Table grapes are a labor and material intensive crop; annual operating expenses in California may exceed $9000 per acre (Peacock et al., 2007). This considerable investment is put at risk by exposure to precipitation within 6 weeks of harvest, which may stimulate the development of rots and molds that render grapes unmarketable (Gerawan and Zweigle, 2004). The most common bunch rot of grape in the San Joaquin Valley is summer bunch rot, a disease complex associated with numerous species of fungi and bacteria, including Aspergillus sp., Botrytis cinerea, Cladosporium sp., and Acetobacter sp. (Bettiga and Gubler, 2013). Botrytis bunch rot may also occur in the San Joaquin Valley, particularly in autumn, when temperatures moderate and clusters become wetted by condensation or precipitation. Canopy management practices that reduce humidity and promote cluster dryness help minimize the incidence of both types of rots (Bettiga and Gubler, 2013). Exposure to precipitation may be avoided by harvesting fruit before 1 Oct., after which time the chance of rain increases considerably, especially in the north-central San Joaquin Valley (Christensen, 2000). However, the grapes of some late maturing varieties may not have achieved optimal market quality by then and prices paid for grapes often increase toward the end of the harvest season, thus providing an incentive to harvest fruit as late as possible.
Some growers cover their grapevines with a sheet of polyethylene film to help prevent precipitation from wetting the clusters of grapes and thereby potentially extend the harvest season (Novello and de Palma, 2008). The films, which are generally ≈1-mil thick and 100 inches wide, are typically deployed in late August or early September and installed in such a way that they form an uninterrupted cover along the entire row length (Gerawan and Zweigle, 2004). In California, the covers are generally supported by the trellis and grapevine canopy and are thus in direct contact with the grapevines’ leaves. The vast majority of growers use one of two different covers, which are green or white colored (Fig. 1). The radiometric properties of plastic covers can affect canopy microclimate (Novello et al., 2000), but there is a lack of knowledge on the effects that covers used in California may have on canopy microclimate, yield, and fruit quality.
Other grape-growing countries, particularly Italy, also use plastic covers, but the timing and method of deployment, and materials used are quite different (Novello and de Palma, 2008). In Italy and some other countries, plastic sheets may be used as a form of protected cultivation, in which the plastic forms a roof and walls around the vineyard (Novello and de Palma, 2008; Novello et al., 2000). Such systems may be installed in winter and kept in place until bloom to promote early ripening (Novello et al., 1999, 2000). Alternately, plastic may be installed later in the season, around veraison, to delay ripening and/or to protect fruit of late-ripening varieties from precipitation (Novello and de Palma, 2008). In such cases, lateral sheets may be retracted, leaving colorless transparent polyethylene sheets attached to frames above the trellising system, with areas between each vine row left uncovered (Novello and de Palma, 2008).
In California, growers use thinner, colored plastics that are installed late in the season, for a relatively short period of time, and in direct contact with the grapevine canopy. Such covers may have different effects on canopy microclimate and fruit quality than the Italian systems. Furthermore, growers may use covers of different colors, primarily green or white, which may differentially affect canopy microclimate, and thus fruit integrity and quality (Novello et al., 2000). Therefore, experiments were conducted to determine the effect of green- and white-colored plastic grapevine covers on several microclimate variables in the fruit zone and on yield, quality, and postharvest spoilage of grapes.
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