No abstract

Melampsora epitea , the causal agent of leaf rust on willow ( Salix spp.) in short-rotation forestry, was sampled over four consecutive years (1993 -96) in Sweden. The pathotype patterns of 332 single-spore isolates were examined for virulence characteristics using a defined set of willow hosts. Thirty-seven pathotypes of M. epitea were identified and grouped into three formae speciales . No race or morph subdivision was observed among the isolates, other than the three formae speciales . For monitoring and studying the ecological and evolutionary dynamics of virulence for M. epitea , an internationally useful naming system for pathotypes is proposed, with a three-digit code referring to virulence on a specified set of standard test clones.

A cell‐free culture filtrate of the plant growth inhibitory bacterial isolate Å313, identified as Pseudomonas fluorescens, was tested for its effect on wheat root elongation in vitro, with and without various pretreatments. The filtrate showed a strongly inhibitory effect on root elongation and could be heated to 100°C for 5 min or incubated at a pH within the range of 4–10 without losing its activity. Unlike other root growth‐inhibitory bacterial metabolites the effect of the filtrate was not reversed by methionine, nor was any inhibitory activity present in the water phase after extraction with chloroform. The inhibitory agent(s) was formed when the bacterium was grown in an artificial medium as well as in root exudates from wheat. Two wheat cultivars differing in reaction to inoculation with living cells of Å313 showed the expected difference in growth reduction when exposed to culture filtrate, indicating a cultivar difference in sensitivity to the metabolite(s) formed by the bacterium. Isolate Å313 invaded intercellular spaces of the root cortex of gnotobiotically grown wheat plants, but did not produce pectolytic enzymes in vitro, nor induce a hypersensitive response in tobacco plants.

Pure species and F1 hybrid families of Salix viminalis and S. dasyclados were tested for resistance to four single uredinium isolates of Melampsora rust in laboratory experiments using excised leaves. Rust isolates were derived from: S. viminalis, S. dasyclados, a S. viminalis x triandra hybrid, and S. daphnoides. Incidence of infection, number of uredinia per leaf, and numbers of spores per uredinium were measured. As expected, the isolate from S. daphnoides did not infect any of the willow species or hybrids tested. For the other three rust isolates that were tested, the parent from which the isolate was derived was susceptible, the other parent was resistant, and hybrids were intermediate in resistance for incidence and uredinia per leaf. These patterns indicate additive inheritance of 'these resistance traits in hybrids. Numbers of spores per uredinium were similar on the hybrids and the susceptible parent for one rust isolate, suggesting dominant inheritance of this trait in the hybrids.

Field surveys and inoculation experiments were carried out in order to determine the most important stem‐canker fungi on willows planted for energy production. The pathogenicity of different fungal isolates and factors influencing infection were studied in the field and in greenhouse experiments, where canker extension was measured after inoculation either on dormant plants or on plants undergoing active growth.