Relationship between early family-selection traits and natural blister rust cankering in western white pine families

Abstract

Blister rust (Cronartium ribicola) cankering incidence was compared among half-sib families of both artificially and naturally inoculated western white pine (Pinus monticola). In the first test, artificially inoculated seedlings of six families had their individual infection spots tallied and any infected needles were removed before cankers developed. These healthy seedlings were classified as high or low spotters and were planted in row plots with four high spotters paired with four low spotters. After 7 years, 2 pairs had equal cankers, 12 had more rust on the high spotters, and 12 had more rust on the low spotters. The mean number of cankers per tree was 22 and 23 for high and low spotters, respectively. In the second test, 203 artificially inoculated western white pine families were ranked for their relative susceptibility to blister rust based on (i) family mean counts of individual infection spots per seedling (spotting) and (ii) the presence of only small cankers (slow canker growth resistance). These results were compared with the natural cankering incidence of the same families in eight plantations. There were few significant differences in spotting incidence among inoculated families. The range in field cankering in the test plantations (15–63 families each) was 22–88%. Seven plantations, including two with families significantly different in infection-spotting incidence, lacked a significant correlation (Spearman, P < 0.05) between percentage field cankering and spotting incidence from inoculations. Although one plantation had a significant correlation (r s = 0.64; P < 0.005) between spotting incidence from inoculation and cankering in a plantation, there was no overall trend across plantations. Incidence of slow canker growth resistance from artificial inoculation ranged from 0 to 18%. There was a significant positive correlation between field cankering and slow canker growth resistance, as determined by inoculation only in three of eight plantations. However, when the families were placed into slow canker growth classes, there was a trend of reduced cankering with increasing class of slow canker growth resistance (mean Spearman's r s value = 0.87 (P < 0.01) across all plantations). These studies indicate that selection on the basis of slow canker growth resistance resulted in less cankering in plantations than selection on the basis of reduced infection spotting.