3.7.3
THE USE OF DEGREE DAYS TO ESTABLISH BIOLOGICAL EVENTS FOR DIDYMASCELLA THUJINA, A FOLIAR FUNGAL LEAF BLIGHT OF THUJA PLICATA SEEDLINGS

HH KOPE1 and D TROTTER2

1Contact Biologicals, 17 Jedburgh Road, Victoria, BC, V9B 1K7, Canada; 2Nursery Extension Services, BC Ministry of Forests, Green Timbers Reforestation Centre, Surrey, BC, V3V 7Z2, Canada

Background and objectives
Keithia leaf blight, caused by Didymascella thujina, is a fungal pathogen, endemic to North America, which parasitizes the leaves and stems of Thuja species. In the Pacific Northwest of Canada and the United States it is the most serious foliar disease of all age classes of western red cedar trees Thuja plicata. In reforestation nurseries it is an economically important disease on 1- and 2-year old seedlings, and in reforestation sites blight affected seedlings are compromised in their growth and survival [1]. The same conditions that make nursery production of seedlings successful high densities, favourable temperatures and moisture, and intensive crop-management techniques are also ideal for D. thujina development. The kinetics of apothecia and ascospore maturation, and subsequent ascospore discharge are closely associated with increasing temperatures. The objective of this study was to establish the degree days (D) required for Keithia leaf blight development under laboratory, nursery and field conditions, with the aim of supplying the information as a pest management tool for managers of reforestation nurseries.

Materials and methods
Ascospore discharge was monitored using replicate glass slides fastened above active apothecia on T. plicata seedlings grown in incubators at 3, 5, 10, 20 and 28C. Each hour the glass slides were replaced and the number of discharged ascospores/h recorded. Temperature effects on ascospore germination were established by incubating replicate Petri plates at 5, 10, 20 and 28C, for 10 days. These trials established the lower and upper temperature thresholds for ascospore discharge and germination. The degree days needed for complete pathogen development was determined under three developmental conditions. First, in a laboratory incubator, replicated trials were carried out at a constant temperature of 19C using T. plicata seedlings intentionally infected with D. thujina. Second, at a reforestation nursery, naturally infected 1-year old T. plicata seedlings were monitored from ascospore germination through to apothecia development. Third, in a forested area, heavily infected 20 year-old T. plicata trees were monitored from ascospore germination through to apothecia development.

Results and conclusions
The lower and upper temperature thresholds, for both ascospore discharge and germination, were identified as >5 and <28C, respectively. The degree days required for complete pathogen development were calculated for the three growing conditions. Incubator grown T. plicata seedlings had a degree day accumulation total of 1071D. At a constant temperature of 19C, fungal growth, apothecia maturation and ascospore release occurred 77 days after infection. Seedlings grown at a reforestation nursery had a degree day accumulation of 1185D. Ascospore germination, fungal growth, apothecia formation and ascospore discharge all occurred within the same growing season. In a forested area, the degree day accumulation totaled 1476D, over two growing seasons. The ascospores infected the leaf tissue on June 23/94 and on June 16/95 ascospores were first discharged. The higher D total for the forest trees could be related to day length, irradiance, tree nutrition or water relations that occur in the field, versus the more constant growing conditions in an incubator or at a reforestation nursery. Predicting Keithia leaf blight development using degree day accumulations make it possible for reforestation nursery managers to identify the stage of development of the disease, and if a critical number of degree days have accumulated, a manager can determine when to apply a fungicide to protect their crop against discharged ascospores.

References
1. Kope HH, Sutherland J, Trotter D, 1996. New Forests 11, 137-147.