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Rural Industries Research & Development Corporation
by Ben Croxford, Guijun Yan and Ralph Sedgley
August 2003
RIRDC Publication No 03/085 RIRDC Project No UWA 69A
Executive Summary
Leucadendron is a major wild flower crop in Australia. To increase the competitiveness of the local industry, a breeding program based on interspecific hybridisation was initiated in 1998. Since this time, thousands of hybrid plants have been produced involving 27 different parental species. These were planted at the farms of contributing growers for evaluation but large losses have occurred at some sites with symptoms indicating that infection by Phytophthora cinnamomi may be responsible.
Phytophthora cinnamomi is one of the most widespread and destructive fungi and is known to infect many Leucadendron species. Our observations and other anecdotal evidence suggest that there is a large amount of variation in susceptibility between different plants and species. A method for evaluating the tolerance of plants would be of great value for ranking existing cultivars and potential new varieties and also to identify tolerant selections which can be used as new varieties or rootstocks.
Therefore, the aims of this research are:
1. to develop protocols for
the screening of Leucadendron genotypes tolerant or susceptible to Phytophthora
cinnamomi, and
2. to rank the important
Leucadendron genotypes used in the breeding program.
Three experiments were conducted to inoculate more than 40 Leucadendron genotypes both as established plants and excised cuttings with Phytophthora cinnamomi pathogen. Both lesion development and plant death at day 12, day 24, day 36 and day 52 after inoculation were recorded to evaluate the tolerance and susceptibility of Leucadendron genotypes.
Intact plants were very slow to show any symptoms with most genotypes remaining without symptoms after 80 days. Treated L. procerum plants began to develop lesions within 10 days but took more than 40 days to die. Other selections showing symptoms include L. galpini (60% by day 40) and Pisa (26.7% by day 40) however none of the other 10 genotypes had shown any symptoms by day 80.
Inoculated excised stems developed lesions within 8 to 13 days after inoculation. Genotypes such as L. procerum (1) and L. rubrum developed lesions in 100% of the stems within 36 days while others such as L. eucalyptifolium and ‘Inca Gold’ showed no lesion development in any stems after 52 days. Several genotypes showed lesion development in some but not all the stems within 52 days.
No lesions were observed on any of the control stems.
The occurrence, length and tangential spread of lesions appear to correlate well with eventual death of excised stems. All cuttings that died in the trial first expressed lesions, those dying first and in highest percentages had the largest tangential and vertical lesion spread. This can be seen in genotypes such as L. procerum and L. rubrum which have a high occurrence of lesions, large mean lesion length, high degree of girdling and high death rate compared to L. laureolum, L. salignum, ‘Pisa’ and ‘436’ which have low occurrence of lesions, short lesion length, small degree of girdling and no deaths at the end of the experiment.
The Leucadendron genotypes used in this trial can be divided into 3 groups based on their relative tolerance to inoculation with Phytophthora cinnamomi. The group with the highest tolerance showed no symptoms following inoculation (no lesions or death). The medium tolerant group developed lesions following inoculations but these lesions tended to be small and had a low mean degree of girdling and all the plants trialled survived at day 52 after inoculation. The susceptible group developed larger lesions, higher degree of girdling and death of stems at day 52.
This project has successfully developed a protocol, using excised stems, to assess the tolerance and susceptibility of Leucadendron genotypes to Phytophthora. The trialled Leucadendron genotypes were ranked as highly tolerant, medium tolerant and susceptible. These results are helpful for the selection of parents in our breeding program to produce highly tolerant new Leucadendron varieties and may also be useful for the growers to make decisions on selecting suitable varieties on their properties.
