When strawberry plants are grown in relatively dry climate, tip burn and calyx burn may become problematic. In Arizona and other dry lands, without proper management, plants would exhibit tip burn.  In other milder or northern climates, tip/calyx burn on strawberries may be an issue only during the time when the greenhouse is heated (therefore lowering relative humidity).  

Tip/calyx burn is a localized calcium deficiency often caused by limited allocation of calcium driven by mass flow (transpiration).  Incidence of tip burn can be associated with the relatively high VPD (or low humidity) in the greenhouse that causes excessive transpiration and lower turgor to limit calcium supply to the growing meristematic tissues. Therefore tip burn may be first noticed as a darker-colored shoot tip, which later develop a necrotic distal end of leaf or calyx. Calyx burn is particularly problematic as it reduces cosmetic (visual) quality of strawberry fruit.

While tip/calyx burn is a calcium deficiency, there is often sufficient amount of calcium in the root zone.  Therefore, simply increasing calcium concentration in the nutrient solution may not solve the problem. A typical solution that growers may attempt is a foliar calcium supply. It is recommended that applications should cover all shoot tips and growing leaves, not symptomatic leaves and calyx (because they are irreversible).  At the University of Arizona, we have tried foliar calcium applications but with limited success. Our current practice is to induce high turgor during the night by nighttime humidity (or VPD) control. However, the general concern is potential disease outbreak due to the high humidity during the night.  We have not found such an issue over five years, presumably due to the relatively dry climate during the day, but we need to continue evaluation for potential risk.

Guttation and tip burn. An earlier study showed that guttation is a plant health indicator for strawberry suggesting a turgor sufficient to supply calcium and can be observed under low nighttime VPD (< 0.1 kPa; Bradfield and Guttridge, 1979). [Note: VPD lower than 0.1 kPa means humidity greater than ~95% at 15C.]

Bradfield, E.G. and C.G. Guttridge. 1979. The dependence of calcium transport and leaf tipburn in strawberry on relative humidity and nutrient solution concentration. Ann Bot 43:363-372.

For this reason, we examined a nighttime application of floating cover over the ‘Albion’ and ‘Camino Real’ strawberry canopy to maintain high humidity (low VPD) inside the cover. A clear polyethylene film was used for the cover and treatment was applied from sunset to shortly after sunrise every day for 40 days (4/16 – 5/26/2010). Guttation was observed for plants under floating cover almost every morning while no guttation was observed for untreated plants. The nighttime high humidity treatment exhibited significantly lower percent calyx burn (16.7%) and tip burn (9.0%) than non-treated control (48.6% and 47.2%, respectively) throughout the experiment.  When not covered, cultivar ‘Camino Real’ had more tip/calyx burn (58.9%) than ‘Albion’ (37.0%). The nighttime average VPD was 0.09 kPa under the floating cover and 0.88 kPa in air inside the greenhouse. No particular difference was observed for disease, as foliage was completely dry during the day (average VPD: 1.6 kPa) For a larger scale operation, automated curtains may be a possibility as a low cost solution of tip/calyx burn for strawberry grown in semiarid greenhouses.  The result of our study was presented by Chieri Kubota at the 2012 ASHS annual meeting.

In 2013/2014, we have successfully implemented the ‘below-trough’ misting instead of floating cover (2009/2010) or deployable/stowable humidity tent over the rows (2010-2013) we practiced before. The below-trough misting (intermittent) was effective to achieve 95% RH (0.09 kPa at 15°C) inside the canopy for 3 hours (5 min misting every 20 min for 3-4 hours after sunset). See the project blog of this topic also.

Kubota, C. and M. Kroggel. 2012. Soilless strawberry production in semiarid climate: Improving fruit quality by nighttime VPD control. Poster presented at 2012 Annual Conference, July 31- August 3, 2012, Miami, FL.

Updated (8/10/14)