Effects of spore availability, spore germinability, and shoot susceptibility on gall rust infection of pine

Temporal changes in three factors (shoot susceptibility, spore availability , and spore germinability) were evaluated to estimate their effects on the infection of lodgepole pine (Pinus contorta var. latifolia) by Endocronarti um harknessii. Germinability of rust spores from sori free of hyperparasite s remained above 80% during the growing season and thus would not have sign ificantly limited infection. In contrast, there was a slight, but statistic ally insignificant, increase in estimated susceptibility as infection incre ased from 94% of maximum at 45% shoot elongation to maximum susceptibility at 90% shoot elongation. This was followed by a precipitous decline in esti mated susceptibility to 57% of maximum when shoots were 95% elongated. Spor e availability was 24% of maximum when shoots were 45% elongated, and thus may have been limiting at the beginning of the growing season. At the end o f the growing season, spore availability declined prior to, or simultaneous with, the decline in susceptibility. The combined effect of both factors d rastically reduced the number of infections estimated to occur once shoots are more than 95% elongated. As shoots elongated, relatively more infection s formed higher on the shoot, supporting the hypothesis that periderm is im portant in limiting infection.