Occurrence of Sclerotium rolfsii Inducing Sugar Beet Root Rot and its Sustainable Management by Acting on Soil Fertility in Western Morocco

In Morocco, and particularly in the Doukkala irrigated perimeter, sugar beet rot caused by Sclerotium rolfsii is a major limiting factor for the productivity of this crop. The objective of this study was to identify the relationship between the frequency of Sclerotium rolfsii infestation and the quantity of viable sclerotia in the soil on the one hand, and with the different physicochemical parameters of the soil in cropped sugar beet fields on the other hand. In total, 1794 soil samples were collected during a four years period in the whole irrigated perimeter. These samples were analyzed for their sclerotial content. In addition, laboratory analysis of physico-chemical parameters was performed for 94 sugar beet fields in 2019. The study showed that the relative frequency of infestation by Sclerotium rolfsii and the number of viable sclerotia oscillate inversely and present a relative frequency of 45.7, 57.3, 42.3 and 49.5% for 2016, 2017, 2018, and 2019, respectively, and a number of sclerotia of 2.01, 1.65, 2.11 and 1.25 in 250 g of soil for 2016, 2017, 2018 and 2019, respectively. The study showed that the number of viable sclerotia was positively correlated with clay and silt content, soil pH, Mg, K, Fe, Cu, and Mn and negatively correlated with sand, SOM, EC, CaCO3, Zn, B, Ca, NH4 and NO3 contents. The observed decrease in viable sclerotia between 2016 and 2019 can be attributed to the decrease of Fe and Cu in the soil. It is possible that SOM intervenes with its humified fraction in the neutralization of the redox potential of Feand Cu which are involved in the production of sclerotia under oxidative stress. The good control of fertilization, especially organic, allows limiting the production of sclerotia of Sclerotium rolfsii. Data analysis allowed obtaining a significant prediction model of the number of viable sclerotia in the soil according to the physicochemical soil parameters with (R2 = 0.95 at P<0.0001) for only infested fields and (R2 = 0.87 at P<0.0001) for all the fields