Agriculture Reference
In-Depth Information
accumulated, resulting in increased microbial number and biomass. Respiration was also
markedly higher in the used pumice (138 ng CO
2
-C g
-1
day
-1
) than in the unused pumice (0.7
ng CO
2
-C g
-1
day
-1
). Microbial biomass and activity were markedly enhanced in the repeated
cultivation, compared to microbial number. A similar finding was reported by Postma et al.
(2000), in which it was demonstrated that used rockwool was suppressive to disease caused
by Pythium aphanidermatum. They considered that the suppression mechanisms were due to
certain members, such as antagonists, of microflora enriched through repeated cultivation and
emphasized that although “clean” and “sterile” environment is addressed in hydroponics,
microflora plays an important role in minimize the risk of Pythium disease. These results may
indicate that microbial buffering capacity functions to make any media, such as rockwool and
pumice, resistant to the attack by different belowground pathogens.
Suppression Mechanisms of Used Pumice -Contribution of Chemical Factors
When unused pumice was mixed with different ratios of used pumice, the disease index
of bacterial wilt decreased with increasing ratios of the used pumice. The number of the
pathogen R. solanacearum YU1Rif43 also decreased with increasing ratios of the used
pumice (Figure 2). To estimate the suppression mechanisms of the used pumice to bacterial
wilt, tomato plants were grown in non-treated and gamma-irradiated used pumice (Figure 3).
Compared with the disease index (DI) 1.0 of the used pumice, it was 3.0 in the gamma-
irradiated used pumice, suggesting that the major suppression mechanism was of biological
origin. Since DI of the unused pumice showed always 4 (all the plants died), suggesting that
even gamma-irradiated used pumice possessed a low degree of disease suppression. To
estimate chemical factors involved in the disease suppression, unused pumice was added with
different rates of Ca and a combination of Ca, Mg and K as their chloride forms (Figure 4).
While the addition of Ca did not affect DI of bacterial wilt significantly at a rate of 200 mg
kg
-1
pumice, Ca significantly decreased DI at rates of 500 and 800 mg kg
-1
pumice.
A combination with Mg and K further decreased DI at both rates, although the different
was not significant due to high variations. When Ca was added with unused pumice at more
than 1000 mg kg
-1
as CaCl
2
, the pathogen died quickly and no disease occurred, but both of
tomato germination and their subsequent growth were inhibited. Ca toxicity to growth and
chemotaxis of bacterial cells is reported (Sakai et al. 1995, 2003). In contrast, suppression of
bacterial wilt by Ca is also reported (Yamazaki and Hoshina 1995; Yamazaki et al. 1999). In
their study, Ca was added into a nutrient solution at rates of 4.4 to 20.4 mmol Ca
2+
L
-1
as a
form of calcium nitrate. Our study suggested that suppressive effects of Ca to bacterial wilt
might be further enhanced with a combination of Mg and K. When 10% of used pumice was
mixed with 90% unused pumice, Ca content of the mixture pumice was 125 mg kg
-1
and DI
decreased less than 2.0 (Figure 2). However, the addition of Ca at a rate of 200 mg kg
-1
did
not decrease DI at all (Figure 4), suggesting that the suppression mechanisms of the used
pumice may be due not to Ca effects, although Ca itself possesses disease suppression
property.
