C. M. Triggs

Sampling factors affecting the use of mineral analysis of apple fruit for the prediction of bitter pit

Abstract Several aspects of the sampling of apple fruit for mineral analysis and prediction of bitter pit incidence were studied. Variability of mineral analyses can be minimised, and precision of bitter pit prediction increased, by sampling fewer fruit per tree from a large number of trees. Less variability in Ca analysis was found by taking two opposite plugs of cortical tissue from each fruit. Fruits from the upper parts of trees had less Ca and a higher incidence of bitter pit. If a high proportion of fruit in the crop are from these sites, then a sampling scheme which excludes them may result in less accurate predictions. The relationship between fruit Ca concentration and fruit size varied with the crop load on tree tree. In heavily laden trees, the decrease in Ca concentration with increasing fruit weight was half that in lightly laden trees. The relationships between fruit size and both bitter pit incidence and Ca concentration were sufficiently strong to be useful in standardising sampling scheme...

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...

Stemphylium disease of asparagus (Asparagus officinalis L.) in New Zealand.

Abstract In New Zealand purple spot of asparagus spears and leaf spot of asparagus fern are caused by Stemphylium vesicarium (Wallroth) Simmons (teleomorph Pleospora sp.). Ascospores and conidia were both trapped above asparagus crops during the spear and fern growing season, September–May. Numbers of ascospores and the frequency with which they were trapped were low during February–April. In experimental fields of ‘Mary Washington 500W’ annual losses in weight of harvested spears as a result of leaf spot were estimated as ranging from 18 to 51% over a 5-year period. Leaf spot was reduced and yield of spears increased by spraying fern with four applications of chlorothalonil or mancozeb during January–March. In 1986–87 three sprays of captafol applied to fern during January–March were as effective as weekly sprays from December to May in reducing leaf spot and increasing spear yield for three cultivars—‘Rutgers Beacon’, ‘Cito’, and ‘Mary Washington 500W’.

Control of bacterial soft rot of calla (Zantedeschia spp.) by pathogen exclusion, elimination and removal

Abstract The incidence and severity of bacterial soft rot of calla (Zantedeschia spp.) tubers at harvest were substantially reduced by planting method, chemical control measures, and removal of infected plant material. Callas grown using a sawdust‐shadecloth method had fewer soft‐rotted plants and tubers than callas grown in soil. Dipping tubers before planting in an aqueous solution of 0.08% copper hydroxide, 0.12% thiram, plus 0.03% benomyl reduced plant and tuber losses from bacterial soft rot. Drenching plants during the growing season with an aqueous solution of 0.12% thiram, 0.08% copper hydroxide, plus 0.15% quintozene gave further control against rots. Weekly removal of rotting foliage and tubers during the growing season also reduced levels of plant and tuber soft rots. This study has demonstrated that effective control of calla soft rot can be achieved using disease management that combines appropriate pathogen exclusion, elimination, and removal methods.

Effects of onion (Allium cepa) plant maturity at harvest and method of topping on bulb quality and incidence of rots in storage

Abstract A field study was carried out over two growing seasons to investigate the effects of physiological maturity of onions (Allium cepa L.) at harvest and different topping methods on bulb colour, skin retention, and the incidence of storage rots. Onion plants were lifted at five different stages of maturity from 0 to 4 weeks after 50% leaf collapse (top‐down). Foliage was removed from the bulbs (topped) either before or after field‐curing. The effects of time of lifting of onion plants and method of topping on the incidence of bulb storage rots were the same in both seasons. Onions that were lifted 3 weeks after 50% top‐down and topped before curing had the greatest incidence of rots in store. Increasing harvest maturity increased the mean skin colour score of onions, and decreased markedly the mean number of intact outer skins. The timing of foliage removal had no effect on mean skin colour score, but onions that were topped before curing had slightly more bulb skins than onions topped after curing. Timing of onion lifting to optimise bulb quality appears to be a trade‐off between skin retention and colour. These results confirm that traditional method of harvesting onions in New Zealand, where onions are lifted at 60–80% top‐down, the bulbs are field‐cured, and the foliage is removed after curing, is the simplest method and best compromise to ensure postharvest onion quality and successful storage.

Effects of cessation of irrigation and time of lifting of tubers on bacterial soft rot of calla (Zantedeschia spp.) tubers

Abstract The incidence and severity of bacterial soft rot of calla (Zantedeschia spp.) tubers at harvest were affected by the time that irrigation ceased and the time when tubers were lifted from the ground. The longer that tubers were left in the ground after the onset of foliage senescence, the greater the levels of soft rot. When irrigation was terminated early, foliage senescence occurred more rapidly than when irrigation was terminated later. Early cessation of irrigation reduced the incidence and severity of tuber rots at lifting without greatly affecting tuber size. Early lifting of tubers reduced the incidence of rots but also reduced tuber yield because tubers were still actively growing.

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.

Effect of plant density and initial corm size on the corm yield of konjac (Amorphophallus konjac)

The effect on annual konjac corm production of planting heavier corms and increasing plant density was investigated at Pukekohe Research Station, New Zealand. Corm yields were 2.2 times the planted corm weights giving a mean corm weight of 741 g harvested from large corms (325 g) and 334 g from small corms (150 g). The mean corm yield increased 1.5 t ha−1 for every 10 g increase in planted corm weight in the 150–330 g range. Increasing the plant density from 33,330 to 106,670 plants ha−1 had a strong linear effect on corm yield. The total yield of corms and offsets from large planted corms increased by 7.2 t ha−1 for each additional plant m−2 to lift yields from 34.9 t ha−1 to 88.1 t ha−1. For small planted corms, total yields rose from 13.7 t ha−1 to 43.9 t ha−1 giving an increase of 4.1 t ha−1 for each plant m−2 increase. Offset production was 12% of the total corm yield grown from large corms and 20% when grown from small corms with no effect from plant spacing.

Factors affecting bacterial soft rot of Zantedeschia tubers

Abstract The effects of wounding of calla (Zantedeschia spp.) tubers (‘Black Magic’), inoculation of tubers with Pectobacterium carotovorum subsp. carotovorum (Pcc) (syn Erwinia carotovora subsp. carotovora), anaerobic incubation of tubers, and temperature at which tubers were maintained on the incidence of tuber soft rot were investigated under controlled conditions. It was found that bacterial soft rot of calla tubers was greatly affected by all four factors. Temperature had the greatest effect on mean disease score, followed by wounding of tubers, inoculation with soft-rotting bacteria, and aerobic or reduced oxygen conditions. No rot occurred in tubers that were not wounded, not inoculated with Pcc, and incubated at 20°C in aerobic conditions. Rots developed in all tubers that were wounded, inoculated with Pcc, and incubated at 30°C. The study clearly demonstrated that Pcc infects tubers through damaged tissue, and infection was greatly enhanced under conditions that impaired host resistance and favoured multiplication of the bacteria.