| Western gall rust caused by Endocronartium harknessii (J. P. Moore) Y. Hiratsuka is common on Pinus contorta Dougl. ex Loud. in western Canada. A study was conducted to determine factors influencing rust infection and seasonal occurrence of the mycoparasite Scytalidium uredinicola Kuhlman et al. Spore viability remained above 90% in both field seasons indicating that pathogen viability does not limit infection. Field shoots elongated to 90%, 95%, and 100% of their final length by the third and fourth week of June, and first week of July, respectively. Susceptibility at these stages was 100%, 60%, and less than 10%, respectively. The decline in spore production and decrease in susceptibility combined to reduce infection once shoots reached 90% of their final length.; The likelihood of S. uredinicola presence increased by a factor of 1.3 for each year of growth and increased by a factor of 2.1 for each 1 cm increase in gall size. The incidence of S. uredinicola was only weakly associated with rust severity in the stand. Scytalidium uredinicola was isolated from the surface of galls and from tissues beneath the periderm throughout the year on galls six years of age or older. Histological study confirmed the presence of S. uredinicola in unruptured sori as early as 24 April 1994. Scytalidium uredinicola reoccurred in the same location over 3 successive sporulating seasons from 1992 to 1994. On parasitized galls, S. uredinicola reduced rust spore germination to 5.5%.; Given that S. uredinicola is active at the beginning of the growing season when lodgepole pine is most likely to be infected by E. harknessii, and that it greatly reduces rust spore viability, its potential as a biological control agent would appear quite great. However, the weak association with rust severity in nature suggests that it may not be an important regulating factor in the pathosystem. Further, failure to establish on young galls may be a significant disadvantage for its use as a biological control agent. |
