Meteorological factors affecting periodicity and concentration of airborne spores ofBremia lactucae

Several systems for forecasting downy mildew of lettuce are based on the assumption that spores infect leaves in the early morning. However, there is no clear indication that spores are present in sufficient amounts at that time. Spores of Bremia lactucae were trapped using a 7-day volumetric spore trap for about 35 days in each of 1997 and 1998. Three periodicity patterns were observed. The most frequent pattern had a peak, representing more than 75% of the daily catch, near 10:00 a.m. The second pattern was similar except that the peak occurred later. The third pattern was observed on days with low numbers of spores and was characterized by a random distribution, though most spores were trapped during daylight hours. Spore release was associated with a drop in relative humidity (RH) and increases in temperature and wind speed. The fluctuations in numbers of airborne spores followed fluctuations in disease severity. Overall, daily spore concentrations and number of spores per lesion were not correlated with daily weather variables. However, when spore concentrations were grouped into three categories (low: <10 spores/L; intermediate: ≤10 to < 100 spores/L; high: ≥100 spores/L), a good correlation (r = 0.79) was obtained with night hours of high RH (>95%). For both years, the first significant spore catch was observed when about 200 degree-night hours of RH > 95% (base 0°C) (DNRH) had accumulated. The relative spore concentration (RSC) was modeled as a function of DNRH and the model that best explained (R 2= 0.97) this relationship was RSC = 1×(1 + e–0.0183×DNRH)1/1−1.005.