Agriculture Reference
In-Depth Information
Biological Control of Bacterial Wilt in Pumice Culture Using a Combination
of Biocontrol Agents and Carbon Substrates
Using different bacterial agents, their suppressive effects on bacterial wilt of tomato were
examined in the presence and absence of carbon substrates added at a rate of 1 mg g
-1
pumice.
Compared to single inoculation, combination of bacterial inoculation and substrate addition
decreased DI more remarkably, especially MelRC2Rif, FN2, N2S6 strains (Figure 10).
Among the substrates added with bacteria, lysine and glutamine were most suppressive to
bacterial wilt. Similar results were previously reported (Posas et al. 2007), in which bacterial
wilt of tomato was markedly suppressed by the addition of glucose, lysine, serine, glutamine
and arginine into soil and concomitantly the pathogen declined more rapidly. In this study,
glucose and xylose were not suppressive, when the substrates were added into pumice
infested with the pathogen, but both amino acids, lysine and glutamine, suppressed bacterial
wilt remarkably (Figure 11). It is interesting to note that the addition of glucose into soil made
the soil suppressive to bacterial wilt, but that into pumice did not. This fact may suggest that
indigenous microbes that utilize glucose and perform as antagonists are present in soil, but
such types of microbes are not present in pumice since microbial abundance is too low in
unused pumice. Lysine was added to pumice along with different bacterial agents (Figure 12).
Combinations of lysine with MelRC2Rif and FN2 were consistently effective at decreasing
bacterial wilt, while those with F3 and CF5b were not so effective, although DIs significantly
decreased. These results suggested that a combination of lysine and biocontrol agents, rather
than a single application of lysine, is the best management in pumice culture to minimize the
risk to bacterial wilt of tomato minimum.
Trials in A Greenhouse
Tomato was grown in a greenhouse using 0.05 m
2
pot containing 3.5 kg pumice, near
commercial size, to which the pathogen was inoculated and antagonists and lysine were added
at 5 x 10
7
cfu g
-1
pumice and 1 mg g
-1
pumice, respectively. Lysine was added at seeding and
after that every two week at a rate of 1 mg g
-1
. In small pot experiments described above, no
inhibitory effect was found to the growth of tomato, but in the larger size of pots, tomato
growth was significantly reduced in the lysine added pots (Figure 13). Symptoms of bacterial
wilt was observed only in control(+), MelRC2Rif and FN2 plots at a disease incidence of 8%,
31% and 6%, respectively, but not in control(-), MelRC2Rif+lysine and FN2+glucose plots,
suggesting that a combination of the antagonists and the substrates be effective at reducing
bacterial wilt incidence. This result was supported by the number of MelRC2Rif and the
pathogen: the former was higher and the latter lower in the MelRC2Rif+lysine plot than in the
MelRC2Rif plot. It is common knowledge that certain amino acids can inhibit plants in
millimolar amounts and lysine is categorized into amino acids with relatively higher
inhibitory effects (Vurro et al. 2006). Therefore, we tested the concentrations of lysine that
are effective at reducing bacterial wilt, but not inhibitory to tomato growth and found that the
addition of lysine at a rate of 1 mg g
-1
pumice was never inhibitory to tomato for one month
cultivation period in the small pot experiments. Lysine addition, however, was inhibitory to
tomato growth in a large pot experiment in 2005. Thus, the amount of lysine was further
reduced and its effect on disease suppression properties was evaluated (Figure 14).
