Genetic diversity within commercial populations of watercress (Rorippa nasturtium-aquaticum), and between allied Brassicaceae inferred from RAPD-PCR

Commercial productivity of watercress (Rorippa nasturtium-aquaticum) can be adversely affected by the pathogenic crook-root fungus, Spongospora subterranea f.sp. nasturti, and watercress viruses. As there are no effective control measures for these diseases, attempts have been made to breed varieties resistant to the crook-root pathogen. This work has been hindered by a lack of knowledge of the genetic base of commercial watercress, and the genetic distance between watercress and allied Brassicaceae which have been identified as candidates for hybridisation programmes. We measured the diversity within these two groups using the RAPD-PCR fingerprinting technique and analysed the data by both distance methods and principal co-ordinate analysis. Little genetic diversity was found within commercial watercress populations. However, watercress formed a unique cluster genetically distinct from other Rorippa species, but equidistant to Cardamine species. It was placed closer to Barbarea verna. This revised version was published online in August 2006 with corrections to the Cover Date.     

The role of resistance breeding in the intergrated control of downy mildew (Bremia lactucae) in protected lettuce

Summary Over the last 30 years, six resistance alleles (Dm2, Dm3, Dm6, Dm7, Dm11 andDm16) located in two linkage groups, have contributed to the control of downy mildew in lettuce crops grown under protection (glass or polythene) in northern Europe. More recently, an as yet genetically uncharacterised resistance factor, R18, has also begun to assume importance. The occurrence of the various combinations of these resistance alleles that exist in commercial cultivars has been dictated by the pathotypes ofBremia lactucae used in their selection but also restricted by linkage in repulsion. In the UK, a pathotype ofB. lactucae insensitive to phenylamide fungicides, such as metalaxyl, emerged in 1978 and became prevalent throughout lettuce production areas in subsequent years. The specific virulence of this pathotype was identical to the previously described phenylamide sensitive pathotype NL10 and cultivars carryingDm11, Dm16 or R18 were resistant. Consequently, an integrated control strategy based on the utilisation of metalaxyl on cultivars carryingDm11 provided effective control in UK until 1987 when a new phenylamide insensitive pathotype began to cause problems. The specific virulence of this second pathotype, which was first reported in the Netherlands and France, was identical to the previously described phenylamide sensitive pathotype NL15. Cultivars carryingDm6, Dm16 or R18, but notDm11, were resistant to NL15; consequently an appropriate change in the cultivar recommendations for use in the integrated control strategy was successfully promulgated. It is predicted that variations of this integrated control strategy involving the use of appropriately selectedDm gene combinations may prove effective for some time. This prediction is based on studies of the status of the avirulence loci in the two phenylamide insensitive pathotypes and of the specific virulence characteristics of phenylamide sensitive components of the pathogen population.     

Correlation of stylar ribonuclease isoenzymes with incompatibility alleles in apple

Fifty-six cultivars of apple were analysed for stylar ribonucleases; proteins were extracted from styles, separated by non-equilibrium pH gel electrofocusing and stained for activity. Excellent correlation was found between the ribonuclease bands revealed and the 11 known incompatibility, S, alleles, in 14 diploid cultivars genotyped in the classic work of Kobel by monitoring pollen tube growth after test crossing, and in 20 cultivars genotyped, at least partially, by more recent DNA methods. For 12 triploid cultivars studied by Kobel, the correlation was good but not perfect. Two apparent minor electrophoretic variants for S10 were noted and, to distinguish them from each other and also from the electrophoretically similar S3, isoelectric focusing was used. Ten cultivars were genotyped for the first time. In all, 14 ribonuclease bands that may correspond to the ‘new’ S alleles, S12 to S 25, were detected but these alleles should be regarded as provisional until confirmed by pollination tests, especially when the electrophoretic differences were only slight. Analysis of stylar ribonucleases is a convenient method of predicting S alleles in flowering material and thereby investigating incompatibility relationships. The polymorphism of the S locus makes it useful for checking the identity and parentage of cultivars. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modelling and forecasting epidemics of apple powdery mildew (Podosphaera leucotricha)

A model developed to simulate epidemics of powdery mildew on vegetative shoots of apple generates two types of output. Firstly, it forecasts disease severity (percentage of host tissue infected) by incorporating effects on disease development of the amount of healthy susceptible tissue and current infectious (sporulating) disease, the level of initial inoculum (overwintered 'primary' mildew) and weather conditions. The effects of weather variables are considered on only two aspects of the fungal life cycle: initial spore germination and the subsequent development during the incubation period. Secondly, the model generates indices of the relative favourability of weather conditions on disease development by incorporating effects of weather on conidial production/dispersal and germination. On each day, forecasts of the (relative) severity of new infection and total current infectious disease are given for both types of output. The model was evaluated by comparing its predictions with the mildew epidemics observed in two unsprayed orchards over four years. In all the years, the temporal patterns of the predicted and the observed disease were generally similar. The pattern of the disease severity forecasts was marginally closer to the observed than that derived from two weather indices. Potential roles of the model in practical management of apple powdery mildew are discussed.     

Super-races are not likely to dominate a fungal population within a life time of a perennial crop plantation of cultivar mixtures: a simulation study

ABSTRACT: BACKGROUND: Deployment of cultivars with different resistance in mixtures is one means to manage plant diseases and prolong the life of resistance genes. One major concern in adopting mixtures is the development of 'super-races' that can overcome many resistance genes present in the mixture. A stochastic simulation model was developed to study the dynamics of virulence alleles in two-cultivar mixtures of perennial crops, focusing on the effects of cost of virulence and pathogen reproduction mechanism. The simulated mechanism of virulence has characteristics of both major and minor genes. RESULTS: Random genetic drift due to repeated population crashes during the overwintering phase led to fixation of a single fungal genotype (in terms of its virulence), often within 100 seasons. Overall, cost of virulence is most important in determining the virulence dynamics under the present model formulation. With cost of virulence incorporated, nearly all simulation runs ended up with a single fungal genotype that can infect only one of the two cultivars. In absence of cost of virulence, most of the simulation runs ended up with fungal genotypes that can infect both host cultivars but in many cases do not contain the maximum possible number of virulence alleles due to random drift. A minimum of 20% sexual reproduction between strains from different cultivars is necessary to ensure that the final fixed strains are able to infect both cultivars. Although the number of virulence alleles in the final genotype and the time to fixation are affected by simulation factors, most of the variability was among replicate simulation runs (i.e. stochastic in nature). The time to fixation is generally long relative to cropping cycles. CONCLUSIONS: A single fungal genotype will dominate a population due to the bottleneck in overwintering with cost of virulence primarily determining whether the dominant genotype can infect both cultivars. However, the dominate genotype is unlikely to accumulate all the virulence alleles due to genetic drift. The risk of emergence and spread of super-races is insufficiently great to prevent the use of cultivar mixtures of perennial crops as a means to reduce disease development provided that host resistance structure in mixtures is altered every cropping cycle.     

Effects of fruit maturity and wetness on the infection of apple fruit by Neonectria galligena

Neonectria galligena can cause European canker of apple as well as fruit rot. Healthy unwounded fruits on potted trees of cvs Cox, Bramley and Gala were inoculated with conidia of N. galligena to investigate the effects of wetness duration and fruit maturity on rot development. Overall, the incidence of fruit rot was influenced more by fruit maturity at the time of inoculation than by duration of wetness (6–48 h). Young fruit were most susceptible to infection, with 50% of fruit infected when inoculated up to 4 weeks after full bloom. The susceptibility decreased initially until c. 2 months after full bloom and then increased gradually until harvest. Almost all preharvest symptoms (eye rot) developed only on the fruit inoculated up to 4 weeks after full bloom. All other rots were observed after six‐month postharvest storage under controlled atmospheric conditions. However, the relative proportion of preharvest eye rots and postharvest storage rots varied greatly among three years. The effect of wetness duration was only significant for fruit inoculated in their early stages of development but not for those inoculated near harvest. Regression models were developed to describe the observed effects of fruit maturity and wetness on the incidence of total nectria rots.     

Biocontrol agents to manage brown rot disease on cherry

European Journal of Plant Pathology - This review focuses on biological control as a management strategy for brown rot disease on cherry. Specifically, the strain Monilina laxa as much of the...     

Identifying resistance in wild and ornamental cherry towards bacterial canker caused by Pseudomonas syringae

Bacterial canker is a major disease of stone fruits and is a critical limiting factor to sweet cherry (Prunus avium) production worldwide. One important strategy for disease control is the development of resistant varieties. Partial varietal resistance in sweet cherry is discernible using shoot or whole tree inoculations; however, these quantitative differences in resistance are not evident in detached leaf assays. To identify novel sources of resistance to canker, we used a rapid leaf pathogenicity test to screen a range of wild cherry, ornamental Prunus species and sweet cherry × ornamental cherry hybrids with the canker pathogens, Pseudomonas syringae pvs syringae, morsprunorum races 1 and 2, and avii. Several Prunus accessions exhibited limited symptom development following inoculation with each of the pathogens, and this resistance extended to 16 P. syringae strains pathogenic on sweet cherry and plum. Resistance was associated with reduced bacterial multiplication after inoculation, a phenotype similar to that of commercial sweet cherry towards nonhost strains of P. syringae. Progeny resulting from a cross of a resistant ornamental species Prunus incisa with susceptible sweet cherry (P. avium) exhibited resistance indicating it is an inherited trait. Identification of accessions with resistance to the major bacterial canker pathogens is the first step towards characterizing the underlying genetic mechanisms of resistance and introducing these traits into commercial germplasm.