6.5.1 Materials and Methods
Lettuce seeds (All Year Round) were planted in Perlite, in speedling®-24's. Trays were left in the potting shed for 24 hours and then moved to the seedling tunnel where they were left for 10 days. Trays were then made up to 24 plants each and moved into the mini trough system. Twelve treatments were used (Table 6.5.1.A) each replicated three times.
Table 6.5.1.A Treatments used in pH trial
Treatment pH Pythium used Trichoderma used
1 4 no no
2 4 yes no
3 4 yes yes
4 5 no no
5 5 yes no
6 5 yes yes
7 6 no no
8 6 yes no
9 6 yes yes
10 7 no no
11 7 yes no
12 7 yes yes
Nutrient solution was made up using Hydrogro® (1 g/Q) and calcium nitrate (0.6g/Q). The electrical conductivity (EC) was adjusted to 1.8ms weekly and pH was adjusted to the relevant levels every second day using nitric acid or potassium hydroxide. Trichoderma was added, on transferral of plants to the mini troughs, at 0.5g/Q (2.5x108 conidia/g).
Pythium was isolated from infected soil one week prior and bulked up on water agar.
Agar blocks (25mm2l,with Pythium mycelial growth, were placed (fungal growth down)
application. Harvesting was done three weeks later and total shoot wet weight was recorded for each replicate. Mean total shoot wet weight was calculated for each treatment and graphed (Fig. 6.5.2.A). Statistical analysis (ANOVA and Student- Newman-Keuls tests) was done using the SAS system for Windows 98, Version 6.1.
6.5.2 Results
In the uninoculated controls maximum yield was achieved at pH 6.0. This was significantly higher than the yields at pH 4.0 and 5.0 with the minimum yield at pH 4.0 (Fig. 6.5.2.A).ln the inoculated controls (inoculated with Pythium) highest yields were recorded at pH 4.0 and 5.0 (Fig. 6.5.2.A). The greatest loss in yield, resulting from Pythium inoculation, was seen at pH 6.0 (Fig. 6.5.2.B). Yield losses caused by Pythium inoculation were significant at all pHs, except 4.0 (Table 6.5.2.A)
Biocontrol activity of Eco--r<ID was greatest at pH 5.0 and decreased with increasing pH with a minimum activity at pH 7.0 (Fig. 6.5.2.B). Biocontrol activity was, however, significant at all pH levels.
Table 6.5.2.A Summary of statistical results from pH trial
pH Trichoderma Pythium Mean wet Student-Newman- added Added weight (g) Keuls (SNK) grouping
4 no no 125 cd
4 no yes 115.67 de
4 yes yes 125.33 cd
.5 no no 138 abc
5 no yes 117 de
5 yes yes 140.33 ab
6 no no 152.33 a
6 no yes 105.33 e
6 yes yes 142 ab
7 no no 150.67 a
7 no yes 105.67 e
7 yes yes 133.67 be
F = 20.81*** P=0.05 CV (%) = 4.74
*Treatments with the same letter are nut-stgnificantly different at P
=
0.05Key to Figs. 6.S.2.A and B
control (u)= uninoculated control
control (i)=inoculated control (inoculated withPythium) biocontrol= inoculated withPythium and treated with Eco-T®
cCll Cl>
~ 40
control(u)
control(i) Treatments
biocontrol
•
pH4•
pH5•
pH6•
~pH?Fig. 2.5.2.A Effect of pH on disease severity and biocontrol activity in hydroponic lettuce. Treatments with the same letter are not significantly different at P= 0.05.
•
pH4•
pH5•
pH6ill ,'
.> '
? , . ' -
pH?
control(i)
Treatments
biocontrol
Fig.6.5.2.B Yield obtained under various treatments expressed as percent yield of uninoculated controls
6.5.3. Discussion
Optimum plant growth in uninoculated controls was at pH 6.0-7.0, as was expected given the effect which pH has on the availability of ions to the plant, as summarised in Fig.1.6.A (from Resh, 1995). The optimum range for the availability of most essential elements lies between pH 6.0 and 6.5.
The greater reductions in yield resulting from Pythium inoculation at higher pHs is supported by the works of Abdelzaher et al. (1997) and Sharma and Gupta (1999).
Abdelzaheret al. (1997) found that for three species of Pythium tested, optimum pH for mycelial growth and zoospore production was 7.0. Sharma and Gupta (1999) showed that sporangial germination of P. ultimum occurred at a pH range of 5.6-7.0 with a maximum at 6.4. In the light of these publications it would have been expected that no reductions in yield would be recorded at pH levels of 4.0 and 5.O. This was true at pH 4.0, where the slight reduction in yield observed was not statistically significant.
However, at pH 5.0Pythium inoculation resulted in a significant reduction in yield. This may have been due to strain specificity, with the strain of Pythium used in these trials being functional over a wider pH range. Another explanation is that raised pH levels about the root zone resulted from using mostly nitrate based fertilizers.
Eco-~treatments functioned best at lower pHs, with the highest level of biological control activity being seen at pH 5.0. At pH 4.0 the positive effects ofTrichoderma were not as noticeable due to the direct negative effects which the low pH levels have on plant growth. The decreasing efficacy of Eco-~with increasing pH is supported by the findings of Harman and Taylor (1988) and Jeong et al. (1997). A narrower range of pH levels needs to be examined in future trials to find the optimum pH for integration with Eco-T® use, which appears to lie between 5.0 and 6.0.
Future studies also need to look at the interactions between pH, form of nitrogen, and Trichoderma application. It is known that ammonium nitrogen reduces the pH aboutthe root zone while nitrate nitrogen increases it. These facts could be used to formulate fertilizers which result in optimum pH levels and increased biocontrol activity. It must also be established whether Trichoderma's liking for ammonium nitrogen is a direct response or an indirect one linked with the resulting decrease in root zone pH.