EVALUATION OF BRINJAL (Solanum melongena) GERMPLASM FOR FOOTROT DISEASE

R.R.K. Karunaratna and I.P. Wickremasinghe

Department of Agricultural Biology, Faculty of Agriculture,

University of Peradeniya, Sri Lanka

and

R.G.A.S. Rajapakse

Plant Genetic Resources Center, Gannoruwa,

Sri Lanka.

ABSTRACT

Although brinjal (Solanum melongena) is a popular vegetable in Sri Lanka, the spread of foot rot disease has recently become one of the major constraints for yield reduction. As the disease is soil borne, it is economically impracticable to control using chemicals or any other control methods. Identification of resistant varieties provides the acceptable mean of controlling the disease. Therefore a study was conducted to identify resistance among commonly cultivated brinjal varieties/accessions in Sri Lanka. Fifteen varieties/accessions were artificially inoculated with Fusarium solani and Rhizoctonia solani. After 8 weeks of transplanting Fusarium solani inoculated plants showed leaf yellowing and Rhizoctonia solani inoculated plants showed similar symptoms after 10 weeks of transplanting. BW11 and SA7MTE2 did not show leaf-yellowing symptoms. These two varieties are showing resistance for foot rot disease, but field studies should be carried out for further confirmation.

INTRODUCTION

Brinjal (Solanum melongena L) is an important commercial vegetable crop in Sri Lanka. It is cultivated throughout the tropics, subtropics and warm temperate climates and also grown in greenhouses. Brinjal is a typical commercial vegetable and its cultivation helps to improve human nutrition and income generation. It is especially important for the Asian countries where it is one of the three most important vegetable species. This region accounts for almost 50% of the worlds' area under brinjal cultivation (AVRDC, 2003).

Well drained soils with a pH of 5.5-6.8 and relatively high temperature in the range of 25&degreec-30&degreec are satisfactory for growth of the plant. The climatic requirements of the brinjal plant accurately match with the prevailing climatic conditions of Sri Lanka. As a result of that it is the second largest cultivated vegetable according to the cultivated extent of land (HORDI, 2002).

Average potential yield of brinjal is 20-30 t/ha under Sri Lankan conditions. National average yield is 7.06 t/ha in 2002. The large yield gap is nearly 2/3 and it is mainly due to pests and diseases. The highest cost (Rs.11000/ha) out of total cost (Rs.24000/ha) is accounted for pest and disease management (www.agridept.gov.lk).

Most common pests in brinjal plant are whiteflies, mites and aphids. The major diseases are bacterial wilt, foot rot disease and anthracnose fruit rot, damping off and phomopsis blight.

The foot rot disease is rapidly spreading and it now has become a serious problem to farmers. It is a fungal disease caused by soil borne fungi Fusarium solani, Rhizoctonia solani, Phytopthora spp., Sclerotium spp. and Pythium spp.

There are several control measures to overcome the disease such as improvement of soil drainage, fungicidal treatment with Thiram and Thiophanate Methyl for poorly drained soil, removal of infected plants and crop rotation with non-solanaceous plants. Even though the soil borne diseases are economically impracticable to control leave the use of resistant varieties provides the only mean to produce high yields.

Therefore the objective of this study was to identify resistant varieties/accessions to foot rot disease.

MATERIALS AND METHODS

Isolation & Identification of the Pathogens

Foot rot affected brinjal plants were collected from the field of Horticultural Research and Development Institute at Gannoruwa. The pieces of collar region obtained from the diseases plants were cultured and microscopic observations were carried out to identify Fusarium solani and Rhizoctonia solani. Length and width of micro conidia & macro conidia were measured and compared with the standards of the C.M.I. Descriptors (1964) for the identification of Fusarium solani. Branching pattern of the mycelium was used to identify Rhizoctonia solani (Dugger, 1915).

Variety Evaluation: Fifteen accessions/varieties (07795, 02887, 00270, 07149, 07694, 08104, 05124, SA7MTE2, 08891, 01247, 07145, 08890, 07157, BW11, SM164) were tested with 10 replicates under greenhouse conditions. Nursery establishment and aftercare practices were done according to the recommendations of Department of Agriculture.

Inoculation of Fusarium solani: Spore suspension of Fusarium solani were prepared using 14 days old cultures with the concentration of 2.0 &multiply 106 spores/ml. The sterile soil (80&degreeC, 6hrs) were inoculated with the spore suspension and incubated (28 &plusminus 1&degreeC, 14 days) in the greenhouse. The incubated soils were mixed with sterile soil in 1: 2.5 ratio. Twenty-one (21) days old seedling were dipped in 2.0 &multiply 106 spores/ml and transplanted.

Inoculation of Rhizoctionia solani: Dried saw dust (5kg) was mixed with soya flour (50g), rice bran (500g) and lime (100g). It was mixed with a solution (5l) containing MgSo4 (1g), K2HPO4 (4.5g), Urea (2.5g) KCl (0.75 g) and Glucose (100g). The sterile mixture (121&degreeC, 15 p.s.i., 20 min) were filled into polypropylene bags and inoculated with 1/4 of petri plate of 7 days old Rhizoctonia solani culture. After 14 days 3 - 4 g of inoculum was placed in a hole (3 - 3.5 cm) of the pot containing sterile soil and subsequently planted the seedling on the inoculum.

Data Collection

Variations of leaf yellowing of different accessions/varieties were observed as in the following scale.

Colonization of the Pathogen in Collar Region

Three replicates per each accessions/varieties were randomly selected. The collar region of these plants were removed and cultured on PDA + 0.01% Strptomycin Sulphate medium. The cultures were checked for the presence of Rhizoctonia solani & Fusarium solani using the light microscope. The collected data were ranked as follows

Data Analysis

The non-parametric data were analyzed using Kruskal-Wallis test using the statistical software package Minitab released 1986.

RESULTS & DISCUSSION

Observation of Symptom Development

Fusarium solani inoculated varietes/accessions showed yellowing of leaves after 8 weeks of transplanting except SA7MTE2, BW11 (Plate 2). Typical symptoms of foot rot i.e. leaf yellowing followed by wilting and death of plants was not observed.

After 10 weeks of transplanting, Rhizoctonia solani inoculated plants showed yellowing (Plate 3) except SA7MTE2, BW11.

Zerlik (1979) showed the experiments carried out in the greenhouse with sterilized soil for soil borne pathogens in which late yellowing had only been recorded for foot rot disease. He concluded it is due to the nature of late genesis of foot rot disease under the greenhouse conditions.

Salt (1979) reported distinctive disease symptoms in soil borne pathogens are often lacking, so that assessing their prevalence is difficult and time consuming. Obligate parasites such as Rhizoctonia, Fusarium and Pythium etc. are detected only by isolation on agar media.

Observation of Variations in Leaf Yellowing

There were differences in leaf yellowing among the accessions/varieties (Table 1).

The differences of the leaf yellowing could be due to the variations of the pathogens–host association among the accessions.

Identification of Resistant Varieties

Table 2 shows the results of the Kruskal-Wallis test for the plants 27 days after transplanting for inoculation of Fusarium solani.

According to the Kruskal-Wallis test varieties SA7MTE2 and BW11 show weak host pathogen association against inoculation with Fusarium solani. Table 3 shows the results of the Kruskal-Wallis test for the plants 20 days after transplanting for inoculation of Rhizoctonia solani.

Statistical analysis reveals that there is a difference of the responsiveness among the different accessions/varieties for the inoculation of Rhizoctonia solani.

Symptom Appearance of Foot Rot Disease

Nagarajan and Muralidharan (1998) reported for an infection process to complete and disease symptoms to appear prevalence of favourable atmospheric or weather conditions are an essential requirement. It includes temperature, humidity, light intensity, soil moisture, soil nutrient content, and soil pH. Colhoun (1973) showed the damage to the host plant by soil borne pathogens greatly upon environmental conditions and on the vigour of the plants.

The present study was conducted under the controlled environmental conditions. The plants were given equally optimum conditions for growth and development. Well-balanced nutrients and adequate water were supplied. Since plants were kept in the greenhouse, the exposure of plants to unfavorably high light intensities were prevented. As a result, it may have led to the enhancement of the vigour of the host plants and no complete visual symptom development.

Environment affects the disease severity primarily and the plants succumb to disease mainly when the environment is especially unfavourable for the host. For many soil-inhabiting fungi including sclerotium rolfsii, Rhizoctonia solani, etc., naturally occurring resistance in the host species of interest is unavailable (Agrios, 1988) or else where it has been reported has subsequently been shown to be equivocal (Sedun et al., 1989) Irwin & Langdon (1982) reported when working with this group of fungi, erroneous or misleading results may be obtained by using excessive or inadequate quantities of inoculum, conducting tests in sterile soil or unnatural environments like greenhouse conditions.

There are specific requirements for the pathogens to grow successfully. Extensive field studies at the Washington State University have demonstrated the distinct relationship between the plant water stress and increase incidence of the Fusarium foot rot (Cook, 1980). Chandniwala (1998) reported that Fusarium spp. increases the severity when Ammonium fertilizer is applied.

Plant water stress and Ammonium fertilizer application are favourable for pathogen development and these conditions were not supplied in this experiment. Instead regular watering and application of nitrate fertilizer were practiced. This would have led to the reduction of the disease symptom expression.

Waller et al., (2002) reported that the pathogens often lost virulence or change genetically when sub culturing. Therefore inoculum is best obtained directly from pathogens sporulating on diseased host, or from cultures maintained by techniques that reduce or prevent the saprophytic growth (on artificial culture media) i.e. lyophilisation and cryogenic preservation. In this study series of sub culturing was necessary after the isolation of pathogen from diseased plant parts for purification and maintenance. Thus it too would have led to reduction of the virulence of the pathogens.

In the field the presence of nematodes could damage the plants leading to severe disease development. Several nematode-fungi disease complexes were known for Fusarium diseases of several plants and thus it could increase incidence and severity of disease when the plants are invaded by different types of (root knot, lesion, sting, reinform, burrowing or stunt) nematodes (Agrios. 1997).

CONCLUSION

The varieties SA7MTE2 and BW11 show the resistance against foot rot disease. Confirmation of the resistance, further studies under field conditions should be carried out.
