Eggplant (Solanum melongena L.) is the most important and widely consumed vegetable in Bangladesh. The annual production of eggplant is about 168000 tons (Anonymous, 2000). An early crop of eggplant is very profitable for the farmers in Bangladesh but the early crop is particularly vulnerable to bacterial wilt. It is the most destructive bacterial plant pathogen especially in the warm regions (Hildebrandt, 1950). A report from India reveals that bacterial wilt can cause 27% loss in eggplant (Reddy, 1986). The main effort to control this disease has been directed towards the development of resistant cultivars (Obraro, 1969). But available information suggests that very few of the cultivars are resistant to this disease. Information from home and abroad indicated that grafting of eggplant with wild Solanum sp. to the causal organism resistant rootstock is an effective technique to control the bacterial wilt. A number of wild species of Solanum were recorded as resistant to the disease. These species were reported as graft compatible to cultivated eggplant varieties (Mochizuki and Yamakawa, 1979a; 1979b; Ali et al., 1990a,b; 1992 and Shethy and Reddy, 1985). Recent reports from Bangladesh indicated that grafting of eggplant on resistant rootstocks like wild Solanum is an effective technique to control bacterial wilt (Ali et al., 1994). Therefore, the present investigation was undertaken to evaluate the graft compatibility of cultivated eggplant varieties with wild Solanum species resistant to bacterial wilt both under artificial inoculation and field conditions and also to find the yield performance of grafted and non-grafted eggplant varieties in the field.
Materials and Methods
Three experiments were carried out at the Bangladesh Agricultural Research Institute (BARI), Joydebpur, during August, 1998 to May 1999 to evaluate the resistance of grafted plants against bacterial wilt and yield performance under field conditions.
Screening of wild Solanum spp. and cultivated eggplant varieties against bacterial wilt (Ralstonia solanacearum): Five wild Solanum species and twenty one cultivated eggplant varieties were screened against bacterial wilt (Table 1). Seeds of the Solanum species and the cultivated eggplants were sown in a nursery bed on 10th August 1998. Before sowing, the seeds were treated with GA3 solution (100 ppm) for 24 hrs at room temperature for quick germination. Ten days after seed germination young seedlings were transferred to 9 cm (diameter) plastic pots containing sterilized soil mixed with 50% organic matter (v/v). The experiment was conducted in the glass house of BARI, Joydebpur. Pure culture of Ralstonia solanacearum was prepared on TZC (tetrazolium chloride) medium by isolating the organism from the eggplant showing typical symptoms of bacterial wilt. The identification of the isolate was confirmed by Cocks Postulate. Root dipping method was followed for inoculation. Five weeks old seedlings were uprooted and cut one-fourth root by sterilized scissors. Then the roots of each seedling were dipped in bacterial suspension for five minutes and were replanted in the same pots. The plants were arranged in the glass house following completely randomized design with three replications and ten plants were inoculated in each replication. The population of R. solanacearum from soil was detected in MS medium by soil dilution method. Plant to plant and line to line distance were maintained 25 and 30 cm, respectively. Data on wilting was recorded at 15 days interval. Final data on wilting was converted into percent wilt and reaction of the genotypes was graded following the scale furnished below (Anonymous, 1974).
Highly resistant (HR) = No wilting
Resistant (R) = 0-20% wilting
Moderately resistant (MR)=20-40%wilting
Moderately susceptible (MS)=40-60%wilting
Susceptible (S) = 60 to 80% wilting
Highly susceptible (HS)=80-100% wilting
Graft compatibility of eggplant varieties on wild Solanum species:
Experimental materials consisted of plants of Solanum torvum and Solanum sisymbriifolium as rootstocks and sufala, singnath and uttara varieties were the scion. Seeds of the two wild Solanum species were sown in plastic tray containing sandy soil on 1st August, 1999. Ten days after germination seedlings were transferred to 9 cm dia. plastic pots containing soil and well decomposed compost in equal proportion. Seeds of the eggplant varieties were sown in plastic tray containing sandy soil and well decomposed on 10th August, 1999. Seedlings were transferred to 9 cm dia polybag 10 days after germination. All care were taken for proper growth of the seedling. Rootstock seedlings (40-50 days old) (3- 4 leaf stage) and seedlings of eggplant varieties at 3-4 leaf stage were grafted. Grafting was done following the steps laid down by Ali et al. (1994).
Reaction of grafted plants planted in sickbed: Grafted plants were transplanted in sick beds previously infected by bacterial wilt pathogens. The sick plots were developed by adding bacterial suspension in soil. The population of R. solanacearum from soil was detected in MS medium by soil dilution method. The population of the pathogen was 1.2 x 106 cfu/g soil before transplanting. Planting was done following a randomized complete block design with three replications. Ten plants were accommodated in a unit plot and the plants were spaced at 30 x 20 cm. Transplanting was done on 10th September, 1999. Data on wilting were recorded at seven days interval. Final data on wilting were converted into percent wilt and reaction of the lines were graded following the scale of Anonymous (1974).
Yield performance of the grafted plants in field conditions: The grafted plants were transplanted in field on 10th September, 1999. The experiment was laid out in randomized complete block design with three replications. Twelve plants were planted per replication on a 1m wide bed at a spacing of 70 cm. The fruit yield per hactre was calculated from per plant yield and also recorded percent of bacterial wilt infected plants. Manures and fertilizers were applied at the rate of cow dung 15 tons, urea 250 kg, TSP 175 kg and MP 150 kg ha-1. The entire cow dung, TSP and half of the MP were applied during final land preparation while the entire urea and half of MP were applied at three equal installments as top-dressing. First and second top dressing were done 15 and 30 days after transplanting respectively and the third dose after first harvest of the fruits. Data in respect of mortality of plants and fruit yield were recorded daily and analyzed statistically for interpretation.
Results and Discussion
Experiment 1: It was revealed from the results (Table 1) that most of the eggplant varieties/genotypes showed susceptible reaction against bacterial wilt. However, one line BL 081 showed resistant reaction while the genotype BL099 showed moderately resistant reactions. Six genotypes BL117, ISD011, ISD001, BLS18, BLS2 and Tarapuri were found moderately susceptible to bacterial wilt. All other varieties/genotypes showed susceptible to highly susceptible reactions. It was revealed that all the wild Solanum species had low wilt incidence. S. sisymbriifolium, S. torvum and S. integrifolium showed resistant reaction. Solanum indicum and S. khasianum showed moderately resistant while S. sanitwongsci showed medium susceptible reaction. Shetty and Reddy (1985) and Ali et al. (1990 a, b) reported that S. sisymbriifolium and S. torvum are the effective root stocks to control bacterial wilt. Mochizuki et al. (1979b) reported that S. torvum showed potential for use as eggplant root stock because of high resistance to bacterial wilt and good fruit yield of the scion.
Experiment 2: In the graft compatibility study, maximum (95.71%) success in grafting was obtained. The variety sufala was more grafts compatible with S. torvum than the other eggplant varieties. It had 95.71% grafting success which was followed by sufala x S. sisymbriifolium (94.29%). Grafting success was the lowest in singnath x S. sisymbriifolium (88.51%). Uttara had grafting success of 92.24% and 90.08% respectively with S. sisymbriifolium and S. torvum. Ali (1992) observed 70-95% success in grafting eggplant varieties with non-tuberous wild Solanum which was similar with the results of the present study. There was no interaction between the rootstock species and the eggplant varieties. There was no significant difference in grafting success among the six graft combinations (Table 2).
Experiment 3: In the sick plot all the grafted plants showed highly resistant to bacterial wilt with no mortality. There was 100% mortality of sufala, singnath (66.67%) and uttara (44.44%) were found highly susceptible and moderately susceptible, respectively (Table 3).
|Table 1:||Reaction of wild Solanum and cultivated eggplant varieties/genotypes against bacterial wilt
|Highly resistant=HR Moderately resistant=MR|
Moderately susceptible = MS Highly susceptible = HS
|Table 2:||Grafting success of eggplant varieties with wild Solanum rootstocks
|Table 3:||Reaction of grafted and non-grafted rootstock in the bacterial wilt infection observed in the sick field (bed)
|R: Resistant MR: Moderately resistant HR: Highly resistant|
S: Susceptible MH: Highly susceptible HS: Highly susceptible
Mochizuki et al. (1979b) reported S. torvum as highly resistant to bacterial wilt showing potential for use as rootstock. He also obtained good fruit yield scion varieties grafted on S. torvum.
Experiment 4: Plants of all eggplant varieties, grafted and non-grafted started flowering within 56-73 days after seed sowing (Table 4). It was generally observed that non-grafted plants bloomed earlier than grafted ones. Flowering was late in singnath grafted with S. torvum and S. sisymbriifolium which was followed by sufala and uttara when grafted with wild Solanum. Uttara grafted on S. torvum and S. sisymbriifolium bloomed earlier than sufala and singnath grafted on S. torvum and S. sisymbriifolium. The earliest an thesis was observed in non-grafted uttara, sufala and singnath which was statistically identical.
|Table 4:||Yield potentiality of eggplant varieties when grafted on different Solanum rootstocks
|In a column, means having a common letters do not differ significantly at 5% level of significance.|
The delayed flowering in grafted plants may be due to the growth of the scion plants that was interrupted or slowed down for a week nearly due to grafting. Similar trends of delayed flowering in grafted plants were also reported by Matsuzoe et al. (1990). Among the scion varieties studies, uttara required 65 days after seed sowing to attain harvestable fruit maturity which was significantly earlier than those of other varieties. In grafted plants delayed flowering, fruit set and fruit maturity were common phenomenon due to grafting shock of the scion. Similar results were reported by Matsuzoe et al. (1990). Among the scion varieties studied, number of fruits per plant were statistically significant. Significantly higher number of fruits were recorded in grafted plants. Among the grafted plants on different root stocks, the highest number of fruits per plant was (57) obtained in S. torvum x sufala followed by S. torvum x uttara (54) and S. sisymbriifolium x sufala (53), S. torvum x singnath (39) and S. sisymbriifolium x singnath (38). Among the scion varieties studied, uttara was the highest yielder (2.77 kg/plant) followed by sufala (2.57 kg/plant) and singnath (2.30 kg/plant) (Table 4). Grafting eggplant on Solanum rootstock significantly increased the fruit yield per plant. Sufala grafted on S. torvum gave the highest yield (4.2 kg/plant) followed by uttara grafted on S. torvum (4.0 kg/plant), Sufala grafted on S. sisymbriifolium (3.8 kg/plant), Singnath grafted on S. torvum (3.8 kg/plant), singnath grafted on S. sisymbriifolium (3.7 kg/plant) and uttara grafted on S. sisymbriifolium (3.7 kg/plant) and they were statistically identical in this respect. Matsuzoe et al. (1990) and Ali (1993) also reported higher fruit yields in eggplant when grafted on Solanum rootstock. It was revealed from the results (Table 4 ) that the duration of fruit harvest was significantly longer in grafted plants than those of non-grafted plants. The highest duration of harvest was recorded in Sufala when grafted on S. torvum which was significantly longer compared with those of all other grafted and non-grafted treatments.
The field reaction of grafted and non-grafted plants against bacterial wilt is also presented in Table 4. It was revealed from the results that grafted plants showed complete resistance against bacterial wilt. Whereas the non-grafted plants showed vulnerability against this disease at different levels. The highest wilting percentage was recorded in the variety sufala (66.67) followed by singnath (54.67) and uttara (26.74). Ali (1993) also reported the similar results.
From the results it is recommended that eggplant could be grafted on Solanum torvum and Solanum sisymbriifolium for controlling bacterial wilt and for higher yield. However, further refinement of the technology is required particularly in respect of age of rootstocks and scion seedlings for grafting and the management of grafted seedlings immediately after grafting.