ABSTRACTUMO
Walnut Blight is caused by Xanthomonas arboricola pv. juglandis. In Zavalla (Santa Fe), walnut production is focused on the grafted plants in nurseries. The objective of this work was to quantify the environmental effect on the epidemic development of walnut blight, in susceptible varieties and in varieties with better behaviour to the disease, and to develop mathematical models to predict the severity of the disease In the locality of Zavalla observations of severity (Sev%) of walnut blight were made every seven days in four walnut varieties (three susceptible (sus=1): Chandler, Davis and Tulare and one with better performance: Franquette (sus=0)), from the end of spring until the last days of February (N=76) , during three crop seasons (2010/11, 2011/12 and 2012/13). Weather regression variables were calculated in the respective 15-day periods prior to each observation of daily epidemic increase rate (Tid%=Sev%t-Sev%t-1/7). The variables best correlated (rk: Kendall Tau-b correlation coefficient) with bacterial levels (severe: Tid% >0.1367; moderate to nil) were Tid%<=0.1367, with 0.1367 being the 40% percentile of the observed data), for both types of varieties (sus=1 and sus=0), were DPr>9 (days with precipitation>9mm; rk=0.434) and DMojro (days without rain, with wetness attributable to dew due to the occurrence of relative air humidity greater than 82%; rk=0.326). The logistic model integrated by DPr>9 and DMojro correctly classified 62 cases of the 76 analyzed in the three crop seasons (prediction accuracy: PPred=81.5%), allowing to quantify the environmental effect in both types of walnut varieties analyzed. The model integrated by DPr>9 and DMojt (summing the days of wetting by dew (DMojro) with those originated by precipitation (DMojpr)) achieved a prediction accuracy of 80.3%. Both meteorological variables would be expressing the bacterial free water requirement for infection (DMojt) and energy requirement in rainfall (DPr>9) to provide bacterial dispersion.
Keywords
Xanthomonas arboricola pv juglandis
; epidemic development; environmental effect;
Juglans regia
; Argentina