Evaluating the Reaction of some Non-hybrid and Hybrid Tomato Cultivars to Bacterial Speck Disease

Introduction: Tomato bacterial speck caused by Pseudomonas syringae pv. tomato, is one of the several tomato diseases in the world. The disease can seriously affect the quantity and quality of this high-consumption crop in its cultivated areas. Disease symptoms included black spots surrounded by yellow halo on the leaves and small block spot on fruits surface. The spots on ripe fruits may surround with yellow haloes. It is difficult to diagnose the disease via symptoms, because there is high similarity among symptoms of bacterial speck and other tomato bacterial leaf spot diseases, especially bacterial spot caused by Xanthamonas spp. and Pseudomonas syringae pv. syringae. The disease is seed-borne, and application of bacterial-free seeds is the most effective strategy to reduce disease damage. Besides using healthy seed and seedling, other strategies such as applying resistant cultivars, crop rotation, drip irrigation and using pesticides are common procedures in integrated disease management Materials and Methods: In this study, the reaction of 24 tomato cultivars including 13 non-hybrid cultivars (Early Urbana 111, Early Urbana Y, King stone, Super 22 TO, CalJ N3, 2270, Rio grenade, Early Urbana, Primo early, Falat CH, Super Chef, Primax and Red Stone, and 11 hybrid cultivars (Hyb. Superset, Hyb. Firenze, Hyb. Comodoro, Hyb. Bellariva, Hyb. 1585, Hyb. Kishmat, Hyb. Eden, Hyb. 8320, Hyb. Monty marker F1 and Hyb. Ferguson F1) was evaluated against bacterial speck disease in greenhouse. Four pathogenic P. syringae pv. tomato strains isolated from tomato fields in West Azarbaijan province, northwest of Iran, used in all experiments. For inoculation, bacterial suspension of 107 CFU ml−1 (OD600) was sprayed on the foliage of tomato seedling at four-five leaf stage. Inoculated seedlings were monitored daily for 21 days, and symptoms were recorded. The indexes of disease severity (DS), time of the first disease symptoms appearance and the area under the disease progress curve (AUDPC) were determined. To assess the disease severity, spots were counted on six leaves of each plant, and the index from zero to six was used, where 0) without symptom, 1) 1-10 spots, 2) 11-15 spots, 3) 16-20 spots, 4) 21-25 spots, 5) 26-30 spots and 6) more than 30 spots on leaves. Experiments were conducted in a completely randomized design, and four pots with four seedlings in each pot were considered for each treatment (cultivar). Statistical analysis of data was performed via Tukey’ HSD test using SAS software (version 9.4). The AUDPC index was calculated using R (version 3.5.2) and Agricolae package. Correlation among studied indexes was evaluated via Spearman’s rank correlation coefficients in SPSS (version 25). Results and Discussion: Analysis variance of data indicated the significance at 1% level among the studied indexes. Positive correlation observed between AUDPC and the time of the first symptoms appearance (r=0.71), as well as the disease severity index (r=0.76), but there was no significant correlation between the time of the first symptoms appearance and the disease severity indexes (r=0.22). According to all three indexes cv. Hyb.1585 determined as a resistant cultivar and cv. Hyb. 8320 were identified as disease susceptible cultivar. However, some cultivars such as cv. Hyb. Firenze was susceptible based on the disease severity index but it considered as a resistant cultivar based on the time of the first disease symptoms appearance index. The results of previous research on tomato bacterial speck disease have shown different degrees of disease severity in various cultivars. So far, several resistant cultivars against this disease have been reported. The response of 93 different tomato cultivars growing in the Mediterranean region of Turkey was examined and seven cultivars showed resistance against P. syringae pv. tomato. Six of these cultivars included Atalay, Party, Petrus, Piccadilly, Prenses and they had the Pto gene, which encodes proteins related to resistance against the disease. Overall, based on the findings of this study, hybrid cultivars of Hyb. 1585, Hyb. Superset, King stone, Hyb. Bellariva and Hyb. Firenze, and non-hybrid cultivar Super Chef showed higher resistance to tomato bacterial speck disease in compare to other studied cultivars. Conclusion: According to the results of this study, the resistance to this pathogen depends on various genetic and physiological factors, as well as plant-pathogen interactions. Application of different disease indexes in evaluating the cultivars reaction to diseases is recommended which can be effective in the final decisions for diseases management.