OPTION 2: Myrtle rust disease severity Index - Jack A. Simpson Biosecurity Australia 23 June 2011
Jack A. Simpson Biosecurity Australia An email from Jack Simpson 23 June 2011 regarding recently circulated ..
"..methodologies prepared by University of Hawai�i at Manoa for assessing guava rust disease intensity on two host species Metrosideros polymorpha and Syzygium jambos, both of which are extremely susceptible. The scoring systems used for the two hosts are rather different. In Australia there are about 88 genera and 2253 species of native Myrtaceae plus a significant, but not yet tallied, number of exotic genera and species. Potentially most of these could be susceptible to myrtle rust. It is clearly not feasible to develop specific methodologies for each genus or species. Furthermore the variation in susceptibility between hosts is enormous, ranging from one or two sori per plant in some Callistemon populations, to near total defoliation in some Rhodamia. The methodology to be used on the ALA website has to be able to encompass this very wide variation in disease intensity between hosts, and perhaps between sites, yet provide accurate and reproducible data."
provides some valuable context to the ongoing discussion on MR measurement methodologies in search of a National consensus standard.
A. Rust virulence on infected leavesThe more virulent a fungus isolate is to a host the larger the lesions on infected leaves. Based on Junghans et al.(2003)
|S0||Immunity or small red fleck or small dead spot indicating strong hypersensitive response|
|S1||Small pustules less than 0.8 mm diameter|
|S2||Pustules 0.8�1.6 mm diameter|
|S3||Pustules greater than 1.6 mm diameter|
B. Disease severity on infected leavesBased on Carvalho et al. (1998) (a) Percentage shoots diseased: 0�5% very resistant; 5.1�10% resistant; 10.1% or greater susceptible. (b) Percentage leaf area diseased: 0�10% very resistant; 10.1�20% resistant; 20.1% or greater susceptible. (c) Number of sori per 2.4 cm2 of leaf surface: 0�50 very resistant; 50�100 resistant; 100 or more susceptible.
C. Rust incidencePercentage of infected plants.
D. Severity of disease on infected plantsDoes the disease affect: (a) plant appearance (attractiveness) (b) plant vigour (c) plant survival (d) is morbidity rapid (months) or slow (years) (e) susceptibility to other pests e.g. borers in trunk, leaf feeding insects, other diseases?
E. Severity of disease in infected plant communitiesAssessment will be subjective in first year or two? Stop planting species/cultivar in garden. Susceptible native plant communities replaced by resistant communities.
F. Effects on wildlifeFruit eating birds, bats, marsupials.
Native Forest Areas
|Forest type||NSW �000 ha||QLD �000 ha||Australia �000 ha|
|Eucalypt||21 148||36 466||116 447|
|Melaleuca||48||5 698||7 556|
|Rainforest||495||1 867||3 280|
|Total potentially susceptible||21 691||44 031||127 283|