1. Introduction
1.1 General introduction
The Norwegian production of potatoes comprises approximately 350,000 tonnes, with a wholesale value of approximately NOK 500 million annually (Statistics, 2014). A major part of the potato production is stored for longer or shorter periods. Approximately 10 percent of the production is lost from Norwegian potato stores annually (Bengtsson et al., 1996). This represents both a major loss of resources in an environmental perspective and a high economic loss for the growers. The extent of the potato storage losses is basically determined by the conditions of the potato at harvest (maturity, mechanical damage and infection of disease and pests) and the conditions and duration of storage (Wustman and Struik, 2008).
In Norway, immature potatoes is a major concern, mostly due to the use of relatively late cultivars in combination with a short and cool growing season (100-110 days). Immature potatoes at harvest can lead to high losses caused by poor skin quality (dehydration, diseases), early sprouting and fry colour problems. The impact of maturity on the quality of potatoes after storage is confirmed in several studies (Herrman et al., 1995; Hogge et al., 1993; Kumar et al., 2004; Wiltshire et al., 2004). However, maturation is a complex process including both haulm maturity and physical, physiological and chemical maturity. They all have different effects on the potato quality and more studies are needed evaluating the effect of different aspects of maturity on the potato quality during storage including the effect of storage disease development. Chemical maturity is the most commonly used indicator in prediction models for potato quality (Hertog et al., 1997a; Sowokinos, 1978). Sowokinos (1978) found the level of sucrose at harvest to be a good indicator of subsequent processing quality and suggested to keep levels below 2.8 mg/g fresh weight. However, other studies investigating the relationship between processing quality and sucrose content did not succeed in using sucrose as a predictor (Briddon and Storey, 1996; Lærke and Christiansen, 2005; Wiltshire et al., 2004). More studies are thus needed, looking at the use of other maturity indicators in addition to chemical maturity. In practice, a model predicting tuber quality during and after storage, using measurements in the last part of the growing season and at harvest would be of great interest to the grower and to the processing industry. It could be useful in guiding decisions on length of storage and the order in which the crop should be processed.
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Storage conditions, such as temperature, humidity, atmospheric conditions and ventilation are important elements in maintaining good quality and reducing loss of stored potatoes. The effect of ventilation strategy has been given little attention. There is thus a need for studies on the effect of different ventilation strategies on potato quality under Norwegian conditions during long-term storage.
Fusarium dry rot is an important storage disease, from which problems seem to have increased during the last decades. In order to support future control strategies, studies are needed exploring which Fusarium species are currently causing Fusarium dry rot in commercial potato production since the last survey was done in Norway (Bjor, 1978). In order to implement effective disease-management strategies for Fusarium dry rot, it is important to understand the impact of different inoculum sources on disease development. Studies of the relative importance of seed and soil-borne inoculum between different Fusarium spp. would be useful. Control strategies commonly include use of resistant cultivars. However, knowledge about resistance to Fusarium spp. among the currently most grown potato cultivars in Norway is limited. The Norwegian breeding company, Graminor, has previously done resistance testing in upcoming varieties, but in these tests, the inoculum was made as a mixture of F. coeruleum and F. avenaceum isolates and therefore no information was gained about resistance towards individual species and other species than F. coeruleum and F.
avenaceum. Latent infections can occur in tubers pre-storage. Diagnostic tools can be used to detect these and validate the tuber storability. However, more knowledge is need to confirm the suitability of these tools on Norwegian Fusarium isolates.
1.2 Aim of the study
The main purpose of this thesis is to contribute with knowledge, which can support storage management strategies of potatoes that helps to maintain quality and reduce loss during storage. Special attention was given maturity of the crop, ventilation strategies and the storage disease Fusarium dry rot. The main objective was met by addressing the following research aims presented in five individual papers (I-V) (Figure 1):
• Study the effect of different maturity levels on storage quality of potatoes during long-term storage (Paper I).
• Investigate the potential of potato maturity indicators measured in the field prior to or at harvest to predict potato quality during and after storage (Paper II).
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• Study the effect of maturity levels in potato tubers on Fusarium dry rot development caused by different Fusarium spp. (Paper IV).
• Examine the effect of ventilation strategies on storage quality of potatoes with different maturity during long-term storage (Paper I).
• Identify Fusarium species currently causing Fusarium dry rot in commercial potato production in Norway, including the extent of regional variation, and the effect of agronomic and storage factors (Paper III).
• Evaluate the resistance of commonly grown potato cultivars in Norway to different Fusarium species (Paper IV).
• Test the suitability of available real-time PCR assays for detection of Fusarium spp.
common in Norway, development of new assays if needed (Paper III).
• Investigate the relative importance of seed- versus soil-borne inoculum of three species of Fusarium (F. coeruleum, F. sambucinum and F. avenaceum) in causing dry rot (Paper V).
Figure 1 Graphical outline of the scientific papers (I-V) included in the thesis.
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