Potato variety characteristics
Marketable Yield: Up to 1990 the marketable grades were 40 to 80 mm but since 1991 they have been 45 to 85 mm. Yields are presented as percentages of the yields of the control varieties - Desiree for maincrop and second early varieties and Home Guard for first early varieties. For maincrop and second early varieties, mean yields are presented from two trials per year conducted in Northern Ireland over a minimum of three years. For first early varieties yields at an early lift, normally mid-end June, are presented in the tables together with yields at maturity.
Tuber Number and Size: Varieties producing a large number of tubers will have a high figure on a 1-9 scale, each point on the scale being an increment of 50,000 tubers/ha giving an increase from 200,000 tubers/ha for 2 to 500,000 tubers/ha for 8 on the scale. Varieties with greater proportions of their marketable yields in the 65 - 85 mm grade (60 - 80 mm pre-1991) compared with the 45 - 65 mm grade (40 - 60 mm pre-1991) will have high figures on a 1-9 scale.
Defects and damage: Low production of defects, namely cracked, green, mechanically damaged during handling at harvest, misshapen, slug or wireworm damaged tubers, is indicated by a high figure on a 1-9 scale. Varieties with good resistance to damage as assessed in controlled tests will have a high figure on a 1-9 scale.
Foliage maturity: Varieties whose foliage matures and senesces early are given a high figure on a 1-9 scale.
Dry matter is presented as a percentage figure.
Freedom from Disintegration: Varieties which disintegrate when cooked will have a high figure on a 1-9 scale.
Mealiness: Varieties which are mealy when cooked will have a high figure on a 1-9 scale.
Cooked Flesh Colour: When cooked, flesh colour is described as white (W), cream (C), light yellow (LY) or yellow (Y).
Freedom from After-Cooking Blackening: Varieties which show little tendency to blacken after cooking will have a high figure on a 1-9 scale.
Freedom from Enzymic Browning: Varieties in which a brown colour does not develop on the surface of peeled potatoes before cooking will have a high figure on a 1-9 scale.
Crisp Colour: Varieties which have a light colour after crisping will have a high figure on a 1-9 scale. Many varieties will have a suitable crisp colour both after harvest and after storage. A few varieties have a suitable crisp colour only after harvest and an extremely occasional variety has a suitable crisp colour only after storage. Other characteristics, such as tuber shape, will also determine overall suitability for crisping.
Varieties are included in chipping tests if they have dry matter contents of greater than 20% or if their breeder has claimed chipping and/or crisping as an end-use. Fry colour and its consistency, internal discolouration and peeling efficiency are assessed on selected varieties after harvest and after storage. Details of the characters and their methods of assessment are provided below and in the Appendix.
Potato tubers are metabolically active during storage and this is most readily seen as they begin to sprout. Storage temperature affects fry colour. Colder store temperatures slow down sprouting and prolong storability but produce darker fry colours. Varieties that produce light fry colours when stored at colder temperatures would be highly desirable. AFBI Crossnacreevy has recently introduced assessment of fry colour after storage at colder temperatures as well as the regular storage regime. Preliminary results can be obtained by contacting the potato team at AFBI Crossnacreevy
The chipping tests include the following assessments:
Fry colour after chipping 500g freshly cut chips at 190°C for 4.5 minutes.
Colour is assessed using the USDA chip colour chart which has seven grades: 000 – 00 – 0 for light coloured chips and 1 – 2 – 3 – 4 for darker coloured chips. The numbers and weights of chips in each of the grades are recorded. This scale is converted into an AFBI 1-7 scale, where 1 = ultra light and 7 = very dark, for ease of computation. The results have been presented as the following characters:
Fry Colour: mean colour of ALL chips
Colour Consistency: a calculated measure of the distribution of chips across all colour grades on a 0 – 3+ scale where 0 = no variation, i.e. 100% of the chips found in one colour grade.
Varieties suitable for chipping should have a golden (USDA: 1, AFBI: 4) colour, possibly light (USDA: 0, AFBI: 3) in a few varieties and occasionally dark golden (USDA: 2, AFBI: 5) in some varieties. Ideally the chips should have no variation in colour (0) or be highly uniform (0.01 – 0.25) or uniform (0.26 – 0.50). Fry colour may deteriorate slightly and colour often becomes more variable during storage.
Internal discolouration after frying
Internal discolouration of the chips may occur. This greying of the flesh is similar to that assessed in after-cooking blackening of steamed tubers. It is assessed by slicing 10 chips and determining the discolouration on a 1 – 9 scale where 9 = no discolouration.
Internal discolouration of varieties should not occur (>9.0) but is often very slight (8.5 – 8.9) or slight (7.0 – 8.4). Internal discolouration frequently becomes worse during storage.
Peeling efficiency is determined from weights of the tubers before and after peeling and is reported as a percentage.
A high peeling efficiency (>80%) is desirable in chipping varieties with many varieties achieving good peeling efficiency (70 – 79%). Peeling efficiency deteriorates slightly in most varieties during storage.
Full details of the scales for all characters are given in the Appendix.
Diseases and pests
Good resistance to disease, including viruses, is indicated by a high figure on a 1-9 scale. For some varieties, resistance to dry rot has been determined separately for Fusarium coeruleum (F.c.) and Fusarium sambucinum (F.s.). Resistances to potato cyst eelworm, Globodera rostochiensis and Globodera pallida, are usually presented as S (susceptible), PR (partially resistant) and R (resistant) but, more recently, a numerical 1-9 scale has been introduced for new varieties similar to that for fungal and viral diseases. This information on diseases and pests mainly summarises results of tests conducted during the two preliminary years of evaluation for National List purposes in the United Kingdom. Resistance scores on some varieties have been based on tests conducted by the National Institute of Agricultural Botany, Cambridge.