ASSESSING THE POSSIBLE CAUSES FOR THE "STRAWBERRY DRIED CALYX DISORDER" IN FLORIDA AND SPAIN

Several preliminary assays and surveys were conducted in Huelva, Spain, and Florida, USA to characterize the possible causes of the “strawberry dried calyx disorder” (SDCD). Field surveys performed in both locations suggest that certain strawberry cultivars might be more susceptible than others to the appearance of SDCD. At both locations, several cultivars are planted concurrently under equal irrigation, fertilization and pest management programs. Field observations indicated the cultivars ‘Strawberry Festival’, ‘Camino Real’ and ‘Palomar’ showed the SDCD earlier and more severely than others. SDCD symptoms consistently developed after exposure to stressful conditions, such as low temperatures and/or light. The symptoms appeared to be associated with high electric conductivity (EC; salt concentration) of hydroponic solutions or soils. A greenhouse study conducted in Florida with 6 cultivars showed salt injury developed after 30 days of treating ‘Strawberry Festival’ and ‘Camino Real’ plants with a fertilizer solution of EC of 1.2 mS/cm, whereas other cultivars did not show visible injury until 45 to 55 days after initial treatment with solutions of EC equal to 1.2 and 2.4 mS/cm, indicating that these cultivars appeared to be more sensitive to high-EC fertilizer solutions than others. Circumstantial evidence seems to eliminate biotic entities as the causal agents because: a) symptoms comparable to SDCD were developed under pathogen-free greenhouse conditions; b) none of analyzed calyx samples from Florida and Spain has yielded positive pathogen results; c) SDCD tends to disappear during the growing season when either fertigation practices are modified or stressful conditions disappear; and d) SDCD distribution in high tunnels and fields seems to be more accentuated in rows closer to entrances and at the end of drip lines. Until the nature of this disorder is fully understood, strawberry growers should exercise caution on their fertilization and irrigation programs during stressful environmental conditions for susceptible cultivars. Reducing the fertilization rates during the days before an expected freeze, while maintaining regular irrigation programs, appears to help to minimize the incidence of SDCD.