sdu faculty of forestry journal special edition 2009 - Orman Fakültesi

SDÜ Faculty of Forestry Journal
However, this result should be taken cautiously because the reduction of the
symptom severity caused by D. pinea when D. scrobiculata was also present, was
not always statistically significant. One possible explanation for this lack of
significance could be related to the great variability recorded between the
repetitions. This great variation could be a consequence of the natural presence of
both pathogens, in a latent state, in the plant material used in the experiments.
Latency has already been described in previous studies as a very common state for
Sphaeropsis sapinea sensu lato (Stanosz et al., 1997; Flowers et al., 2001; 2003;
Stanosz et al., 2005; Maresi et al., 2007), which was isolated from asymptomatic
plant tissues in a percentage range of 20-85%. Even in the present study, D. pinea
and D. scrobiculata occurred asymptomatically in several of the non inoculated
seedlings (data not shown). Despite this natural presence of the pathogen in the
seedlings of our experiments, control seedlings did not become diseased and did not
show any symptom of the disease. This result was predictable since it has been
proposed by several authors that activation from this latency to a pathogenic state may
take place under different conditions of stress (Stanosz et al., 1997; Smith et al., 2002);
and seedlings used in the present study were grown in optimal conditions. However in
the case of inoculated seedlings, the fungal spreading and colonization may cause
stress to the plant that could activate the latent infections of the pathogen, if present. In
such cases, the symptom severity recorded could be overestimated.
In general terms, when two of the endophytes, 08-10 and 08-13, were also
inoculated in the seedling, symptom severity casused by the D. pinea isolates was
lower than that recorded when D. pinea was inoculated alone. This fact suggests a
potential antagonism between those two endophytes and D. pinea when they cooccurred
in the same plant tissue. However, that reduction of the symptom severity
caused by D. pinea was only statistically significant on the less aggressive isolate
of D. pinea (p2). Endophytes have been demonstrated to be suitable candidates as
biological control agents, as they seem to be part of the defence system of trees
(Barklund and Unestam, 1988; Ranta et al., 1995) and have already been shown to
be antagonistic to many fungal pathogens, including Diplodia spp. (Holdenrieder
and Greig, 1998; Roy et al., 2001; Campanile et al., 2007).
Among the endophytes used in the present study, isolate 08-10 was identified as
Trichoderma atroviride Karst. Since the potential of this genus as a biocontrol
agent of plant pathogens was first recognized in the early 1930s (Weindling, 1932),
continuous studies have demonstrated the effectiveness of this species in the
biological control against a number of plant pathogenic fungi, including Diplodia
spp., on several forest hosts (Knudsen et al., 1991; Mousseaux et al., 1998;
Campanile et al., 2007; Schubert et al. 2008). T. atroviride is a mycoparasite
which, once it recognizes and attacks the fungal host, uses it’s nutrients killing the
host before or just after invasion (Chet et al., 1998). The endophyte 08-13, which
was also able to reduce the symptom severity caused by Diplodia on pine
seedlings, was identified as Rosellinia subiculata (Schwein.) Sacc. This fungal
endophyte has been shown to produce sordarin, which is an antibiotic with
antifungal properties against a number of plant pathogenic fungi (Bills et al., 2002).
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