CROP LOAD MANAGEMENT OF SEVEN EUROPEAN CIDER APPLE CULTIVARS: EFFECTS ON BIENNIAL BEARING AND FRUIT QUALITY

284 pagesIn the United States and Canada, rapid growth in production and sales of alcoholic or “hard” cider has brought about a rapid increase in the number of acres planted with high-tannin apple (Malus ×domestica) cultivars, mostly of English and French origin. These cultivars, valued for the “mouthfeel” they confer to alcoholic cider, are often prone to “biennial” bearing, which is largely caused by seed-derived gibberellins suppressing floral bud formation, and thus return bloom. Crop load management, via bloom or fruitlet thinning, is often used to mitigate biennial bearing, as are bloom-promoting plant growth regulator (PGR) sprays. Four field experiments assessed the innate bearing habit, response to thinning, and response to bloom-promoting PGR sprays of seven high-tannin cider cultivars, as well as the effects of crop load on fruit maturity and juice quality. In a three-year hand-thinning experiment conducted in Lyndonville, NY (Chapter 2), biennial bearing index (BBI) correlated negatively with cumulative yield both within and among cultivars: ‘Chisel Jersey’, ‘Dabinett’, and ‘Harry Masters Jersey’ had low BBI and high cumulative yield overall, while ‘Michelin’ had higher BBI and somewhat lower cumulative yield; ‘Brown Snout’, ‘Binet Rouge’, and ‘Geneva Tremlett’s Bitter’ were highly biennial and unproductive overall. Hand-thinning reduced BBI over three years, but rarely achieved comparable or greater cumulative yields over three years. Fourth-year bloom data indicate that, had treatments been re-imposed, thinning would result in comparable or improved cumulative yield, and lower BBI, for four of seven cultivars. This experiment, and an identical experiment performed in the low-crop “off” year only on a different set of trees, indicate that thinning in the “off” year is not effective at promoting return bloom. The same two experiments found that crop load correlated negatively with polyphenol (i.e., tannin) content, soluble solid (i.e., sugar) content, titratable acidity, and nitrogen content, of juice over three years, for all seven cultivars (Chapter 3). The relationship between maturity and crop load was cultivar-specific. A concurrent three-year experiment (Chapter 4) found that PGRs were not effective at promoting return bloom in any of the same seven cultivars. Likewise, a two-year experiment (Chapter 4) conducted in Lansing, NY found that PGR sprays had no appreciable bloom-promoting effect either compared to or in combination with hand-thinning, in ‘Chisel Jersey’ or ‘Brown Snout’, but that crop load did have a significant negative effect on return bloom. A survey of commercial cidermakers in the United States and Canada (Chapter 5) found that biennial bearing affects apple supply for a majority of cidermakers growing their own high-tannin fruit, but only affects a minority of non-grower cidermakers working with high-tannin cider cultivars. Thinning and other horticultural practices to promote annual bearing were not commonly performed by high-tannin cider apple growers. The most mentioned cultivars by both apple growers and cideries who only purchase fruit or juice were the reputedly annual ‘Dabinett’ and the notoriously biennial ‘Kingston Black’. Inconsistent supply was the leading reason given by cidermakers who choose not to use high-tannin fruit in their cider. The work presented in this thesis demonstrates: the efficacy of hand thinning in certain high-tannin cider cultivars at promoting annual bearing, improved cumulative yields, improved juice quality, and overall tannin yield; the long-term profitability of thinning in high-density cider orchards for some cultivars; and the need for further research to identify effective midsummer PGR strategies that can be used with fruit thinning to promote return bloom in biennial high-tannin cider cultivars. Thinning to 9 fruit/cm2 TCSA can improve long-term productivity in ‘Brown Snout’, ‘Chisel Jersey’, ‘Dabinett’, and ‘Harry Masters Jersey’, and can improve long-term tannin production in all seven cultivars described in this thesis