Production and release of asexual sporangia in Plasmopara viticola

To study the effect of environmental conditions on sporulation of Plasmopara viticola lesions under vineyard’s conditions, unsprayed vines were inspected every second or third day, and the numbers of sporulating and non-sporulating lesions were counted, in two North Italian vineyards from 2008 to 2010. Infected leaves were removed so that only fresh lesions were assessed at each field assessment. Sporulation was studied at two scales, across field assessments and across the seasonal population of lesions. Frequencies of sporulating lesions were positively correlated with the numbers of moist hours in the preceding dark period (i.e., the number of hours between 8.00 p.m. and 07.00 a.m. with relative humidity ≥80%, or rainfall >0 mm, or wetness duration > 30 min). In a ROC analysis, predicted sporulation based on the occurrence of ≥ 3 moist hours at night-time provided overall accuracy of 0.85. To study the time-course of sporulation on lesions which were not washed by rainfall, numbers of sporangia produced per mm2 of lesion were estimated on individual cohorts of lesions, over the whole infectious period. The numbers of sporangia per mm2 of lesion increased rapidly during the first 4 days after the beginning of sporulation and then tapered off prior to a halt; the time-course of cumulative sporangia production by a lesion followed a monomolecular growth model (R2=0.97). The total number of sporangia produced by a mm2 of lesion increased as the maximum temperature decreased and moist hours in the dark increased. To study the release pattern of the sporangia, spore traps were placed near grapevines with sporulating lesions. Sporangia were airborne in 91.2% of the days, over a wide range of weather conditions and also in no rainy periods. The results provide quantitative information on production of P. viticola sporangia that may help refine epidemiological models used as decision aids in grape disease management programs.