Colour development of apple with reflective mulches and biostimulants

The aim of this research was to improve fruit quality and fruit colouration using four-year-old ‘Braeburn Hillwell’ apple trees on M9 (3×1 m spacing) by employing reflective ground covers and biostimulants at Klein-Altendorf, nr. Bonn, Germany (50°N). Two reflective ground covers were spread on both sides of tree rows seven weeks before anticipated harvest and two biostimulants were applied twice – four and two weeks before harvest. Adjacent trees of the same row without reflective ground cover or without biostimulants served as control. For each treatment, ten replicate trees plus one border tree either side were employed and all trees hand thinned to the same number of fruit at the end of July, i.e., one month prior to the treatment. Apple trees with Extenday^® and Lumilys^® yielded from 69 to 44% well-coloured fruit (with 75-100% fruit colouration) compared with 16-26% in the control. No differences in sugar content (°Brix) and fruit firmness (kg cm^-2) were observed as a result of the reflective mulches. Starch breakdown in fruit exposed to seven weeks reflective mulch was significantly delayed compared with the control resulting in retarded maturation of fruit with reflective mulches. In 2015, a year with delayed maturation, biostimulants showed neither effects on inner and external fruit quality nor on fruit with >75% surface colour. Trees treated with the biostimulants Wuxal^®ATRiun or Sunred^® did not improve the percentage of well-coloured fruits (75-100% fruit surface with red colouration) compared with the untreated control; Sunred^® improved the portion of fruit >25% colouration by 15%. Similarly, inner and external fruit quality showed no significant differences in fruit firmness, sugar content and starch degradation. An economic analysis showed an economic advantage, if the reflective mulches are used more than once a year.