to DAS-ELISA. In the fourth method, both live and dead cells are detected, but not quantified, by PCR amplification of target sequences using specific primers for E. c. atroseptica regardless of serogroup.     

The reliability of leaf bioassays for predicting disease resistance on fruit: a case study on grapevine resistance to downy and powdery mildew

This study was designed to assess the reliability of grapevine leaf bioassays for predicting disease resistance on fruit in the field. The efficacy of various grapevine quantitative trait loci (QTLs) for conferring resistance to downy and powdery mildew was evaluated in bioassays and in a 2‐year field experiment for downy mildew. The resistance genes studied were inherited from Muscadinia rotundifolia (Rpv1 and Run1) and from American Vitis species through cv. Regent (QTLRgP and QTLRgD). In bioassays, genotypes carrying Run1 blocked powdery mildew development at early stages. Genotypes combining Run1 with QTLRgP displayed no greater level of resistance. For downy mildew, genotypes carrying Rpv1 and/or QTLRgD were more resistant than the susceptible cv. Merlot, and showed a high level of leaf resistance in the field (<10% severity). Disease levels on bunches were much higher than those on leaves, with a high variability between Rpv1 genotypes (1–48%). A Bayesian decision theory framework predicted that an OIV‐452 threshold of 5 in leaf bioassays allowed accurate selection of grapevine genotypes (P = 0·83) with satisfactory disease severity on bunches. Therefore, this study validates that the use of early bioassays on leaves, as currently performed by grapevine breeders, ensures a satisfactory level of resistance to downy mildew of bunches in the field.