Optimum temperatures for handling tubers
Tuber temperatures between 12 and 18°C are considered best for harvesting and handling tubers. Tubers warmer than 18–20°C and under drought stress are susceptible to black spot bruising. The optimum day/night temperatures for potato growth are 25/12°C. Harvesting when tuber pulp temperature exceeds 18°C or soil temperature is greater than 20°C increases the risk of microbial rots, especially in damaged tubers.
Optimum soil temperature for harvest is 12–18°C. Harvest temperatures greater than 25°C or lower than 5°C can lead to bruising.
Soil temperature is primarily controlled by air temperature and solar radiation. The drier the soil, the closer the soil temperature will be to air temperature. High soil temperatures can be reduced by more frequent irrigation to provide evaporative cooling.
Tuber development declines as soil temperature rises above 20°C and practically stops above 30°C. The number of tubers per plant is higher at low soil temperatures while bigger but fewer tubers are set at higher temperatures.
Seasons with long periods of high temperatures increase the rate of vine senescence at the end of the season, exposing the soil to more sunlight and higher temperatures. Tables 1 and 2 show average soil temperatures for some of the main potato producing areas in WA.
For Manjimup and Pemberton, the major ware production areas, the high minimum soil temperature in March which is close to 20°C is considered undesirable for potato handling due to increased risk of microbiological decay (Table 2). The soil temperature in March ranges from 18 to 29°C in Manjimup. This means a high risk of bruising and rots if tubers are harvested at this time.
The main harvesting months at Busselton will have reduced risk of rots and bruising as minimum soil temperatures are within the range of 10–15°C which is considered the best for handling and harvesting tubers (Table 2). However, maximum soil temperatures in the warmer months are above 20°C and harvesting during the hotter parts of the day could lead to increased black spot bruise damage and increased risk of rots during storage.
Soil temperatures at Myalup from May to September will cause low risk of rots and bruising (Table 1). However, maximum temperatures in November and December are above 20°C and harvesting during the hotter parts of the day could lead to increased black spot bruises and risk of rots during storage.
Soil temperatures in the Perth area show reduced risk of rots and bruising during September (Table 1). Risk may increase later in the year as soil temperatures rise.