Four methods of overwintering container-grown ornamental nursery stock were evaluated. Canopy temperatures of plants unprotected or covered with a thin layer of ice closely followed air temperatures during subfreezing conditions; minimum ambient air temperature was – 16C. Canopy temperatures under white copolymer film fluctuated widely, while canopy and growth medium temperatures of plants continuously irrigated during subfreezing temperatures did not drop below 0C. Root injury of all species and foliar injury of azalea (Rhododendron × ‘Due de Rohan’), euonymus (Euonymus japonica Thunb. ‘Microphylla’ H. Jaeg.), and pittosporum [Pittosporum tobira (Thunb.) Ait.] were least when plants were continuously irrigated or covered with white copolymer film. Foliage of Japanese holly (Ilex crenata Thunb. ‘Compacta’) and dwarf Burford holly (Ilex cornuta Lindl & Paxt. ‘Burfordii Nana’) was not injured with any treatment. Bark splitting of azaleas was most severe under the thinly iced and white copolymer-covered treatments, less severe in the unprotected treatment, and least with continuous irrigation.
The effects of greenhouse environment and outside weather factors on epidemiology and spore release of powdery mildew of roses (Rosa hybrida L.) caused by the fungus Sphaerotheca pennosa (Wallr.) Lev. var. rosae Wor. were studied. Mildew epidemics were correlated to outside dew point temperatures, and mildew incidence could be reduced to low levels by sulfur vaporization and adopting cultural procedures designed to dry out the greenhouse environment prior to nightfall during seasonal periods when mildew was likely to be prevalent Highest spore release occurred in the period between 1200 to 1600, and was correlated with external variables.