Estimation of the unrestricted risk

Once the above assessments are completed, the unrestricted risk can be determined for each pest or groups of pests. This is determined by using a risk estimation matrix (Table 2.5) to combine the estimates of the probability of entry, establishment and spread and the overall consequences of pest establishment and spread. Therefore, risk is the product of likelihood and consequence.

The SPS Agreement defines the concept of an ‘appropriate level of sanitary or phytosanitary protection (ALOP)’ as the level of protection deemed appropriate by the WTO Member establishing a sanitary or phytosanitary measure to protect human, animal or plant life or health within its territory.

Like many other countries, Australia expresses its ALOP in qualitative terms. Australia’s ALOP, which reflects community expectations through government policy, is currently expressed as providing a high level of sanitary or phytosanitary protection aimed at reducing risk to a very low level, but not to zero. The band of cells in Table 2.5 marked ‘very low risk’ represents Australia’s ALOP.

2.3 Stage 3: Pest risk management

Pest risk management describes the process of identifying and implementing phytosanitary measures to manage risks to achieve Australia's ALOP, while ensuring that any negative effects on trade are minimised.

The conclusions from the pest risk assessment are used to decide whether risk management is required and if so, the appropriate measures to be used. Where the unrestricted risk estimate exceeds Australia’s ALOP, risk management measures are required to reduce this risk to a very low level. The guiding principle for risk management is to manage risk to achieve Australia’s ALOP. The effectiveness of any proposed phytosanitary measure (or combination of measures) is evaluated, using the same approach as used to evaluate the unrestricted risk, to ensure it reduces the restricted risk for the relevant pest or pests to meet Australia’s ALOP.

ISPM 11 (FAO 2004) provides details on the identification and selection of appropriate risk management options and notes that the choice of measures should be based on their effectiveness in reducing the probability of entry of the pest.

Examples given of measures commonly applied to traded commodities include:

• 
  options for consignments – e.g., inspection or testing for freedom from pests, prohibition of parts of the host, a pre-entry or post-entry quarantine system, specified conditions on preparation of the consignment, specified treatment of the consignment, restrictions on end-use, distribution and periods of entry of the commodity
  
• 
  options preventing or reducing infestation in the crop – e.g., treatment of the crop, restriction on the composition of a consignment so it is composed of plants belonging to resistant or less susceptible species, harvesting of plants at a certain age or specified time of the year, or production in a certification scheme
  
• 
  options ensuring that the area, place or site of production or crop is free from the pest – e.g., pest-free area, pest-free place of production or pest-free production site
  
• 
  options for other types of pathways – e.g., consider natural spread, measures for human travellers and their baggage, cleaning or disinfestation of contaminated machinery
  
• 
  options within the importing country – e.g., surveillance and eradication programs
  
• 
  prohibition of commodities – if no satisfactory measure can be found.
  

1.3Assumptions used to estimate unrestricted risk

Biosecurity Australia took into consideration the following information on commercial production practices when estimating the unrestricted risk of pests likely to be associated with fresh taro corms. Additional information on taro varieties is presented in Appendix D.

Large corm taro (Colocasia esculenta var. esculenta) is traditionally marketed with a short tuft of petiole bases attached, but with most of the leaf material removed. This protects the apical meristem to ensure the corm stays physiologically active, which delays the development of storage rots. The roots and soil are also removed from the corms during harvest or at the packing house. The corm of this variety is large and cylindrical, up to 30 cm in length and 15 cm in diameter, lacking hairs, with few cormels (stem tubers). The central corm is harvested as the main crop, with the lateral cormels being removed and discarded or used as planting stock.

Estimates of the unrestricted risk assume that the petiole bases are present on corms of the large corm variety. The quality control of the removal of leaves and roots may vary, so it is assumed that occasionally a small number of corms with a few minor stems or roots may be packed for export. During transport and storage, the feeder roots desiccate and the outer parts of the corm begin to dry out, affecting the feeding of external pests such as root aphids and nematodes.

Existing import conditions for large corm taro in Australia require topping to remove all petiole bases and the apical growing points to prevent the corms being propagated. If the petiole bases and the apical growing points are excised, the remaining lateral buds on the corm will usually not sprout. This topping is an additional quarantine measure, and is not considered when assessing the unrestricted risk.

Fresh corms of the small corm type (Colocasia esculenta var. antiquorum) are not currently permitted entry to Australia, following the introduction of emergency measures in 2006. The central corm of this variety is small, globoid, and surrounded by cormels (daughter corms). The harvested crop is actually the lateral cormels and daughter corms, as the central corm is generally considered inedible. Removal of the apical growing point does not affect the propagability of small corm taro. After the apex has been removed, these daughter corms will still sprout readily from lateral buds in the corm, and so may be propagated easily. The potential propagation of small corm taro is considered when assessing the unrestricted risk.

See Figures D.1 and D.2 in Appendix D for diagrams identifying the key features that differentiate the corms of the large and small corm varieties of taro.

Estimates of the unrestricted risk in this report assume that soil has been removed from the corms during harvest and grading operations.

1.4Taro cultivation practices

Taro is an adaptable crop that tolerates flooding, salinity and shading. Cultivation practices vary from region to region. Detailed accounts of cultivation practices in countries of Asia and Oceania are given in Wilson and Siemonsma (1996), Onwueme (1999) and Vinning (2003).

Propagation is almost entirely by vegetative means, with four types of planting material used (Onwueme 1999):

• 
  side suckers from the harvested crop used directly or sprouted under nursery conditions
  
• 
  small corms from the harvested crop used directly or sprouted under nursery conditions
  
• 
  headsetts of large corm taro (the apical 1–2 cm of the corm with the basal 15–20 cm of the petiole bases attached), which are planted straight after harvest
  
• 
  corm pieces of small corm taro, around 30–50 g in weight, either planted directly, or sprouted under nursery conditions.
  

Two main production systems are used in the Asia and Oceania regions: flooded or wetland production, and dryland or upland production. Cropping densities range from 4000 to 49 000 plants per hectare. In SE Asia, densities of 6000 to 36 000 plants/ha in dryland cropping and 27 000 to 40 000 plants/ha in flooded cropping are usual.

Flooded taro production utilises a paddy system similar to that for rice. It requires a heavy soil and cool running water (warm or stagnant water can lead to corm rot). Either irrigation or diversion/adaptation of natural rivers, streams or swamps may be used. After establishment of an impoundment area, the ground is flooded just before or just after planting. The water level is gradually raised during growth so that the base of the plant is always under water. If water is drained for fertilising, it is raised again within a few days. Maturity is reached in 12–15 months.

The advantages of flooded production are higher corm yields (about double) and minimal weed infestation, and year-round production is possible. Disadvantages are the high infrastructure and operational costs, a longer time to maturity, and increased susceptibility to root and corm rots, particularly if water is stagnant. Intercropping is usually not possible.

Dryland taro is grown in fields and relies mainly on rainfall. Sometimes minimal furrow or sprinkler irrigation is used, designed to keep the soil moist but not flooded. The rainy season must last 6–9 months and deliver 2500 mm of rainfall unless supplemented by irrigation. Planting is on the flat, or on ridges 70–100 cm apart. Intercropping between coconut and oil palms, fruit trees, coffee or cocoa is common. Mulching is necessary, as is weed control during the first three months until the canopy closes. Maturity is reached in 5–12 months.

Advantages are faster maturity, lower investment in infrastructure and operational costs, lower incidence of root and corm rots, and production is not dependent on large quantities of water. Disadvantages are that the planting time is dictated by the onset of the rainy season, it results in lower yields and weeding is necessary during the first three months.

1.5The global taro industry

Food and Agriculture Organization of the United Nations (FAO) estimates for taro production in 2009 (FAO 2011) are shown in Table 3.1. Although the major taro-producing countries are in Africa and Asia, taro forms a relatively small part of the diet there. In the Pacific, however, taro is a staple, particularly in the Cook Islands, Fiji, Futuna, Niue, Samoa, Tahiti, Tonga and Micronesia (Vinning 2003).

World trade in taro is estimated at about 145 000 tonnes per annum (Vinning 2003), largely based on supplying expatriate populations (Pacific Islanders, Hispanics and Chinese) with a traditional staple. The exception is Japan, where imports are consumed largely by the indigenous population. Export statistics for the major taro exporting countries in 2008 are listed in Table 3.2.