Controlled environment strategies for tipburn management in greenhouse strawberry production

Greenhouse strawberry production is gaining interest in North America where field production is the primary source of fruit. Issues of off-season greenhouse strawberry production include tipburn, an environmentally induced localized calcium deficiency. Tipburn often increases when nighttime relative humidity (RH) is low (high vapor pressure deficit, VPD) from nighttime heating or venting with dry air in arid climate. High nighttime humidity (<0.1 kPa VPD) effectively increases Ca supply in emerging leaves and suppressed tipburn in strawberry. We adopted the concept by applying floating plastic covers over plants during night and demonstrated the efficacy to mitigate tipburn of two cultivars (L'Albion' and 'Camino Real') grown in greenhouse. This strategy evolved into under-trough (under-bench) misting, shortening time of exposure to high humidity. The misting system runs intermittently at night to increase RH to 95% and efficacy observed over three winter seasons (2013-2016) in greenhouse (Tucson, AZ, USA). Without humidity control, high tipburn incidence and/or severity was observed. However, with >3 h nighttime high RH (<.1 kPa VPD), guttation was observed and leaf/calyx tipburn severity was low in 'Albion' (5.0±1.1 and 3.0±0.8% respectively) in 2013-2014. However, during 2014-2015, consecutive cold nights in November and December resulted in continuous nighttime heating, eliminating high humidity despite misting, which resulted in severe leaf and calyx tipburn. An additional strategy was implemented for remainder of season, covering plants on nights of anticipated continuous heating. In 2015-2016, a modified strategy of humidification and covering was developed. If 2 consecutive nights were below the 3-h, 95% RH minimum (0.1 kPa VPD maximum) requirement, plants were covered for the subsequent 2 consecutive nights, resulting in acceptably low levels of tipburn incidence (<10%). These practices suggest plants tolerate low humidity nights if Ca supply to the growing shoot tip is periodically restored. Implementation using computer-based nighttime humidity control is discussed.