More research is required to better understand the underlying genetics and biochemical pathways for red pigment (i.e. anthocyanin) development in the skin of full red (FR) and blush (B) pear cultivars.
Until this knowledge builds, there are limited options in the orchard that can be recommended for growers to manipulate blush levels.
Blush development is restricted when fruit are not exposed to light. In the orchard, dappled sunlight should reach the ground at all times as an indicator of good light penetration through the canopy.
Canopy training and pruning methods should open the foliage for light exposure.
Maintaining canopies to allow maximum light exposure is not difficult but foliage thinning can have the counter-effect of exposing fruit surfaces to excess radiation, thereby increasing skin temperature, accelerating anthocyanin degradation and raising incidence of fruit sunburn.
Over-tree evaporative water cooling
Under hot conditions, over-tree cooling can help lower fruit temperatures; and for those cultivars where anthocyanin synthesis declines in the weeks immediately prior to harvest, cooling may reduce anthocyanin degradation leading up to harvest.
In South Africa, Wand et al. (2005) found that over-tree evaporative water cooling starting two weeks before harvest usually improved Rosemarie (B) fruit blush. However, its effect was relatively small compared to colour change in response to fluctuating temperature in warm production areas.
Cultivar Rosemarie responds well to lower temperatures but is inhibited under hotter conditions.
In comparison, evaporative cooling had no significant effect on cultivar Forelle (B) fruit colour or mass but reduced firmness and total soluble solids when started early in the season (Wand et al. 2005).
Cooling is less necessary for Forelle and Flamingo (B) because they tend to retain colour stability under hot conditions, possibly through on-going anthocyanin synthesis (Steyn et al. 2004).
In trials in the USA, Dussi et al. (1997) applied evaporative cooling treatments with over-tree sprinkler irrigation to Sensation Red Bartlett (FR) pear trees during the final 30 days of fruit development when orchard air temperatures were greater than 29C.
Fruit from cooled trees were more red and less yellow than fruit from non-cooled trees and in one season, cooled fruit had a greater portion of surface blush.
However, fruit firmness between 100 and 150 days after full bloom decreased more rapidly in fruit from cooled trees. Dussi et al. (1997) recommended that cooled fruit should be harvested earlier than non-cooled fruit to maintain postharvest quality.
Research in Australia on evaporative cooling has focused on apple and mainly to prevent sunburn damage. Results showed that fruit surface temperature can be reduced by 10C to 15C by applying overhead watering during the daytime.
Watering can be pulsed and timed to avoid wetting the understorey excessively.
Over-tree netting (continued next month)
See this article in Tree Fruit Jan 2016
