Rainfall measurement (Rot risk assessment)

The most significant factor influencing rotting in stored fruit is rainfall. Rainfall varies considerably from place to place and is most influenced by height. Summer rainfall is particularly localised. Therefore, to get an accurate assessment of rot risk in store, growers need to obtain daily rainfall from a representative orchard on the holding.

This can be obtained from either an automatic weather station if one is in use, or achieved using a simple rain gauge as follows:

  • Where possible, the gauge used should be a standard copper gauge with a five-inch collecting funnel. Readings should be taken using a tapered glass measure calibrated in mm.
  • The site chosen for the gauge should be level, clear of overhanging objects, which might drip into the funnel, and there should be no objects close to the gauge which could cause eddy effects and thus inaccurate catches when rainfall events occur in windy conditions.
  • The gauge should be installed so that the rim of the collecting funnel is one foot above ground and should be level and firm. A ring of gravel around the gauge will prevent large raindrops from splashing back into the funnel from the ground when rainfall is heavy.
  • While it is not essential that the gauge is read daily or at the same time each day, it is standard practice for met. stations to take daily readings at 0900 hr GMT in order that readings are comparable. It is probably a good idea if growers follow the same practice.

Average rainfall
Total monthly rainfall (mm) for 1998, 2005 and 50 year average for East Malling

Month 1988 2005 50 year average
April 93.6 49.2 44.5
May 14.0 33.0 45.8
June 64.4 6.2 49.7
July 23.4 39.0 46.4
August 8.4 51.2 52.0
September 89.0 32.6 63.7

The rainfall criteria are based on rainfall for south east England. The table above shows 50 year monthly average rainfall for East Malling to illustrate average rainfall figures on which to judge the rot risk based on rainfall. On this basis, 1998 would be considered May and August below average and 2005, below average for June but average for the other months. Obviously regions which normally have higher average rainfall than south east England will inherently have a higher rot risk.

Collected rainfall information is used as follows:

Rainfall criteria

Fungal rot Rainfall criteria
Botrytis Rainfall June – harvestscore as average
Neonectria Rainfall blossom – harvestscore as average
Gloeosporium Rainfall in month prior to harvestscore as average
Phytophthora Rainfall in 15 days prior to harvestlow or no rain = low risk
20 mm or >= high risk