According to studies performed at the Finnish Forest Research Institute (Metla) the conidia of Gremmeniella abietina, the fungus that causes Scleroderris canker spread in rainy weather. Dispersion occurs throughout the growing season, but the main dispersal takes place from the start of the growing season until the end of July or beginning of August. During the first growing season in nursery, seedlings are most susceptible towards the end of the growing season during bud development and later during the second growing season when the seedlings are at the height-growth phase. The results of the study will be useful for forest nurseries when they plan control of Scleroderris canker.
The conidia dispersal of G. abietina starts in spring when the temperature sum (the sum of daily mean temperatures when over 5 º C with threshold of 5 subtracted) is 100 d.d. In central Finland 80% of conidia have already been dispersed by mid July (temperature sum about 800 d.d.). Rainfall is important for dispersal of spores, though small amounts are also released in rainless weather. A monoclonal antibody was developed for detecting conidia of G. abietina. It serves a tool to determine the time and level of conidia dispersal.
Nursery seedlings are most susceptible towards the end of their first growing season, at the time of bud development. During the following growing seasons, seedlings are very susceptible to the disease at the height-growth phase. A high number of spores, humidity and cool weather after infection, as well as a slow start for growth increase the severity of the disease. Frosts in late summer and early autumn make the damage worse.The A type of the fungus is found both in seedlings and adult trees throughout Finland . The B type is most often found in nurseries in northern Finland . In addition to asexual spores (conidia), both types produce also sexual spores (ascospores), but these are more common to the B type. Spores can be very different in shape and size, but the A and B types cannot be distinguished based on these characteristics.
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