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Water Potential of Ergot Honeydew and Its Influence upon Colonization by Microorganisms. Barry M. Cunfer, Assistant Professor, Department of Plant Pathology, University of Georgia, Georgia Experiment Station, Experiment 30212; Phytopathology 66:449-452. Accepted for publication 18 September 1975. DOI: 10.1094/Phyto-66-449.

Water potential of Claviceps purpurea honeydew exuded outside the glumes of male sterile barley florets was rarely higher than 100 bar and as low as 750 bar. Within the glumes where the sphacelium was growing the water potential averaged 17 to 35 bar. This range of water potential was favorable for C. purpurea mycelial growth and conidial germination (minimum growth 52 bar) as determined on sucrose-amended agar media. Low water potential of honeydew outside the glumes prevented germination of ergot conidia within the sugar-rich droplet. During a simulated rainy period in a mist chamber water potential of honeydew outside the glumes increased to 10 to 30 bar. After the plants were returned to sunny conditions, water potential decreased to less than 100 bar within 4 hours. Conidia of Fusarium heterosporum and other Fusarium spp. which colonized honeydew did not germinate at less than 110 to 120 bar on sucrose-amended agar. Mycelial growth ceased at 150 to 160 bar. Therefore, except during conditions of rain or heavy dew, low water potential of honeydew outside the glumes is a major factor preventing colonization by Fusarium and most other microorganisms.