摘要或動機

Purpose Every fall, I collect highbush cranberries (Viburnum edule) to make jelly. In 2012, for the first time, I observed highbush cranberry leaves covered in striking patterns of raised purple dots (telia) caused by the pathogenic rust fungus Puccinia linkii. I investigated the distribution and effects of this rust for several reasons: little is known about P. linkii, highbush cranberries are an important food source for wildlife and people, and foliar pathogens may increase with climate change in sub-boreal forests. First, I investigated the patterns of telia within plants. Second, I compared the variation in infection severity among plants, hypothesising that younger plants, those in dense populations, and those in low-elevation riparian areas would be most infected. Finally, I studied the effects of P. linkii on its host, hypothesising that highly infected plants would produce fewer berries and produce berries with less sugar, and that infected leaves would die sooner. Procedures This study investigated P. linkii in mixed coniferous-deciduous forest sites near Smithers, British Columbia. I marked a total of 41 randomly-selected focal V. edule plants in four sites. To examine within-plant patterns of infection, I photographed four leaves of each plant and used a graphic analysis program to examine the size, density and coverage of telia. To assess among-plant patterns, I compared infection severity (5 classes of telia coverage), to three ecological variables: host density within 5m, position on a moisture gradient, and plant maturity. To investigate the effects of P. linkii on its host, I compared infection severity to the number of berries produced, the proportion of malformed and infected berries, and the sugar content of mature berries as measured with a handheld refractometer. I measured leaf mortality in fall. Results P. linkii produced a characteristic pattern within each plant: higher leaves consistently had fewer but bigger telia than lower leaves. Across sites, plants were significantly more infected in areas of high host density. Within sites, young plants and plants and in moister ecosystems were significantly more infected (increases of 1.1 ± 0.2; F1,128=44.8, P<0.001 and 1.7 ± 0.5; F1,145=95.9, P<0.001 severity classes respectively). There was no relationship between infection severity and the total number of berries produced. However, highly infected plants produced 20 times more malformed and infected berries than plants with low infection severity. In addition, plants with higher levels of infection produced berries with about 15% less sugar (F1,63 = 19.4, P<0.001). Highly infected leaves had significantly more area of dead tissue (F1,97 = 21.5, P<0.001). Conclusions Although the long-term effects of P. linkii on V. edule are unknown, this study suggests that the pathogen stresses plants. Climate change is projected to lead to moister springs in the study area, potentially increasing infection severity. Increased infection may reduce berry quantity and quality, potentially reducing food availability for over-wintering birds and small mammals, fall food for bears, and jelly for people.