Probability of spread
The likelihood that P. arbutifolia will spread, based on a comparison of factors in the area of origin and in Australia that affect the expansion of the geographic distribution of P. arbutifolia: MODERATE.
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Phyllosticta arbutifolia is a common disease of apples across all apple production areas in China (Ma 2006), suggesting that it tolerates a wide range of climates.
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The host range of P. arbutifolia is restricted to apples, pears and Crataegus spp. (CAB International 2008; Farr et al. 2008; Ma 2006; Yoder 1990). These hosts are available in Australia, both in commercial orchard districts, suburban and rural areas.
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The ability to cause infection of apple trees within 80 m from an infected tree by wind-blown rain (CAB International 2008) provides the potential for this pathogen to be spread from one orchard to nearby orchards (Ma 2006).
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Disposal of infected fruit via commercial or domestic rubbish systems may aid the spread of the pathogen.
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On fruit, pycnidia, which have already functioned in the season, fill up and become typical pycnosclerotia in the autumn, in which stage they overwinter. However, the overwintering pycnosclerotia on mummified fruit and fallen leaves give rise to pycnidiospores in the spring, but their role as inoculum is probably minor; many overwintering pycnosclerotia become sterile (Gardner 1923; Ma 2006).
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Heavy rains and extended wetting periods promote exudation, dissemination, and germination of conidia. The radius of infection in wind-blown rain from a 10 m tree was estimated to be 80 m, with 100% infection occurring within 12 m from the infected trees (CAB International 2008), suggesting that this pathogen has the potential to rapidly establish and spread.
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Transport of infected plant seedlings and other planting materials with cankers may aid the long distance movement of P. arbutifolia (CAB International 2008) to uninfected orchards.
The potential movement of symptomless infected planting materials, moderated by the requirement of rain and an extended wetting period for spore exudation, dissemination, and germination, support a risk rating for distribution of ‘moderate’.
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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 P. arbutifolia will enter Australia as a result of trade in the commodity from the country of origin, be distributed in a viable state to suitable hosts, establish in that area and subsequently spread within Australia: LOW.
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Consequences
The consequences of the establishment of P. arbutifolia 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.
Reasoning for these ratings is provided below:
Criterion
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Estimate
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DIRECT
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Plant life or health
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D – Significant at the district level
Phyllosticta arbutifolia affects apple trees at both the vegetative growth and fruiting stages. Phyllosticta arbutifolia causes a serious blotching of apples which reduces fruit quality and yield (Ma 2006). In the USA, losses caused by P. arbutifolia were reported in the past to vary between 5 and 10%. However, the economic importance of P. arbutifolia has declined, probably due to the regular fungicide treatment of apple and pear orchards against diseases such as apple scab (Venturia inaequalis) (CAB International 2008).
The disease also causes damage on leaves, buds, twigs, and branches of susceptible apple cultivars (Yoder 1990), causing defoliation and development of cankers on twigs and branches (Ma 2006).
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Other aspects of the environment
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A – Indiscernible at the local level
Unlikely to affect native flora, because its host range is restricted to species of Aronia, Malus, Pyrus and Crataegus spp. (Farr et al. 2008; Ma 2006; Yoder 1990).
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INDIRECT
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Eradication, control etc.
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D – Significant at the district level
Programs to monitor and eradicate P. arbutifolia, should it reach Australia, would be costly. The disease is usually controlled by planting disease free nursery stock or using resistant cultivars (Ma 2006) and following a regular fungicide program for summer disease control (Yoder 1990). Eradication of cankers formed on branches and twigs by using fungicides can be costly (Yoder 1990).
Existing 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 the pest may be incurred by the producer.
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Domestic trade
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D – Significant at the district level
Presence of P. arbutifolia in apple commercial production areas would result in the implementation of interstate quarantine measures, causing loss of market and subsequent significant industry adjustment at district level.
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International trade
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E – Significant at the regional level
Phyllosticta arbutifolia has been listed as an A1 quarantine pathogen by EPPO (EPPO 2005b) and is also of quarantine significance for Comite de Sanidad Vegetal Del Cono Sur (COSAVE). Its presence in apple production areas of Australia would make it more difficult for Australia to access these markets.
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Environment
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B – Minor significance at the local level
Additional fungicide applications or other control activities would be required to control this disease on susceptible crops and these may have minor impact on the environment.
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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 apple blotch
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Overall probability of entry, establishment and spread
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Low
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Consequences
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Moderate
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Unrestricted risk
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Low
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