Influence of incubation-period humidity on the development of brown rot blossom blight of sour cherry

When detached sour cherry (Prunus cerasus) blossoms were inoculated with conidia of Monilinia fructicola and subjected to a standard 8-h wetting treatment at 20 degrees C, blossom blight incidence was proportional to relative humidity (RH) when RH was held constant during the subsequent 6-day incubation period (frequency = 1.0 at the maximum RH of 92%; frequency = 0.38 at the minimum RH of 57%). Similarly, when a primary incubation period at 87% RH was followed by a secondary incubation period at 54% RH, blossom blight incidence was proportional to the number of hours at the higher level (frequencies of 0.94, 0.80, and 0.38 with primary incubation of 6 days, 36 h, and 12 h, respectively). When intact blossoms on potted trees were exposed to inoculation and wetting treatments, disease incidence was consistently high on trees that subsequently were incubated in a controlled environment chamber (20 degrees C, 90 to 95% RH) but was extremely variable when trees were incubated under variable ambient conditions. Ambient incubation temperature had little effect on disease incidence 9 days after inoculation, whereas ambient RH had a pronounced effect: the frequency of blighted blossoms was 0.53 to 0.61 when the number of hours at RH >90% was approximately two to six times that at RH <60%, whereas this frequency was only 0.02 to 0.07 when the number of hours at RH >90% was approximately one-third the number at RH <60%. After 48 h at a constant RH of 89 or 57%, the water potential of excised uninoculated blossoms was -1.15 and -1.93 MPa, respectively; however, growth of M. fruticola on osmotically adjusted potato dextrose agar was unaffected by changes in water potential within this range. Thus, although RH during incubation has an important influence on blossom blight development, the causal mechanism remains uncertain.