The likelihood that G. yamadae will establish, based on a comparison of factors in the source and destination areas that affect pest survival and reproduction: MODERATE.
Gymnosporangium yamadae is established in China, Japan, South Korea and North Korea (CAB International 2008; Farr et al. 2008). Environments with climates similar to these countries exist in various parts of Australia where apples are produced.
Gymnosporangium yamadae has a restricted host range including Malus species and its alternate host species Sabina and Juniperus spp. (CAB International 2008; Farr et al. 2008). These host plants are grown in Australia, in commercial orchard districts, suburban and rural areas.
On the apple host, aecia develop after dikaryotization of spermatia produced in pycnia. The aecia form aeciospores which infect the alternate hosts. The fungus overwinters on Sabina and Juniper and forms teliospores in spring. The basidiospores released from the germinated teliospores infect apples (OEPP/EPPO 2006).
The temperature range for germination of basidiospores of G. yamadae is 7-30 ºC with an optimum temperature of 16-20 °C (Fukushi 1925; Guo 1994). These temperatures are found across the apple growing regions of temperate Australia for much of the year.
The occurrence of suitable temperature and moisture conditions for spore germination and infection in some parts of Australia, moderated by the need for proximity of alternate hosts, support a risk rating for establishment of ‘moderate’.
Probability of spread
The likelihood that G. yamadae will spread, based on comparison of factors in the area of origin and in Australia that affect the expansion of the geographic distribution of the pests: HIGH.
Gymnosporangium yamadae is a serious disease of apple in China and Japan (Aldwinckle 1990; Guo 1994), and it tolerates a wide range of climates.
Hosts of G. yamadae are present in Australia in commercial orchard districts, suburban and rural areas.
No species of Gymnosporangium is known to be endemic to Australia or associated with native Cuppressaceae.
Under natural conditions basidiospores (from Juniperus and Sabinato Malus)and aeciospores (from Malusto Juniperus and Sabina )are dispersed by wind (CAB International 2008). The basidiospores can be wind dispersed for distances of 2.5–5 km (CAAS 1992). Wind dispersal will promote rapid spread of this pathogen (Guo 1994).
In addition, aeciospores of G. yamadae can be carried on fruit, stems and leaves of infected apple plants during trade and transport (CAAS 1992). Spread by humans may also occur by transporting infected plants of Juniperus species (Ma 2006). However, infection of apple trees does not persist after infected leaves or fruits have fallen in the dormant stage (CAB International 2008).
The long distance dispersal of spores by wind, and the potential movement of symptomless infected planting materials support a risk rating for distribution of ‘high’.
Overall probability of entry, establishment and spread
The overall probability of entry, establishment and spread is determined by combining the probabilities of entry, of establishment and of spread using the matrix of ‘rules’ for combining qualitative likelihood shown in Table 2.2.
The overall likelihood that G. yamadae will enter Australia as a result of trade in apple fruit from the country of origin, be distributed in a viable state to suitable hosts, establish in that area and subsequently spread within Australia: LOW.
The consequences of the establishment of G. yamadae in Australia have been estimated according to the methods described in Table 2.3.
Based on the decision rules described in Table 2.4, that is, where the consequences of a pest with respect to one or more criteria are ‘E’, the overall consequences are estimated to be MODERATE.
Gymnosporangium yamadae is considered a serious disease of apples in China and Japan (Aldwinckle 1990).
Gymnosporangium yamadae infects leaves and stems of Sabina and Juniperus spp., and leaves, stems and immature fruit of apples (Guo 1994; Ma 2006). Gymnosporangium yamadae causes damage by defoliation (Aldwinckle 1990; Guo 1994). Infection of young fruit causes fruit drop and a reduction in apple fruit yield and quality (Guo 1994).
Unlikely to affect native flora because its host range is restricted mainly to Malus and Pyrus species, and alternate hosts Sabina and Juniperus spp. (Wang and Guo 1985). However, G. yamadae would be detrimental to ornamental Juniperus spp. No species of Gymnosporangium is known to be endemic to Australia or associated with native Cupressaceae.
Eradication, control etc.
D – Significant at the district level
Programs to monitor and eradicate G. yamadae, should it reach Australia, would be costly. Existing Integrated Disease Management (IDM) programs may be disrupted due to possible increases in the use of fungicides. Costs for crop monitoring and consultant’s advice regarding management of this pathogen may also be incurred by the producers.
Removal of Sabina and Juniperus spp. has been recommended as the most important Integrated Disease Management (IDM) measure for G. yamadae management. In Liaoling province of China, no Sabina and Juniperus spp. plants were allowed to be planted within 5 km of apple orchards (Guo 1994). However, this measure may be not practical in Australia as Sabina and Juniperus spp. plants are often grown in private gardens and public parks.
D – Significant at the district level
The presence of G. yamadae in commercial apple production areas in Australia would result in the implementation of interstate quarantine measures and potential loss of markets.
E – Significant at the regional level.
Gymnosporangium yamadae is one of the non-European Gymnosporangium spp. listed as A1 quarantine organisms by EPPO (EPPO 2005b). It is also listed as a quarantine pest by the Interafrican Phytosanitary Council. Its presence in commercial apple production areas in Australia would limit market access for Australian apples to overseas markets which lack this pest.
B – Minor significance at the local level
Additional fungicide applications or other control activities would be required to control this disease on susceptible crops. These may have minor impact on the environment.
Unrestricted risk estimate
Unrestricted risk is the result of combining the probability of entry, establishment and spread with the estimate of consequences. Probabilities and consequences are combined using the risk estimation matrix shown in Table 2.5.
Unrestricted risk estimate for Japanese apple rust