Browsing by Author "Aked, Julia"
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Item Open Access Alternative chemical control of anthracnose and crown rot of banana caused by Colletotrichum musae(2000-11) Khan, Sabir Hussain; Aked, JuliaChemical alternatives to the fungicides currently used to control Colletotrichum musae infections on banana fruit have been investigated. Four isolates of C. musae cultured from anthracnose lesions on imported banana fruit were screened against the fungicides thiabendazole (TBZ) and imazalil incorporated into malt extract agar (MEA). Two of the isolates showed some tolerance to TBZ even at 1.24 mM (250 ppm), whereas all four isolates were very sensitive to imazalil at concentrations of > 0.017 mM (5 ppm). One susceptible (CM100) and one tolerant (CM103) isolate (to TBZ) was selected. Eleven chemicals were evaluated over a range of concentrations for their effectiveness in inhibiting the germination and mycelial growth of these isolates in vitro (on MEA). The chemicals were the antioxidants ascorbic acid, propionic acid, benzoic acid, butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl paraben (PP), propyl gallate (PG), dimethyl sulphoxide (DMSO), thiourea and tannic acid and the natural plant product azadiractin. BHA, azadiractin, benzoic acid, PP and PG gave the highest levels of fungal inhibition in vitro. BHA at 5 mM completely inhibited the germination and mycelial growth of CM100 and CM 103 at 25°C. BHA (1 mM) in combinations with benzoic acid (1 mM), PP (1 mM) or imazalil (1.68 pM) completely inhibited germination and mycelial growth of CM103 at 14 and 25°C. The effectiveness of imazalil was not affected by the pH of the medium whereas the effectiveness of BHA was greatest at pH 3.5 and 6.5 and benzoic acid was more effective at lower pH values. C. musae was shown to produce nine extracellular enzymes in liquid medium which have not been reported before including lipases, phosphatases and hydrolases. BHA, imazalil and TBZ caused differential inhibition of the production or activity of these enzymes. The toxicity of selected chemicals was tested against anthracnose or crown rot infections of fingers or hands of banana fruits. Inoculations of C. musae were made with either mycelium in wounds in the banana peel or with conidia on the intact surface of the fruit or on the cut stalk or crown tissues. Of the alternative chemicals tested on their own, BHA and azadiractin gave the greatest inhibition of anthracnose and crown rot infections with dip treatments at 1 mM giving control levels ranging from 10-39% compared with 17.6-38.3% for imazalil (0.84 mM = 250 ppm). BHA (5 mM) in combinations with imazalil (0.84 mM), PP (10 mM) or benzoic acid (10 mM) suppressed anthracnose lesions (CM103) by 64.7, 54.9 and 35.3 %, and crown rot development by 70.5, 62.3 and 42.6 % at 25°C respectively. At 14°C these same combinations gave complete control of anthracnose lesions and crown rot development. TBZ was less effective at controlling crown rot (CM100 or CM 103) than imazalil alone or when in combinations with BHA. Combinations of BHA with imazalil but not with TBZ showed synergism both in vitro and in vivo. The cost/litre of the best trial treatment (BHA at 5 mM with imazalil at 0.84 mM) was less than that of imazalil at 1.68 mM (500 ppm). This suggests that food grade phenolic antioxidants such as BHA may be useful adjuncts to currently used fungicides such as imazalil for commercial control of banana postharvest diseases.Item Open Access Storability of sweet potatoes (Ipomoea batatas (L.)) under tropical conditions: physiological and sensory aspects.(Cranfield University, 2000-03-01) van Oirschot, Quirien; Aked, Julia; Rees, DeborahThe shelf-life of the sweet potato storage root under tropical marketing conditions limits its potential for marketing. This research aimed to identify the physiological characteristics that affect the shelf-life of sweet potato cultivars when they are exposed to tropical marketing conditions. Weight loss was the key limiting factor in storability under the conditions studied. The range in weight loss was large amongst the 39 cultivars tested, and varied between 5 to 15% per week. Weight loss related to the marketable appearance. It consisted mainly of water loss and only 10% was due to respiratory metabolism. Sweet potatoes with high rates of water loss were more susceptible to rotting. The role of periderm characteristics (thickness and permeability), root-size, root surface area/mass ratio and shape were investigated. Although significant differences were observed among cultivars, these characteristics could not account for the variation in storability. The level of damage severely affected the rates of weight loss, with transpiration rate through damaged areas many times higher than through undamaged periderm. Breakage was found to be the most severe form of damage, having a great impact on weight loss for 14 days. Cultivars differed in susceptibility to damage after standardised damage treatments. Susceptibility to breaks was greater for long thin roots. Skinning injury was negatively related to the periderm thickness. Wound healing ability was a major factor for the shelf-life of sweet potato cultivars. It was demonstrated that lignification of wounds as measured by phloroglucinol staining, correlates with reduced susceptibility to weight loss, water loss and microbial attack. A lignin index was used to express the probability that lignification occurs. Cultivars differed significantly in their lignin indices under tropical marketing conditions. A high dry matter content generally coincided with a low lignin index. This relationship was consistent for 19 cultivars tested. Sensory evaluation of five sweet potato cultivars resulted in five distinct sensory profiles. During storage some of the cultivars lost some of their flavour but little changes were observed for textural properties. It was concluded that changes in sensory aspects are not a limiting factor for storage of sweet potato.