The present study has been undertaken to
determine the efficacy of some antagonistic fungi isolated from the rhizosphere
of wheat plants grown in Summer region, Diwaniya Governorate, Mid Iraq for the
biological control of Helminthosporium sativum, the causal organism of
root rot disease of wheat in vitro. Three different species from the
genus Trichoderma: T. harzianum, T. pseudokoningii and T. lignorum,
in addition to Stachybotrys atra and Penicillium sp. were
isolated from the rhizosphere of wheat plants. Laboratory experiments indicated
that T.  harzianum and S. atra
were highly antagonistic to the pathogen when grown together on potato dextrose
agar in Petri plates. Microscopic examination of the mycelia showed that hyphae
of T. harzianum were parasitized H. sativum, coiled around its
hyphae and caused its lysis, but did not penetrate inside the hyphae. However, S.
atra was invaded colonies of H. sativum and caused severe hyphal
damage. In the experiments of culture filtrates of the antagonistic fungi, T.
harzianum and S. atra, were able to suppress growth of H. sativum,
if incorporated in the medium and proved to be effective in controlling the
pathogen.  Results
of effect of filtrate on spore germination showed that about 80 and 95% of them
are unable to germinate with high concentrations (15 or 20 %) of culture
filtrates of T. harzianum and S. atra respectively. However, the
other antagonists: T. pseudokoningii, T. lignorum and Penicillium
sp. were less effective in inhibiting spore germination of the pathogen. Results
of antagonistic effect of the culture filtrates on wheat seed infection, except
T. lignorum, showed that seed colonization by the root rot fungus was
decreased significantly at the concentrations of 15 and 20 % compared with
control. Seed coating with antagonistic fungi was the best biological seed
treatment for reducing seed rot and diseased seedling caused by H. sativum.
Antagonistic fungi have no side effects on seed germination except Penicillium
sp. which reduced the seed activity and germination as compared with the other
antagonists and with the control. It can be concluded that T. harzianum
is a strong mycoparasite and S. atra is a good antagonistic agent to
control H. sativum, but in fields may be their activity are conditioned
by soil environment specially the microflora