Pearson correlation analysis was conducted to identify pairwise relationships between diseases. Multivariate analysis was performed using stepwise regressions with single plant data to determine the influence of agro-ecological and varietal variables on disease severity. Some class data were coded ordinal (ecozones, which are closely correlated with the rainfall, soil texture, according to increasing percentage of sand), while data for variety mixtures were treated as nominal variable (see the earlier sections). Partial sum of squares (SS2) was used to assess the relative contribution and to test the significance of the included variables (testing the hypothesis that the slope is zero or otherwise).