A.J. Inman1, P.A. Beales, C.R. Lane, and C. Brasier
Investigations are being done on the potential foliar susceptibility of UK ornamental, hedgerow and woodland under-story plants to European and US isolates of P. ramorum. Whereas leaves and shoots of ornamentals are an important means of introduction and spread, those of under-story and hedgerow species represent potentially important sources of inoculum for tree epidemics. The work is complementary to the UK Forest Research Agency's studies on leaves of European tree species. Three representative European isolates (Germany/UK, ex. Viburnum/ Rhododendron) and three US isolates (ex. Quercus, Rhododendron, Vaccinium) were selected.
In preliminary studies on the development of a detached leaf assay, mini-inoculators (modified microcentrifuge tubes) containing mycelial plugs were used with two UK isolates to investigate the effect of leaf age, leaf surface and wounding/non-wounding on infection of Rhododendron. Mini-inoculators were held in place on the leaves using modified hair clips. Infection occurred only on wounded leaves, regardless of leaf age or leaf surface inoculated. A zoospore dipping method (K. Park/E. Hansen) was compared with the mini-inoculators. Two isolates (UK/USA) were tested on 15 plant species: leaves were dipped into a zoospore suspension either by their tip end (unwounded) or by their petiole end (wounded). Infection only occurred on the wounded petiole end. Mini-inoculators gave comparable, but more consistent, data to the dipping method.
The mini-inoculator test using detached, wounded leaves was selected for large scale host tests as it is easy to use and allows quantification. Infection was recorded after 7 days at 20°C and a lesion index rating assigned (cf. Linderman et al., 2002): 0 = no lesion; 1 = lesion not extending beyond the plug (<5 mm diam.); 2 = lesion extending 1–2 mm beyond plug (c. 7 - 9 mm diam.); 3 = lesion extending 3 - 7 mm (c. 11 - 19 mm diam.); 4 = lesion extending >8 mm (>21 mm diam.).
There were few differences between the European and American isolates in their ornamental and non-tree host range and pathogenicity, though there were some differences in aggressiveness between individual isolates. Plant species (>51 species from >19 families) were assigned to susceptibility categories based on their mean lesion index for all isolates, as follows: virtually immune (<0.49); resistant (0.5 - 1.49); slightly susceptible (1.5 - 2.49); moderately susceptible (2.5 - 3.49); highly susceptible (>3.5). Potentially highly susceptible species included those in the following genera: Sambucus (Caprifoliaceae); Rhododendron, Camellia, Pieris, Leucothoe (Ericaceae); Syringa (Oleaceae); Eucalyptus (Myrtaceae). Moderately susceptible species included: Viburnum, Symphoricarpus (Caprifoliaceae); Arctostaphylous (Ericaceae); Fuchsia (Onagraceae); Photinia (Rosaceae); Ulmus (Ulmaceae). Slightly susceptible species included: Acer (Aceraceae); Gaultheria and some azaleas (Ericaceae); Malus and Prunus (Rosaceae). Of the potentially susceptible species, Acer, Fraxinus, Sambucus and Ulmus are important hedgerow or under-story species in the UK. Hedgerow or under-story species considered potentially resistant or immune included: Alnus, Corylus, Crataegus, Ilex, Rubus fructicosus and Viburnum opulus. With the exception of Rhododendron and Viburnum, none of these laboratory-tested hosts showing physiological leaf susceptibility are recorded as natural hosts in the UK. Hosts showing susceptibility will be investigated further using attached leaves; stem infection will also be investigated. The data will contribute to UK/EU risk assessments for P. ramorum.
