Under netting, light levels are generally lower and in summer, the shade created by netting can significantly reduce fruit skin temperatures (continued from last issue).
Over-tree netting
These two impacts of netting can have a bearing on colour development (Figures 1 and 2). When blush pears are actively synthesising red pigments, heavy shading by nets could reduce colour development by cutting light exposure.
Conversely, for cultivars with the developmental model proposed by Steyn et al. (2005) where highest anthocyanin concentrations are attained in immature pears and colour tends to fade towards harvest, shading from nets in the few weeks before harvest may offer protection against blush degradation caused by excessive exposure to solar radiation.
These impacts are likely to be cultivar specific. For example, benefitting Rosemarie that slows colour development in response to higher temperatures and providing less effect on Forelle and Flamingo which may have capacity to maintain anthocyanin synthesis in hot conditions prior to harvest.
Dussi et al. (1995) found that shading Sensation Red Bartlett pears in the month before harvest slowed the decrease of anthocyanin levels and colour fading
During developmental stages when pears are accumulating anthocyanin, the colour of nets and resultant transmitted light quality (i.e. wavelength) could impact on synthesis. However, more information on anthocyanin or red colour synthesis in response to specific light wavelengths needs to be generated before applied field solutions can be acted upon.
Information to date suggests that exposure to ultraviolet (UV) radiation is important for blush development and that blocking UV with nets and spray-on sunscreens could have negative consequences on colour development.
(continued next month)
See this article and images in Tree Fruit February 2016
