sdu faculty of forestry journal special edition 2009 - Orman Fakültesi

SDÜ Faculty of Forestry Journal
apothecia) on the leaf surface opened, and contained a grayish milky substance. At
this time, Norway maples were abundantly producing and shedding pollen, and
small samaras were formed, with leaf sizes averaging 10 cm × 15 cm. By late May
in 2007 and 2008, a few partially filled or empty asci were observed, with
ascospore release through the tips, and paraphyses becoming curled beside asci
after spore release. In early June, Norway maple leaves reached their full size (20
cm × 24 cm), and 10% (2007) to 30% (2008) of the asci had fully discharged their
spores. By the beginning of July 2008, nearly all the asci were empty and this
sporulation period was longer than that observed in 2006 (Hsiang and Tian, 2008)
because of much drier conditions. However in 2008, the sporulation period was
shortened with full spore release by mid June because of much wetter conditions.
In 2007, we found a few tar spots on trees tentatively identified as sugar maple
(A. saccharum). We confirmed the identity of these maple trees based on DNA
sequencing of a chloroplast gene. The fungal DNA was also sequenced, and it
turned out to match R. acerinum, the European species. This was very surprising
since the European fungal species should not occur on a native North American
maple species. The same trees were visited again in 2008, and samples were
collected during the growing season. These also yielded DNA confirmed as
belonging to A. acerinum. In 2008, we also collected specimens of A. campestre
and A. negundo from Ontario, and other samples of A. saccharum from Quebec.
All specimens were infected with tar spot caused by R. acerinum as identified by
sequences of the ITS region. This result was not unexpected for A. campestre since
both the host and pathogen are European species. However, this was another
unexpected result for A negundo, and A. saccharum. The first species is considered
a weed, but the second species is the major source of maple syrup. The occurrence
of a European tar spot species on North American maple species raises the
possibility of pathogenic adaptation to native species that could result in epidemics
more widespread than currently seen on Norway maples.
In Hsiang & Tian (2008), we predicted that based on spore production periods of R.
acerinum, "the practical implication is that fungicide protection against tar spot, if
necessary, needs only to be applied during a very short period, which begins near the
end of full leaf expansion in Norway maple". We tested this hypothesis in spring 2008
with application of fungicides at different times. We found that a single fungicide
application in late May or early June was efficacious in reducing the number of spots
per leaf assessed on September 2, 2008, from over 20 to none for many of the
fungicide tested. All fungicides were found to significantly suppress tar spot compared
to the water control when applied at either of these two times (May 20 or June 4).
Applications of fungicides on May 6 or June 30 were not effective in reducing tar spot,
while the application on June 17, which was just at the end of the spore production
period, was effective for some fungicides but not others. The implication of these
results is that if fungicides are used, only a single application between late May and
early June is needed for control of tar spot of Norway maple in Southern Ontario, but
this may depend on weather and growth, with another application possibly necessary if
early June weather is dry.
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