Direct observation of the surface of seed tubers by confocal
laser-scanning microscopy demonstrated that P. oligandrum
hyphae not only colonized R. solani hyphae, but also colonized
the surface of R. solani sclerotia (Fig. 4), suggesting that
P. oligandrum also has mycoparasitic activity against sclerotia of
R. solani. Al-Hadani and Cooke (1983) reported that co-inoculation
with P. oligandrum greatly reduced sclerotium formation by
R. solani. Furthermore, Rey et al. (2005) showed that this oomycete
displays mycoparasitic activity against sclerotia of B. cinerea and
Sclerotinia minor Jagger, which are anatomically different types of
sclerotia. However, hyphal development of both P. oligandrum
and R. solani was not observed on seed tubers inoculated with
both organisms at 2 weeks after planting (Fig. 4). Benhamou
et al. (1999) reported that, at a later stage of the interaction,
P. oligandrum and R. solani hyphae both suffered some damage,
characterized by a marked loss of turgor and a pronounced
collapse. Walther and Gindrat (1987) showed the opposite, an
antagonistic activity of R. solani against P. oligandrum. Our
competitive PCR analysis indicated that R. solani population was
not detected until 7 days after planting; however, 14 days after
planting, the R. solani population did increase on two tubers
(Table 1). The mutual antagonism between these fungi appears
to explain the failure of P. oligandrum to completely