Analysis of Cronartium flaccidum lesion development on pole-stage Scots pines

Historical and current lesion development and sporulation of Cronartium flaccidum was investigated in a stand of artificially seeded pole-stage Pinus sylvestris in northern Finland. An average of 6.5 lesions developed per infected tree, most of them occurring on a minority (25 %) of the trees. During the monitoring period of five years, fresh aecia appeared mainly in 7–10-year-old shoots, the age of the shoots bearing aecia varying between 3–20 years. Aecia appeared for the first time most frequently in 5–10-year-old shoots. Infection waves occurred, whereas lesions were formed most frequently in shoots formed in various years through the 1980s. After the lesions started to sporulate, sporulation in most lesions that finished sporulating during the monitoring period lasted for 1–2 years. The aecia in between 47 % and 59 % of the infected shoots developed annually over a longer length in proximal direction than in distal direction next to the previous year’s infection. The aecia-bearing distal part of the shoot was longer in between 19 % and 37 % of the shoots.