A ThesisOrange (Citrus sinensis L.), mango (Mangifera indica L.) and tomato (Lycopersion esculentum Mill.) are among the important horticultural crops in Tanzania. Fruit production in Tanzania is constrained by several pre-harvest factors which cause production of inferior quality. The major pre-harvest causes of low fruit production and quality are pests, diseases, weather conditions, especially drought stress, and improper agronomic practices. Hexanal formulation is relatively a new technology that has been reported to reduce pre-harvest and postharvest losses of fruits and vegetables in temperate and subtropical climates. However, there is limited information on the effect of pre- harvest field application of hexanal on marketable yield and quality of selected fruit varieties grown in Tanzania. Similarly, the effects of field hexanal application on pre- harvest yield losses of selected fruits are also not well known. Therefore, the objectives of this study were to: (i) determine the key fruit attributes used by buyers in selecting fruits; (ii) determine the effects of pre-harvest hexanal application on market fruit attributes, and (iii) determine the effects of field application of hexanal on pre-harvest fruit losses of selected major crops in Tanzania. To achieve objective 1, a study was conducted at Muheza district in Tanga region and Mkuranga district in Coast region to identify key orange and mango fruit attributes preferred by buyers. A total of 179 participants were engaged to evaluate the fruit attributes that influence marketing, longevity and endurance of Orange and Mango. Of the total participants, 50.3% assessed orange fruit attributes and 49.7% assessed similar attributes for mangoes. Data were collected by interviewing consumers, farmers and traders of oranges and mango using open and close ended questionnaires. Results revealed that buyers strongly associate fruit colour, freshness, firmness, spots free and medium size with marketability of mango and orange fruits. Moreover, buyers strongly correlate fruit freshness, colour, spots free and firmness with shelf life of mango and orange fruits whereas buyers further associate spots free, freshness and firmness fruit attributes with endurance of mango and orange fruits. It is recommended that sellers of orange and mango fruits should apply technologies which can improve fruit freshness, firmness and colour, and reduce pest incidences. To achieve objective 2, an experiment was laid out in a Completely Randomized Design in a 4 x 4 factorial arrangement using well established and maintained farmers‟ of orange, mango and tomato orchards. The factors A was hexanal concentrations (0.01, 0.02, 0.04% and control - untreated fruits) and factor B was time of hexanal application prior to fruit harvest (7, 21, 42 and 60 days to harvest) for orange varieties (Early Valencia, Jaffa and Late Valencia) and mango varieties (Apple, Palmer and Keitt). For tomato (Mwanga, Rio Grande and Tanya) cultivars, the factor A was hexanal concentrations (0.01, 0.02, 0.04% and control - untreated fruits) and factor B was time of hexanal application prior to fruit harvest (7, 14, 21 and 28 days to harvest). Each fruit species was treated as an individual factor to achieve this objective. Orange: Results show that application of hexanal at 0.01% improved fruit marketable yields by increasing fruit firmness and marketable yield of Early Valencia, Jaffa and Late Valencia varieties. The orange marketable yield increased by 19.28, 26.21 and 30.74% over the controls for Early Valencia, Jaffa and Late Valencia, respectively. Similarly, fruit firmness increased by 9.07, 9.64 and 10.12 N/mm 2 over the control for Early Valencia, Jaffa and Late Valencia, respectively. Mango: Results indicated that application of hexanal concentration 0.01% increased mango marketable yield by 24.54, 19.64 and 20.40% over the controls for Apple, Palmer and Keitt varieties, respectively. Likewise, fruit firmness increased by 11.19, 9.97 and 10.05 N/mm 2 over the control for Apple, Palmer and Keitt varieties, respectively. Tomato: Results show that hexanal application 0.01% increased tomato marketable yield by 23.38 and 23.10% over the controls for Mwanga and Rio Grande cultivars, respectively. Similarly, hexanal application at 0.01% increased fruit firmness by 5.03, 5.77 and 5.19 N/mm 2 over the control for Mwanga, Rio Grande and Tanya tomato cultivars, respectively. To achieve objective 3, an experiment was laid out as in objective 2 above. Orange: Results show that application of hexanal at 0.01% reduced number of dropped fruits by 22.62, 37.73 and 46.31% over the untreated fruits (control) for Early Valencia, Jaffa and Late Valencia orange varieties, respectively. Similarly, application of hexanal at 0.01% reduced non-marketable yield by 21.39, 26.10 and 30.74% over the control for Early Valencia, Jaffa and Late Valencia orange varieties, respectively. Likewise, application of hexanal at 0.01% decreased incidences of pest damage by 21.59, 22.50 and 24.86% over the control for Early Valencia, Jaffa and Late Valencia, respectively. Mango: Results further show that application of hexanal at 0.01% reduced number of dropped fruits by 25.94, 20.77 and 22.58% over the control for Apple, Palmer and Keitt mango varieties, respectively. Moreover, application of hexanal at 0.01% reduced non- marketable yield by 24.82, 19.59 and 21.40% over the controls for Apple, Palmer and Keitt mango varieties, respectively. The application of hexanal at 0.01% also reduced incidences of pest damage on fruits by 27.93, 17.05 and 19.58% over the control for Apple, Palmer and Keitt mango varieties, respectively. Tomato: Results show that application of hexanal 0.01% reduced non-marketable yield by 23.24, 23.27 and 28.39% over the control for Mwanga, Rio Grande and Tanya tomato cultivars, respectively. Similarly, the application of hexanal at 0.01% also decreased incidence of pest defects by 22.53, 22.00 and 23.02% over the control for Mwanga, Rio Grande and Tanya tomato cultivars, respectively. General conclusion and recommendation: According to farmers, traders and consumers the main fruit attributes which influence purchase preference of mango and orange are freshness, colour, spots free and firmness. Pre-harvest application of hexanal at 0.01% improves fruit attributes namely firmness and freedom of fruit skin from pest damage of orange, mango and tomato treated varieties. The application of hexanal at 0.01% further increases marketable yield of oranges, mango and tomato treated varieties by reducing dropped fruits, number of non-marketable yield and incidences of pest damage. It is recommended that farmers should apply hexanal at 0.01% from 42 to 7 days before fruit harvest in order to improve fruit attributes to enhance marketing, and increase marketable yield of orange (Early Valencia, Jaffa and Late Valencia), mango (Apple, Palmer and Keitt) and tomato (Mwanga, Rio Grande and Tanya).