Overcoming adverse effects of hailnets on fruit quality and microclimate in an apple orchard

With the increase of hailstorms as a possible result of global warming, fruit crops are increasingly grown under hailnets. This results in lesser fruit quality in terms of colouration, fruit mass, firmness, starch and taste, i.e. sugar and acid, and vitamin content under hailnet due to altered microclimate and light deprivation. Reflective mulches (Extenday and Daybright) were spread in autumn 2006 to improve fruit quality and light utilisation under hailnet at Klein-Altendorf Research Station near Bonn, Germany. A monophosphate (Seniphos) was applied twice for the same purpose; untreated apple cv. 'Elstar' trees served as control.

Under the translucent, 'white' hailnet, humidity was increased by ca. 6%, air temperature reduced by ca. 1.6 °C, soil temperature increased by ca. 0.5 °C and light reduced by 11-15% resulting in lesser fruit quality of 2.5% less sugar and less taste. The two reflective mulches increased light reflection at 45° and 90° angles by 2.5- to 6.3-fold. No differences in fruit ripening and firmness were observed, but fruit from trees under hailnet with reflective mulch contained up to 2.4% (from 13.3 to 15.7%) more sugar than those of the control (uncovered grass alleys). A less negative normalised differential vegetation index (NDVI) of - 0.3 on the red compared with - 0.5 on the green fruit side indicated more chlorophyll in the outer, sun-exposed, red side of the apples relative to the shaded side. Monophosphate-treated fruits maintained the peel chlorophyll with a greener ground colour of 92-97° hue and a NDVI of - 0.3 as in the grassed control. Fruit in the lower canopy with reflective mulch were darker red (a value 30; 22° hue) relative to the grassed control with a = 25 and 43° hue (light red), expressed in a 4-fold increase in normalised anthocyanin index (NAI), but showed enhanced chlorophyll breakdown (NDVI from - 0.2 to - 0.5); similarly, the monophosphate increased the NAI by up to 2.5-fold.

The effect of both reflective mulch cloths was most pronounced on apple fruit in the lower canopy under hailnet, which developed large vitamin C contents and a dark red top colour relative to the grassed control with enhanced chlorophyll breakdown. Both reflective mulch cloths increased the percentage of class I fruit with > 25% colouration by 12% (from 82 to 94%) without and under hailnet by 23% (from 69 to 89%) relative to the grassed control resulting in financial net gains of up to 1300 € ha(-1) Copyright © 2007 Society of Chemical Industry.