The effect of cane management techniques on raspberry yellow rust (Phragmidium rubi-idaei)

The effects of various cane removal treatments on the epidemiology of Phragmidium rubi-idaei were studied in four experiments in infected raspberry plantations in Wales and Scotland. In 1981, removal of 15 cm high young canes either by cutting or by spraying dinoseb-in-oil reduced infection of a second flush of canes (replacement canes) compared to that of the first flush of canes on untreated plots of cv. Mailing Delight in Wales and of cv. Glen Clova in Scotland. In 1982, removal of 20 cm high young canes reduced infection of cv. Mailing Delight in Wales but in Scotland only removal of 60 cm high canes by cutting reduced infection of replacement canes. Removal of all fruiting canes and old cane stubs reduced rust infection of young canes, but removal of successive flushes of replacement canes did not reduce infection of lateral shoots on the fruiting canes. Dinoseb-in-oil applied to the bases of fruiting canes before emergence of young canes had no effect on infection of young canes in Wales or Scotland. The production of basidiospores was inhibited in vitro by dinoseb-in-oil at concentrations over 1.0 μg a.i./ml.     

An interpretation of the frequencies of host specific phenotypes of Phytophthora infestans in North Wales

Race surveys of Phytophthora infestans over 4 years in North Wales have shown that races unselected by host resistance are frequent. In most cases frequencies of complex races are those expected from the frequencies of simple race characters if these assort at random. These results suggest that recombination between races may be unrestricted. The significance of unexpected deviations from the predicted frequencies are discussed.     

Phenotype-phenotype analysis: field application of the gene-for-gene hypothesis in host-pathogen relations

The frequency of occurrence of a virulence phenotype in a pathogen population is equivalent to its probability of occurrence in that population. From the compound probability law it can thus be shown that the expected frequency of occurrence of phenotypes with combined virulences is a product of the observed frequencies of the individual virulences. In many host-pathogen interactions, observed and expected frequencies of combined virulences are closely similar. Deviations from expectation, in both directions, do occur however, and these may be of crucial importance in disease control.     

Stem Lesions, Canker and Dieback of Skimmia japonica

Stem lesions, cankers and dieback of shoots were common on bothyoung and mature plants of Skimmia japonica following the exceptionallyhot and dry summer of 1976. Phoma macrostoma var. macrostoma, Phomopsis skimmiae, Fusariumlateritium and a species of Diplodia were regularly isolatedfrom necrotic tissue. P. macrostoma var. macrostoma was isolatedmost frequently and produced more necrosis and larger cankersthan other species in inoculated plants. All four species, however,were regarded as weak parasites only causing substantial damagein drought-stressed plants or when large inocular were implanteddirectly into host tissue. Skimmia japonica, Rutaceae, stem lesions, canker, dieback, fungal parasites     

Inheritance of resistance to yellow rust (Phragmidium rubi-idaei) in red raspberry

The inheritance of resistance in red raspberry (Rubus idaeus) to yellow rust (Phragmidium rubi-idaei) was studied in a diallel cross among the cultivars Boyne, Meeker, Mailing Jewel, Glen Prosen and Glen Clova. The progenies and clonally propagated parents were exposed initially in a rust-infected plantation where the incidence and severity of infection was assessed at the telial stage. The following spring leaves on new canes of the same plants were inoculated with urediniospores in a glasshouse at 18 ± 2 °C. The latent period and number of uredinia per cm² of abaxial leaf surface were determined 13 and 18 days after inoculation. Cv. Boyne developed only chlorotic flecking on inoculated leaves in the glasshouse and no sporulation was observed. It is postulated that this reaction was determined by a major gene which is designated Yr. Some of the other parents showed incomplete resistance of the slow rusting type, and segregants of Boyne selfed that were susceptible also showed low levels of rust, which suggests that Boyne carries both major- and minor-gene resistance. Diallel analyses of both the field and glasshouse data from the susceptible segregates indicated that the degree of incomplete resistance present was determined predominantly by additive gene action, though small but significant non-additive effects also occurred: cv. Meeker was the most resistant parent and contributed the most resistance to the progenies. The Meeker progenies also segregated for gene H, which determines cane hairiness and is known to be associated with resistance to other diseases. Segregants without hairs (genotypes hh) were found to be the more resistant to rust.     

In vitro spore germination and infection of cultivars of Rubus and Rosa by downy mildews from both hosts

The cardinal temperatures for in vitro germination of conidia of imported and indigenous isolates of downy mildew from hosts in the genera Rubus and Rosa were similar. A high percentage of conidia germinated above 2°C and germination remained between 80% and 90% up to 15°C or 20°C, depending on the isolate. The highest incidence of disease on leaf disks of Tummelberry (blackberry × red raspberry) inoculated with an isolate of Peronospora rubi occurred at c. 15°C, with infection over a range from 2°C to 28°C. Tests on leaf disks in vitro, and leaflets of primocane and lateral shoots in plastic tunnels, with three hybrid berry (blackberry x red raspberry), six blackberry and nine red raspberry cultivars showed the hybrid berries to be most susceptible. In a plastic tunnel infected drupelets of red raspberry fruits developed more slowly and failed to ripen evenly compared with uninfected drupelets. Similar malformation of infected fruits occurred in a plantation of Tummelberry. An isolate of P. rubi attacked severely both Tummelberry and rose cv. Can Can. Fluorescence microscopy after staining with aniline blue showed that leaf disks of Tummelberry were extensively colonised by intercellular mycelium of P. sparsa isolated from rose, even though sporulation was sparse or absent. This supports the view that P. rubi and P. sparsa may be conspecific. Oospores of P. rubi were found routinely within leaf disks of Rubus cultivars inoculated in vitro and once in naturally infected leaflets of Tummelberry.