S. L. Lewthwaite

Free sugar composition of sweetpotato cultivars after storage

Abstract The concentrations of fructose, glucose, sucrose, and maltose in sweetpotato (Ipomoea batatas (L.) Lam.) roots, following an 8‐month storage period, were assessed by high performance liquid chromatography (HPLC). Imported cultivars and breeding lines were compared in both the raw and cooked state against New Zealand standards. Strong linear relationships were demonstrated between concentrations of the sugars (fructose, glucose, and sucrose) in cooked roots, and the corresponding sugars in raw roots (R2 of 93.5, 93.8, and 88.8% respectively). The relative proportion of fructose to glucose 0.44: 0.56 was very stable across all cultivars, and independent of the total concentration of the three sugars—fructose, glucose, and sucrose. The negative relationship between sucrose and the levels of fructose + glucose was significant (P < 0.001) excluding cultivars ‘Jewel’ and “Toka Toka Gold’. All of the clones produced considerable amounts of maltose during cooking, which was significantly (P < 0.001) rela...

In vitro elimination of onion yellow dwarf and shallot latent viruses in shallots (Allium cepa var. ascalonicum L.).

Abstract Two shallot (Allium cepa var. ascalonicum L.) cultivars, ‘Mikor’ and ‘Jermor’, were used to test a protocol for in vitro virus elimination. Basal explants were prepared, surface sterilised, and placed onto a medium consisting of Murashige and Skoog (M & S) salts and vitamins with the addition of 3% sucrose, 1.0 mg/litre benzyladenine, 50 mg/litre ribavirin, and 0.8% agar. The explants underwent 5–6 days of continuous heat therapy: 4 h light at 35°C; 4 h dark at 31°C. When the shoots were 2–3 cm long they were excised, transferred to shoot inducing medium without ribavirin (the anti‐viral chemical), and grown under normal tissue culture conditions of 24°C under fluorescent lights with a photoperiod of 16 h. Finally, the plantlets were introduced to a bulb inducing medium (M & S salts and vitamins, 120 g/litre sucrose, 5 g/litre activated charcoal) for 2 months in vitro before being tested for onion yellow dwarf and shallot latent viruses by ELISA (enzyme‐linked immunosorbent assay) and transferred...

Virus disease surveys of ware potato crops, Franklin County, North Island, New Zealand

Abstract Surveys for virus disease in potato (Solanum tuberosum L.) ware crops were completed over three successive seasons. Overall mean incidence of viruses in plants derived from pathogen‐tested (PT®) seed lines and non‐PT seed lines was 2.2 and7.6% (1991),4.1 and 11.2% (1992), and 2.5 and 6.5% (1993) respectively. The levels of potato leaf‐roll virus and severe mosaics were particularly low in crops grown from PT® seed. Virus incidence was generally low apart from individual crops with high potato virus X, potato virus S (PVS), or potato virus Y (PVY) levels (these crops were often from saved seed tubers). The incidence of PVYN appears to have been contained through the use of PT® seed, but the identification of PVSA and the rediscovery of potato virus A and potato virus M are of some concern. Suggestions for improvements to seed quality control are discussed.

‘Karaka’: A new fresh market potato with high resistance to Globodera pallida and G. rostochiensis

Abstract ‘Karaka’ is a new potato cultivar with high resistance to all populations of potato cyst nematode (Globodera pallida and G. rostochiensis) that occur in New Zealand. Its main market niche is as an early to early maincrop fresh market potato, and it has some potential for french fry production.

‘Gladiator’: A new potato cultivar with high resistance to potato cyst nematode and powdery scab suitable for french fries and fresh market

Abstract ‘Gladiator’ is a new maincrop potato (Solanum tuberosum L.) cultivar with resistance to both potato cyst nematode species (Globodera rostochiensis and G. pallida) and powdery scab (Spongospora subterranea). It has long oval, block‐shaped tubers suitable for french fry manufacture when mature and for fresh market when still immature.

Sprout suppression in sweetpotato roots following immersion in sodium hypochlorite solutions

Abstract The effect of varying sodium hypochlorite (NaOCl) concentrations and immersion times on sprout suppression in sweetpotato (Ipomoea batatas (L.) Lam.) roots was examined. Treatments included immersion times of 20, 60, and 180 min by solutions containing 0, 0.33, 1.0, 3.0, and 9.0% NaOCl by volume, in all combinations. The aim of the experiment was to find combinations of NaOCl concentrations and immersion times that would minimise sprout production while maintaining root quality. Root quality was defined by the frequency of surface lesions and weight loss during storage. Sprout production was significantly (P < 0.05) reduced, but at the expense of root quality. No combination of factors gave comprehensive sprout inhibition while maintaining root quality at an acceptable level.

Laboratory evaluation of sweetpotato (Ipomoea batatas) resistance to sclerotinia rot

Abstract Sweetpotato (Ipomoea batatas) resistance to sclerotinia rot (pink rot), caused by the fungus Sclerotinia sclerotiorum, was evaluated using a laboratory technique. Excised stem lengths of four commercial sweetpotato cultivars were pierced, then inoculated with mycelial disks from S. sclerotiorum cultures. The inoculated stems were incubated in moist chambers for 48 h at 20°C and then assessed for rot severity. Of the four cultivars, ‘Toka Toka Gold’ was most susceptible to the fungus and ‘Beauregard’ most resistant. The cultivars ‘Owairaka Red’ and ‘Northland Rose’ had moderate levels of resistance. Correlations between laboratory and field results were good for ‘Toka Toka Gold’ and ‘Beauregard’. Although plant growth habit affected the disease responses of ‘Owairaka Red’ and ‘Northland Rose’, laboratory and field results for these cultivars were similar. Laboratory tests will prove useful in determining the level of disease resistance in diverse sweetpotato germplasm.

‘Moonlight’: A new dual‐purpose main crop potato (Solanum tuberosum) cultivar

Abstract ‘Moonlight’ is a new early‐main to main crop potato (Solanum tuberosum) cultivar with high yield potential that has been developed for both the fresh market and French fry production. Tubers are oval and have white flesh. ‘Moonlight’ has moderate resistance to the cream potato cyst nematode (Globodera pallida) and has the H1 gene for resistance to G. rostochiensis. It also has moderately high resistance to powdery scab (Spongospora subterranea) and bacterial soft rot (Erwinia sp.)

‘Dawn'—a new early‐maincrop fresh market/crisping potato cultivar

Abstract ‘Dawn’ is a new, high‐yielding, early‐maincrop potato (Solanum tuberosum L.) cultivar suited to the fresh market and crisp manufacture. Tubers are round‐oval in shape and have white flesh. This variety has moderate resistance to late blight (Phytophthora infestans) and to powdery scab (Spongospora subterranea).

‘White Delight’: A new maincrop fresh market potato cultivar

Abstract ‘White Delight’ is a new, high yielding, maincrop potato (Solarium tuberosum L.) cultivar suitable for the fresh market and crisp manufacture. Tubers are round—oval in shape and have white flesh. This variety has moderate resistance to late blight (Phytophthora infestans) and to bacterial soft rot (Erwinia sp.).