Installation of netting is a long-term investment, but will it have a long-term benefit? Installation of netting by apple growers has been a widespread response to combat adverse weather events.
In addition to providing protection from hail, netting alleviates sun damage by reducing exposure of fruit to solar radiation.
The cost of netting is weighed against these benefits over many years. Current netting products are expected to have a life span of 10 years while netting structures may last 30 to 40 years.
The effectiveness of netting in preventing sun damage of fruit may diminish if future climate predictions of increased air temperatures eventuate.
Studies undertaken in January 2013 will aid modelling of fruit temperatures and evaluate the potential of netting to reduce sun damage in future climates.
Netting
Netting provides protection by shielding fruit from solar radiation. This lowers fruit surface temperatures and reduces fruit exposure to UV-B radiation.
Fruit surface temperatures greater than 46C to 49C (varies depending on variety) for 60 minutes have been shown to cause sunburn browning, while sunburn necrosis results when fruit temperatures exceed 52C for 10 minutes.
Photooxidative sun damage occurs when previously shaded fruit is suddenly exposed to solar radiation. UV-B radiation is known to contribute to sunburn browning, but the interactions of fruit temperature and UV-B radiation exposure are not well understood.
Effects of netting on temperature & radiation
A block of Royal Gala in North Shepparton provided ideal conditions to examine the effects of netting on fruit surface temperatures and exposure to UV-B radiation.
The site consisted of a partially netted Royal Gala block. The absence of netting over part of the block enabled comparison of responses from trees of the same age, variety and rootstock that were subjected to similar management with the main exception being the presence or absence of netting.
Surface temperatures of 60 fruit in each of the non-netted and netted areas of the block were monitored by inserting thermocouples under the fruit skin (Figure 1). Weather data was collected within both areas of the block and in an open area beside the block (Figure 2).
Results
Fruit surface temperature
Maximum daily fruit temperatures were consistently higher in non-netted areas of the block compared with netted areas (Figure 3).
Over the monitoring period, the maximum fruit surface temperature recorded in the non-netted area was 54.1C. By contrast, the maximum fruit surface temperature recorded under netting was 49.6C.
A greater proportion of sensors installed in the non-netted area recorded fruit surface temperatures in excess of 46C (85%) than under netting (45%), and 10 % of sensors in the non-netted area recorded temperatures in excess of 52C.
Fruit surface temperatures exceeded orchard air temperature by as much as 17.5C in the non-netted area and by 14.3C under netting.
Air temperature
Continued next month
See this article and the graphs in Tree Fruit August 2013
