sdu faculty of forestry journal special edition 2009 - Orman Fakültesi

SDÜ Faculty of Forestry Journal
discharge. Maturity of perithecia varied substantially. There were some samples
that were 100% old and empty, then there were some samples with only young
perithecia incapable of discharging the ascospores, and there were whole range of
samples that had a mixture of young, mature and old perithecia. Generally, the
maturity tests showed that areas of canker with a lower density of perithecia have
more young perithecia, while those with higher density have more old perithecia.
Because the maturity test showed that there was no strict rule of maturity of
perithecia among different densities, the results of ascospore discharge could not be
shown by low, middle, and high class densities of perithecia but only as average
value. The range of ascospore discharge was from 360,000 to 718,000 ascospores
cm –2 h –1 while the average discharge was 506,000 ascospores cm –2 h –1 . These
results are comparable to the results of Lachance (1971) and Johnson and Kuntz
(1979) who reported 450,000 ascospores cm –2 h –1 .
On average, a perithecium discharged 2520 ascospores per hour i.e. 315 octads. The
average ascospore discharge (506,000 ascospores cm –2 h –1 ) was used for calculating the
total ascospore discharge of cankers which represents the potential of disease spread.
One Eutypella canker could discharge from 65 million to 3.3 billion ascospores per
hour with an average 1.0 billion ascospores per hour under favorable environmental
conditions. This kind of ascospore production represents enormous inoculation
potential.
4. DISCUSSION
This research pointed out some of morphological aspects of Eutypella canker of
maple that had not been analyzed before or were hypothesized and not checked, while
others are checked again. A total of 23 Eutypella cankers were analyzed in detail. This
is the required minimum for reliable statistics and it is comparable to other dissectional
studies (Gross, 1984a).
While ascospore discharge of Eutypella canker can be enormous under favorable
environmental conditions, the question is raised as to why the disease is not more
frequent. Specific demands of the fungus for successful colonization are the main
reason for generally low occurrence of Eutypella canker. The fungus needs an exposed
xylem for successful infection, i.e. branch stubs or bark wounds. Since the branch stubs
represent a small area and ascospores are disseminated only over short distances (25
m), it is hard for infection to take place despite enormous ascospore production. There
are some other environmental limitations. Ascospore discharge is greatest at
temperatures between 24 and 28 °C (Lachance, 1971; Johnson and Kuntz, 1979).
Laboratory tests show no ascospore discharge and dissemination at temperatures below
4 °C or higher than 36 °C. Free moisture (rainfall) induces mature perithecia to
discharge the ascospores. At least 3 mm of rain has to penetrate the tree canopy to
initiate discharge (Lachance, 1971; Johnson and Kuntz, 1979). Spore ejection begins
about 2 hours after rain has started. High relative humidity alone is not sufficient to
induce discharge of spores but it can influence the rate of drying of bark on cankers
and prolongs discharge after periods of rainfall (Johnson and Kuntz, 1979).
158