SCREENING OF TOMATO VARIETIES FOR RESISTANCE TO MAJOR FUNGAL DISEASES AND BACTERIAL WILT

The resistance of local and introduced tomato varieties to early blight (Alternaria solani Sorauer) and late blight (Phytopthora infestans (Mont.) de Bary) were evaluated in the field and to bacterial wilt (Ralstonia solanacearum) were tested under artificial infection in greenhouse conditions. Three varieties (De-brao krasnii, De-brao Rozovii and Sultan F1) showed the moderate resistance to early blight, while the only two varieties (De-brao krasnii and De-brao Rozovii) were moderate resistant to late blight. The remaining samples showed susceptibility to disease. The tested varieties showed a different degree of resistance to the bacterial wilt. Four varieties: Rozovii Gigant, Krachodarskii Krasnii,Tarasenko and Lagidnii appeared to be resistant; Two varieties: Fakel and Polbig showed the moderate resistance and the rest ten varieties were susceptible to the disease.


tomato, Ralstonia solanacearum, early blight, late blight, resistance
The cultivated tomato (Solanum lycopericum) is one of the most popular and widely used foods in Georgia. It was introduced in the second half of the XVIII century in Georgia and now it is grown in all regions up to 1700 meters above sea level. Of the several major diseases infecting tomato, late blight (caused by Phytophtora infenstance) and early blight (caused by Alternaria solani Sorauer) are prevalent and severe diseases, reducing yields significantly. Average yield losses of a tomato crop caused by these pathogens are 40-50%, but during severe outbreaks of diseases, the affected foliage area reached 70-100% [1,2].
Very harmful, restricted distributed quarantine disease of tomato is bacterial wilt (caused by Ralstonia solanacearum). In tomato, bacterial wilt was identified for the first time in 2011 in Chkorotsku and Kutaisi regions (west Georgia). Where it caused up to 100% plant loss in greenhouse-and field-grown crops [3]. Since then, several cases of this disease have been documented on potato in home gardens [4]. Natural climatic conditions of  Georgia are favorable for development tomato diseases, however, the level  of damage depends on the varieties, on the infection time and severity, crop  grown season climate, etc. Chemical control of the disease has commonly been used in Georgia, but it is expensive and despite their usefulness, pesticides could pose potential risks to food safety and the environment. Adoption of disease resistant varieties/hybrids is the most environmentally friendly practical way to solve this problem, especially to control Ralstonia solanacearum.
Successful production of tomato depends on the choice of resistant varieties adapted to a concrete location. The purpose of this study was to evaluate the level of resistance of tomato varieties introduced recently in Georgia to above mentioned diseases.
Plots with three times replication were 1 m wide ќ 3 m long. Intra-row spacing was 50 cm and inter-row spacing -70 cm. Late blight and Early blight severity was evaluated weekly on the basis of the percent leaf area infected using unified scale of resistance of vegetable plants (Table 1) [5].
The tomato varieties were also tested to Ralstonia solanacearum under artificial infection in greenhouse conditions with temperature 26-28°C, humidity -60-80%. The tested plants were grown in plastic pots contained sterilized mixture of peat, moss and soil (1:1:1). As a inoculum, a mixture of strains of Ralstonia solanacearum spread in Georgia was used. The strains of pathogen were subcultured and multiplied on the Casamino Acid-Peptone-Glucose (CPG) media. Plants in the phase of three to four leaves were inoculated with a suspension (10 9 cfu/ml) of 24 hours cultivation cultures of Ralstonia solanacearum by injection into the plants stem. Control plants were inoculated with sterile water. The experiments were carried out in three replicates with 10 plants in each replication. After the first visible disease symptoms tested cultivars were observed weekly. Disease severity was recorded at an interval of 7 days after inoculation according to 0-5 scale developed by [6], where RT (reaction type), 0 -means no symptom, 1 -one leaf wilted, 2 -two or three leaves wilted, 3 -the majority of leaves wilted except the top two or three leaves, 4 -all leaves wilted, and 5 -plant dead. Also, disease severity has been calculated using the following formula:

Unified scale of resistance of vegetable
where DI-diseases index, Ni = number of infected plants with a scale, I -RT's from 0 to 5, N=total plant tested. On the base of disease index, the level of resistance of cultivar was determined as follows: when DI is less than 10% the cultivar considered as resistant (R), when DI =10-20%, cultivar is moderately resistant(MR), DI = 21-40%, cultivar shows moderately susceptibility -MS and when DI is more than 40%, cultivar is susceptible -S.

RESULTS
In accordance with results of the field trials only two varieties: debravo krasnii, de-bravo rozovi were moderate resistant to late blight and three varieties (de-bravo krasnii, de-bravo rozovi, sulthan F1) showed moderate resistance to early blight. The most of tested tomato cultivars were susceptible to both diseases ( Table 2).
The results from greenhouse experiments indicated that potato varieties