Summary of the session on national potato production and the powdery scab situation

Transcription

1 Summary of the session on national potato production and the powdery scab situation S. Wale, SAC, Ferguson Building, Craibstone Estate, Bucksburn, Aberdeen AB21 9YA, UK Although powdery scab is favoured by a cool, wet climate, it is not confined to wet maritime climates. The disease can be found in hot, dry countries where production takes place in winter or at high altitude and where irrigation is applied. Brief overview of occurrence of powdery scab: Europe: Reported in Denmark, Germany, Holland, Ireland, Spain, Switzerland and the UK. It has been identified in Sicily (Italy). In France, apart from breeding plots, the disease has not been recorded. Middle East: Reported in Israel and Turkey. Both countries take extreme care when importing seed to check for freedom from the disease. North America: Reported in both Canada and the USA. In the latter it is causing some concern in elevated areas of Colorado and in Massachusetts. It has also been found in Idaho. South America: As the center of origin of the potato, it is not surprising that powdery scab has been found in South America. It has been confirmed in Bolivia, Brazil, Colombia, Ecuador, Guatemala, Mexico and Peru. Australasia: Both New Zealand and Australia consider powdery scab as a major disease of potatoes. It has been reported in Papua New Guinea. Asia: Powdery scab has caused considerable concern in India and Pakistan, but mainly in seed production. It has also been recorded in Japan. Africa: It is reported in Burundi, Kenya and South Africa. Recent unconfirmed reports suggest it is also present in Egypt and at high altitude in tropical central African countries. How powdery scab impacts on profitability Trials have indicated that powdery scab has little or no effect on yield There is limited (unpublished) UK evidence to suggest that seed with moderate levels of powdery scab can reduce vigour slightly by reducing stem number and daughter tuber numbers. Work in New Zealand on extreme levels of seed infection, indicated that, in their situation, a major effect on crop vigour was possible. As a blemish disease, the main losses are through rejection of tubers in order to achieve seed and ware market standards Financially, this rejection of tubers means a direct loss of marketable crop and extra cost to grade to the necessary standard. No quantification of these losses have been made but seed grower estimates in Scotland of an extra 10/tonne grading costs have been quoted. For processing, infected tubers will impact on the extent of peeling required. There may be need for increased waste treatment after peeling to ensure sporeballs of the fungus do not contaminate land where the waste is spread. Seed growers experience other costs as a result of attempts to avoid or reduce powdery scab. They may have to rent land away from their own farm that has not grown potatoes in order to escape the disease. This incurs both a rental cost and extra travelling costs. High grade seed growers particularly are affected in this way. Additionally, there may be added costs to apply 3

2 chemical or cultivation control measures. Finally, seed growers have an extra cost of grader hygiene after grading an infected stock to prevent cross contamination. With ware production, for the washed high quality, pre-pack market, virtual freedom from blemishes are required and grading out all infected tubers may be necessary. For the unwashed bag market, a low level of infection is tolerated. The European and Australian situation An accurate assessment of both occurrence and financial loss due to powdery scab has not been made in Europe. In northern Europe, winters have become milder in the last decade and the prevalence of groundkeepers ( volunteer potatoes ) has increased. The persistence of potatoes as groundkeepers between potato crops will have exacerbated soil contamination by sporeballs and prevented a decline in soil contamination. Another factor in the rise of powdery scab in Europe has been that, until recently, breeders did not screen for resistance to the disease. UK Production: c. 6m tonnes annually from around 140,00 ha. Approximately 50% table, 10% seed and 40% processing (no starch production). In the UK, powdery scab is a much greater problem in the cooler and wetter northern and western areas. Recent studies have demonstrated that in south and east England, powdery scab can occur but is of much less significance. In 1998, losses from powdery scab in England were estimated at 1m but in Scotland where total production is 20% of that in England, losses were estimated at 6-8m. Historically, powdery scab has been recorded as a disease since intelligence records began in the 1920 s. Sporadically, it achieved greater significance when more susceptible cultivars came to prominence (e.g. in the early 1950 s). However, it became a very significant disease of potatoes from the late 1970 s onwards for several reasons. Firstly, in the 1970 s, the cultivar Pentland Crown rose rapidly to dominate the potato acreage. For the fresh market, the cultivar had a uniform shape, had a good skin finish and was high yielding. This cultivar was highly susceptible to powdery scab and it seems certain that the disease was spread on seed to all potato growing areas of the UK and soil inoculum increased as a result. The continuing practice of feeding discard ( brock ) or low value potatoes to livestock as a cheap food source is also likely to have increased soil contamination. The sporeballs are largely unaffected by passage through the animal gut and contaminated slurry or manure is spread on fields. Secondly, partly as a result of the importance of powdery scab on this cultivar, the disease was separated from common scab in certification. Prior to this both common and powdery scab were lumped together as scab. Indeed, the visual similarities of the two diseases may have prevented powdery scab being recognised as a disease of growing importance for many years. Thirdly, from the 1970 s onwards the proportion of ware (table) potatoes sold washed increased markedly and skin blemishes became increasingly important. Fourthly, although Pentland Crown declined almost as fast as it rose in popularity, following behind was a plethora of cultivars many of which were susceptible to powdery scab. In consequence, of the cultivars listed in the NIAB variety handbook, two-thirds can be considered susceptible or moderately susceptible. Combined with an increased occurrence of inoculum, the disease has persisted as a major one in northern and western UK. In the UK, past and recent (2000) studies on the relationship between the incidence and severity of infection on seed tubers and that developing on the progeny crop have failed to show a clear relationship. Where inoculum is present as sporeball contamination of tubers severe infection 4

3 can still develop. This suggests that, in the UK at least, conditions at the critical stage of tuber initiation are more important than the level of inoculum. The recent studies, also suggested that the certification standard for powdery scab imposed by the Scottish Executive Rural Affairs Department (SERAD) is as good a baseline standard for this disease as any. Spraing caused by Potato Mop Top Virus (PTMV) or Tobacco Rattle Virus (TRV) is an increasing problem is table crops. The relative importance of the two viruses is uncertain although tuber diagnostic tests in one laboratory suggests that PTMV and TRV are of equal occurrence. Denmark Production: 1.4m tonnes annually. 21% table, 10% seed, 69% starch In surveys, powdery scab sporeballs have been found throughout Denmark but tuber disease is only important in a few susceptible cultivars such as Kennebec. Thus powdery scab is a minor problem in Denmark, even though conditions at tuber initiation can be conducive to the disease. However, spraing caused by PTMV is a major problem. Spraing caused by TRV can also occur and differentiation between the two viruses is difficult. Most problems occur on coarse sandy, black soil with a high humus content. The cultivar Saturna is very susceptible to PTMV (but not TRV), showing arcs or spots in the flesh. Frequently 15-20% of tubers show infection and this leads to high rates of rejection. It has proved difficult to produce spraing free tubers of Saturna. Symptoms of PTMV develop in store. PTMV symptoms can occur on tubers without powdery scab symptoms present. [Saturna is widely grown in the UK where it is highly susceptible to powdery scab but spraing symptoms are rarely seen.] Switzerland Production: 0.6m tonnes from 15,000 ha. 31% table, 22% processing, 4% seed, 36% stock feed, 5% flake, 1% export (to Italy). Imports make up 17% of consumption for ware and 7% of seed. Main cultivars are Agria (21%), Bintje (18%), Desiree (8%), Ernestolz (5%), Saturna (8%) and Charlotte (9%). Seven out of 13 cultivars occupying 49% of the cropping area are susceptible or moderately susceptible to powdery scab. All processing cultivars are susceptible. Seed is typically grown m above seas level. In the lowland area east of Lausanne, 37% of potato production occurs. In this area powdery scab is now almost endemic sporeballs per gram of soil is defined as a high level of risk. Powdery scab has become a problem because there are too many infested soils, too many susceptible cultivars, short rotations, poorly drained land and irrigation. The seed tolerance of no more than 1% tubers showing more than 5 pustules (i.e. 99% of tubers can have powdery scab with < 5 pustules) is considered too high. The spread of disease on seed is an endemic problem. The absence of mercury as a permitted tuber treatment since the 1980 s may have contributed to the increase in powdery scab. Root galling does not relate to powdery scab tuber infection. The amount of galling strongly depends on the time of assessment. Galling is less easy to detect later in crop development. France Production: 4.3m tonnes for processing and table from 104,000 ha. Of this 1.7m tonnes for the fresh market, 1.1m tonnes for processing and 0.6m tonnes for export. A large tonnage ( m) is produced in gardens m tonnes of starch production is concentrated in three factories. 0.28m tonnes seed (15,000 ha) and 0.22m tonnes early production (25,000 ha) mainly in Brittany (55%), the Atlantic coast (9%), east of Bordeaux (27%) and the south of France 5

4 (9%) ,000 tonnes of seed is exported annually. Some seed of specific cultivars is imported from Scotland. Over 200 cultivars are grown. Powdery scab is not a problem so far in France. There have been no reported outbreaks in field crops. This may be due to an unfavourable climate for the disease. There have been occasional outbreaks (2-3 instances in the last 13 years) in seed production areas. This has been attributed to contaminated culture substrates. Holland Production: 6m tonnes from 170,000 ha. Seed - 40,000 ha (1m tonnes production of which 70% is exported), table - 80,000 ha (3m tonnes), starch - 50,000 ha (2m tonnes). 50% of seed is produced from tissue culture and 50% from clonal selection. For at least 45 years powdery scab has been wrongly assigned as common scab. This has given the impression that powdery scab is a minor disease but this is not the case. The farmers perception of powdery scab is that it is more prevalent in wet years, it mainly occurs on sandy or peaty soils and correct irrigation scheduling reduces it. Only 5-10% of the seed lots are absolutely free of any scab. A rapid development of symptoms occurs between haulm destruction and harvest. A survey of scab during showed that almost all were infected or contaminated with sporeballs. This survey included 45 different cultivars and 100 seed lots. For certification, the inspection services describe scab and they do not differentiate between common and powdery scab. It is believed that there is a constant build up of inoculum in soil and it is not possible to predict where a problem may occur. Germany The starch region adjacent to Holland has a significant powdery scab problem. Spain In the cooler areas of northern Spain near the French border, powdery scab has been recorded. Australia Production: 1.1m tonnes from 40,000 ha. 55% processing, 5% seed, 40% washed table market. Because of quality demands, powdery scab is of most significance in the table market. First record of the disease was in It was found in the state of Victoria in Powdery scab used to be found in the traditional growing areas but can now be found in new production areas (acid sands). Powdery scab sporeballs have been found in dust samples from potato stores, even in areas where the tuber disease has not been observed. The disease was exacerbated by irrigating with borehole water. Soil temperatures drop rapidly from o C to o C when applying this cold water. Powdery scab has been less of a problem in the last three seasons which have been dry and hot, except where potatoes were grown in winter and irrigated. Cultivar susceptibility Very high Mod-high Low Atlantic Very low Kennebec, Shepody Desiree Russet Burbank, Russet Ranger 6

5 Severe galling of the roots of Russet Burbank has been recorded and is suspected to be causing a yield reduction. The spread of sporeballs with soil particles during wind-storms is strongly suspected. Spraing caused by PTMV (or TRV) has not been recorded in Australia. Summary points Powdery scab occurrence and severity is greater in northern European countries. It appears that the incidence and severity of powdery scab is less in warmer countries unless crops are grown at high altitude (where conditions are more favourable) or conditions are made favourable by agronomic practices such as irrigating with cold borehole water or winter cropping. Some countries have a zero tolerance for powdery scab. These include Hungary, Turkey and Cuba. It seems likely that as markets develop in countries to which the EU exports, more emphasis will be placed on tuber quality and blemishes such as that caused by powdery scab will become economically more important. Thus the importance of powdery scab on export seed is likely to increase. Within the EU there is a lack of a common export standard for powdery scab No objective assessment has been made of the incidence and severity of powdery scab across the EU. The extent of soil contamination also remains undetermined. No objective assessments of losses due to powdery scab or spraing have been made. Summary of seed certification standards All seed certification in the countries below is based on inspection of an un-washed sample. There is no science behind the standards, they are set to what is achievable practically. The differentiation between common and powdery scab on a visual inspection can be difficult. The epidemiological significance of standards is not understood. It is not clear whether it is more meaningful to have standards based on incidence or incidence and severity. Country Australia Denmark England France Powdery scab standard Maximum of 2% infected tubers by number ( a single pustule counts as infected). This forms part of a group tolerance of 3% total defects including disease. There is option to vary this standard if the buyer accepts a greater tolerance In the scab category (which does not distinguish between powdery and common scab and includes black scurf): Deep infection (>3mm): maximum of 1% tubers with >10% surface area Superficial: maximum of 3% tubers with >20% surface area Maximum of 5% spraing (tubers are quartered) No cankers. Maximum of 2% tubers with >12.5% surface area 0.2% tubers with powdery scab (1 tuber in 25 kg) 7

6 Holland Scotland Switzerland No cankers. Standards based on scab which can be either powdery or common scab. 2.5 class (Inside EU): 1/3 of tubers can have up to 8 lesions/pustules 1.5 class (Outside EU): 30% tubers free of scab, 70% with a maximum of 3 small lesions/pustules 1 class (Middle east): 50% tubers free of scab, 50% with 1-2 lesions/pustules only No cankers Within an overall group tolerance of 4% tubers showing defects, a maximum of 3% tubers with >12.5% surface area is permitted. Within an overall group tolerance of 4% tubers, a maximum of 2% tubers showing external symptoms of spraing is permitted Not more than 1% tubers with > 5 pustules (i.e. 99% tubers must have less than 5 pustules 8

7 Countries where powdery scab has been recorded 9