Relationship between Verticillium dahliae inoculum and sunflower wilt in Argentina

Leaf mottle and wilt (LM) of sunflower caused by Verticillium dahliae is the most important disease in Argentina. LM is a monocyclic disease where microsclerotia in the soil are the primary inoculum source and have an important impact on symptom expression (incidence and severity) and yield. The inoculum (microsclerotia/g of soil) and disease relationship is affected by several factors, such as temperature and soil moisture. The objective of this work was to determine the regression model that best fits the relationship between V. dahliae microsclerotia in soil and sunflower LM, over a wide environmental range. Sunflower plants were exposed to different microsclerotia densities in pasteurized and non-pasteurized soils in pots under greenhouse and field conditions. The microsclerotia in soil and LM relationship regression models were also tested in the field with natural soil borne microsclerotia infestation, in different sunflower cultivars and locations. After the flowering stage (+/- 8 days), disease severity was affected by inoculum density at every evaluation time (P-value <= 0.0009). A 45% increase in the area under the disease progress curve was shown by soil pasteurization. Power regression model showed the highest concordance and Pearson correlation coefficients, parameter significance and no difference with the lowest Akaike index criterion value among all tested models in all experiments. This study provides a model (LM = 11.6*inoculum density<^>0.57) which describes the inoculum density and LM relationship. The use of this model might help farmers to predict an LM risk before sowing and to choose the appropriate field and the resistance level of the sunflower hybrid to be used.