Initiation of stem infection in western white pine by blister rust [Cronartium ribicola]

Cronartium ribicola, the causal agent of blister rust, initiates infection of white pines by penetrating open stomata. Because resistance responses may occur in the needles, thereby preventing or delaying subsequent colonization of the branch or stem, it is important to determine when normal stem infection is initiated. In the past, this has been determined by observing orange lesions on the branch or stem. However, undetected latent infections are probably present earlier. The objective of the present study was to obtain a better estimate of when stem infection is initiated in western white pine (Pinus monticola) in British Columbia. Groups of seedlings with all inoculated foliage removed at consecutive monthly or semi-monthly intervals were observed for subsequent canker development. The group first developing cankers would determine the shortest interval for initiating stem cankers after inoculation. After 4 years of separate inoculations in September, it was determined that latent stem infections could be present as early as in May in the coastal region of British Columbia and in June in the interior region of the province. Often, stem cankers were initiated more than a month earlier than when seedlings had been classified as "cankered" by observation of discolored stems. Consequently, the putative resistance response "early needle shedding" in P. monticola seedlings would be ineffective because needle shedding would have occurred after stem infection had already been initiated. Placing, for 1-week intervals, potted P. monticola seedlings beneath Ribes nigrum plants infected with C ribicola resulted in some infection from June through October in the interior of British Columbia. Because natural infection may occur earlier than in September, stem-canker initiation could also occur even earlier than determined from a September-only inoculation. Thus, an early-needle-shedding resistance response would be even less effective. These data are consistent with the notion that P. monticola lacks such a resistance response.