Segmented linear Model to characterize tolerance to tomato yellow leaf curl virus and tomato leaf curl virus in two tomato cultivars under greenhouse conditions

To investigate the amount of yield losses caused by Iranian  isolates of tomato yellow leaf curl virus (TYLCV-[Ab]) and tomato leaf curl Karnataka virus (ToLCKV-IR) in Rio Grande (RG) and Grosse Lisse (GL) tomato cultivars, tomato seedlingswere separately agroinoculated with  an infectious clone of either virus and disease development was monitored over four time intervals. After the emergence of symptoms, disease severity (DS) was evaluated visually using an ordinal rating scale and the data was converted to a ratio scale. Losses caused by the viruses were estimated by measuring vegetative indices including wet and dry weight and height of the aerial and underground parts of the plants. Initial DS in plants infected by ToLCKV-IR was less than those infected by TYLCV-[Ab]. Descending order of rates of disease increase for the studied pathosystems was as follows: ToLCKV-IR-infected GL>TYLCV-Ab-infected GL> TYLCV-[Ab]-infected GL> ToLCKV-IR-infected RG. With delayed inoculation, DS was reduced in both cultivars inoculated with either of the two viruses. A segmented linear model showed a very good fit to the data of relative biomass and duration of exposure of all pathosystems. The estimated parameters of the model were used to evaluate different aspects of the damage curve. The highest crop loss disease threshold was observed for GL cultivar irrespective of the inoculated virus. Similar results were achieved when the desensitization disease level was evaluated. Curvilinear tolerance or slope did not show any significant difference between the infected-GL plants to either virus and also TOLCKV-IR-RG, while TYLCV-[Ab]-RG showed the highest curvilinear tolerance. Findings of this study can be applied in screening for tolerant tomato plants.