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 leaves

The 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 leaves

Based 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 incidence

Percentage of infected plants.

D. Severity of disease on infected plants

Does 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 communities

Assessment 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 wildlife

Fruit 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
Eucalypt plantations 93 64 991