D.J. Royle

Identity and host alternation of some willow rusts (Melampsora spp.) in England

Six species of Melampsora: M. salicis-albae, M. larici-pentandrae, M. amygdalinae, M. capraearum, M. ribesii-viminalis and M. epitea , were identified among 26 willow leaf samples collected mainly in SW England. Identification of species was based on the morphology of the uredinial and telial stages as well as on the results of inoculations of the aecial hosts. M. epitea occurred alone in 18 leaf samples, and together with M. ribesii-viminalis in one further sample. Basidiospores produced from teliospores of the autoecious M. amygdalinae infected Salix triandra (a species of tree willows). All other species were heteroecious, with M. larici-pentandrae, M. capraearum and M. epitea producing spermogonia and aecia on Larix decidua (European larch), and less readily on L. kaempferi (Japanese larch). M. ribesii-viminalis and a form of M. epitea from S. purpurea , occasionally formed spermogonia but not aecia on Ribes spp. M. epitea, M. capraearum , and M. ribesii-viminalis occurred on cultivars of shrub willows which are favoured for short-rotation, coppiced energy cropping in the U.K.

Distinction between stem- and leaf-infecting forms of Melampsora rust on Salix viminalis using RAPD markers

The RAPD technique was used to distinguish two forms of Melampsora rust on Salix viminalis . Fifteen isolates of the stem-infecting form and 19 of the leaf-infecting form collected from the U.K. were tested with 10 arbitrary primers. All the primers generated polymorphic bands and 46 putative loci were examined. Seven primers gave a total of eight bands specific for the stem-infecting form and four primers produced five bands characteristic of the leaf-infecting form. Cluster analysis using Unweighted Pair-Group Method with Averaging (UPGMA) revealed 62% disagreement between the two forms. The RAPD band patterns of the stem- infecting form isolates were almost identical, while those of the leaf-infecting isolates showed up to 29% variation.

A comparative study of stem- and leaf-infecting forms of Melampsora rust on Salix viminalis in the U.K.

A form of Melampsora rust was found to cause stem cankers on Salix viminalis clones in south-west England. Comparative studies suggested that the rust differed from the leaf-infecting form (M. epitea var. epitea f. sp. larici-epitea typica) by causing infection of both young stems and shoot tips, having distinct host specificity, and existing predominantly as the uredinial stage. After incubation for a period at low temperature the rust produced only poorly developed hypophyllous telia similar to those of the leaf-infecting form. In the field, this form of rust overwintered in buds or cankered stems of infected willows. Survey results of willow plantations in the U.K. in 1991 showed that the stem-infecting form occurred in most study sites including Northern Ireland and Scotland. The leaf-infecting form was present in all locations, whereas M. ribesii-viminalis was occasionally found only in south-west England.

Variation in responses of spores of Septoria tritici and S. nodorum to UV-B irradiation in vitro.

The effect of UV-B radiation (280–320 nm) on germination of spores and extension of germ-tubes of Septoria tritici (two isolates from the U.K., two from Tunisia) and S. nodorum (two isolates from the U.K.) was examined in vitro . Germination of spores after 48 h incubation was unaffected in both species by 4 h irradiation per 24 h period (UV-B DNA , 3 kJ m −2 d −1 ), but was inhibited in S. nodorum by continuous UV-B irradiation (UV-B DNA , 18 kJ m −2 d −1 ). Germination was unaffected by background white light (400–700 nm) for 16 h per day. After 48 h in the dark, germ-tubes were longest in S. nodorum (both isolates) and shortest in the two Tunisian isolates of S. tritici . By comparison with continuous darkness, germ-tube extension in all isolates was strongly inhibited by continuous UV-B. Germ tube extension of the U.K. isolates, but not of the Tunisian isolates, was also inhibited by 4 h UV-B both with a dark background and within a 16 h white light day. The time in the 24 h cycle when germination started, i.e. incubation began, and when exposure to either UV-B radiation or white light occurred, affected the response of the fungi. Differences in sensitivity to UV-B between the S. tritici isolates suggest that adaptations to different fluxes in contrasting natural environments may occur.

Pathogenicity to willow rust, Melampsora epitea, of the mycoparasite Sphaerellopsis filum from different sources

Using leaf disc techniques, nine isolates of the mycoparasite Sphaerellopsis filum were examined for pathogenicity to Melampsora epitea, the most important species of rust in renewable energy willow plantations. Five isolates were derived from Melampsora spp. on willows and poplar, one from Puccinia coronata on couch grass, two from Phragmidium violaceum on blackberry and one from Triphragmiopsis laricinum on larch. Two inoculation experiments were carried out. In the first, S. Filum and rust were applied simultaneously on to leaf discs of Salix burjatica cv. Korso, in the second, leaf discs were inoculated with rust initially, then the resulting rust uredinial pustules were inoculated with S. filum. In both experiments, all S. filum isolates from Melampsora spp., and that from P. coronata, developed pycnidia on willow rust. No pycnidia were produced from the other three isolates. Among isolates which produced pycnidia, the frequency of rust pustules colonized by S. filum and suppression of rust spore production differed markedly. In the first experiment, 55-99% pustules were colonized and rust spore production was suppressed by 64-98%. In the second, the proportions were 33-97% and 53-73%, respectively. Suppression of rust sporulation was closely correlated with the frequency of rust pustules bearing S. filum pycnidia. The results suggest that S. filum is composed of pathogenically specialized populations differing widely in their virulence.

Eudarluca caricis, the teleomorph of the mycoparasite Sphaerellopsis filum, on blackberry rust Phragmidium violaceum

Eudarluca caricis, the teleomorph of Sphaerellopsis filum, was found on blackberry rust (Phragmidium violaceum) in south west England and its morphology is described. Cultures derived from ascospores produced pycnidial conidiomata and conidia typical of S. filum, confirming the connection between the meiotic and mitotic stages of the fungus. Microconidia were also found in the cultures derived from ascospores. Conidiomata developed on the majority of uredinial pustules of P. violaceum 10 d after inoculating detached, uredinia-bearing blackbery leaves with the conidia from the cultures derived from ascospores.

Hybridization in larch-alternating Melampsora epitea (M. larici-epitea)

Crossing and selfing experiments were carried out with six field collections and five isolates belonging to three formae speciales, larici-epitea typica (LET), larici-daphnoides (LD), and larici-retusae (LR), of willow rust, Melampsora epitea . European larch ( Larix decidua ) was inoculated with basidiospores produced on overwintered telial leaves and the resulting spermagonial lesions on larch needles subsequently paired in vitro . In two crosses between LR and LD involving a total of 439 lesion pairs, only two cultures obtained were identified as hybrids. These two cultures were non-pathogenic to the maternal host Salix burjatica cv. Korso and weakly pathogenic to the paternal host S. daphnoides cv. Meikle. No identifiable hybrids were obtained from 56 lesion pairs between LET and LD. In a cross between LET as receptor and LD as donor, one-third of the lesions formed aecia. In the reciprocal combination, however, less than 1% developed aecia. All F 1 cultures between LET and LR were weakly pathogenic to the parental hosts S. viminalis cv. Mullatin and S. burjatica cv. Korso. When three of the F 1 cultures were used to produce telia, only one developed mature teliospores. Subsequent selfing experiments showed that this culture was predominantly self-sterile, with only 1 % of lesions producing aecia. In selfing and crossing two pathotypes within the same f. spp. , 20–40 % of needles produced aecia and the rate of aecial formation was similar in both directions. The results obtained suggest that M. epitea is heterothallic and the sexual compatibility is controlled by a pair of alleles at a locus. It is concluded that the three f. spp. are genetically different populations and ecologically fit new pathotypes cannot arise easily as a result of hybridization between them. Within a f. sp. , however, many pathotypes exist or will occur due to exchange of genes for virulence during the sexual life-cycle.