How to achieve good red colour in apples and pears

This checklist of pre-plant decisions and management considerations will help growers increase pack outs with the required red colour. By Wiehann Steyn.

• Plant redder strains of existing cultivars.
• Don’t plant apple cultivars that colour up with difficulty in areas where night temperatures are not conducive to inducing red colour development.
• Choose appropriate sites at farm level — warm sites or those that induce excessive vigour are not the best for red colour development. Rosemarie pears were found to develop better red colour on light, sandy/stony soils compared to heavy clays. This might be due to an indirect effect on vigour.
• Red colour development is a high irradiance response. This means that the extent and intensity of red colour depends on the amount and duration of light that strikes the fruit skin. Shading will reduce red colour development. The following considerations and practices can ensure that fruit throughout the canopy are exposed to sufficient light for good red colour development:
  • Choose appropriate training systems that won’t shade a large proportion of fruit.
  • Smaller trees on dwarfing rootstocks should have a higher proportion of fruit exposed to enough light, but note that even small trees can be overly dense. In pears, more dwarfing rootstocks were found to increase red colour irrespective of tree size, i.e. fruit receiving similar light levels will be redder on a more dwarfing than vigorous rootstock.
  • Use pruning to ensure adequate light distribution throughout the canopy. This may involve both winter and summer pruning. In apples, Regalis application can improve light distribution by reducing shoot extension growth.
  • Since maximum red colour is achieved before harvest in pears — typically in November to December in Forelle and Rosemarie — good light exposure of fruit at this time would maximise red colour potential. Towards harvest in these pears, some level of shading may actually reduce red colour fading. Summer pruning closer to harvest may increase red colour loss in blushed pears.
  • In very lucrative cultivars, try breaking out spur leaves that are shading fruit shortly before harvest.
  • Aim to leave one fruit per cluster to prevent between fruit shading — this may not always be realistic from a yield perspective.
  • Shading may reduce red colour on the southern side of east to west rows, while exposed fruit on the northern side of such rows may be lost to increased levels of sunburn.
  • Reflective mulch is used in many apple-production areas to increase red colour development. It is unsure why we are not using reflective mulch locally, but it may relate to cost and durability under local orchard -floor conditions.
  • Shade netting could reduce red colour development, due to less light reaching the fruit. But shade netting may prevent the most exposed (and potentially reddest) fruit from developing sunburn, thus neutralising the negative impact of shading. Note that light management under nets requires special attention.
  • Sunburn is always a risk as good red colour requires exposure to light. Drought stress, and other factors that increase sunburn, can decrease the amount of fruit with good red colour.
• Excessive nitrogen levels may decrease red colour development by increasing vegetative growth and shading. Excess nitrogen also directly decreases production of the red pigment. High fruit peel nitrogen levels increase the green ground colour resulting in an (unattractive) muddy red blush.
• Low temperatures induce production of the red pigment. Apart from appropriate site selection, the following temperature-related tools may improve red colour pack outs:
  • In apples, red colour can increase dramatically with the passing of a cold front prior to harvest. If fruit maturity allows, delaying harvest until after the passing of a cold front will increase colour pack outs. The same strategy would work for Rosemarie and maybe also for Forelle pears, but not all pear cultivars respond to low temperatures.
  • Following on the previous point, delaying harvest maturity by chemical means (applying Retain or Harvista) may increase the likelihood of low-temperature stimulation of red colour development in later apple cultivars. However, such applications can also decrease red colour development if fruits are harvest ready before low temperatures occur.
  • Evaporative cooling can increase red colour synthesis and prevent the breakdown of anthocyanin at high temperatures. However, evaporative cooling is unlikely to be a viable method to increase red colour under conditions of water scarcity — in other words, this is not an option in South Africa.
• Fruit from trees with poor carbohydrate reserves will generally develop poorer red colour. Overcropping may decrease red colour pack outs. This is due to shading between fruit in a cluster, by reducing the carbohydrate building blocks of anthocyanin, and by increasing sunburn levels. Girdling apple trees prior to harvest may increase red colour development, but is not a consistent treatment. It isn’t a miracle treatment, and potential side effects need to be considered.
• There are no silver-bullet chemicals available to turn green apples red under low light conditions and in the absence of low temperatures. Improved red colour upon application of certain nutrients can often be attributed to remediation of a shortage. Chemically advancing ripening in apples can increase fruit red colour, but may have serious undesirable side effects on storability and internal fruit quality.