Influence of Environment and Inoculum Density on the Incidence of Brown Rot Blossom Blight of Sour Cherry

Wilcox, W. F. 1989. Influence of environment and inoculum density on the incidence of brown rot blossom blight of sour cherry. Phytopathology 79:530-534. When potted Montmorency sour cherry trees were inoculated in full Montmorency trees were inoculated with either 50, 500, or 5,000 bloom with 5,000 conidia/ml of Monilinia fructicola, placed into mist conidia/ ml of M. fructicola, placed into mist chambers for 8-24 hr at the chambers at 8, 12, 16, or 20 C for 0-24 hr, then incubated for 7 additional abovementioned temperatures, and subsequently incubated at 20 C and days at 20 C and 60-70% relative humidity (RH), blossom blight incidence 85-90% RH, blossom blight incidence was influenced by inoculum was proportional to the temperature and duration of the wetting period, concentration in addition to temperature and wetting duration. Production Disease incidence was nil without wetting and minimal with 3 hr of wetting, of conidia of M. fructicola was assessed for 3 yr on mummified regardless of temperature, but was 5, 7, 28, and 72% with 5 hr of wetting at Montmorency fruits collected regularly from orchard trees between bud 8, 12, 16, and 20 C, respectively. Disease incidence increased to 21, 78, 90, break and harvest, yielding only 2-18% as many conidia during the sample and 98% after 10 hr of wetting at the same respective temperatures, and periods before petal fall as during the peak sampling period before harvest. reached a level of 85-97% after 24-hr wetting periods, regardless of These data suggest that under New York conditions, mummified fruits are temperature. The influence of time and temperature on the rate of conidium often a more important inoculum source for initiating fruit rot than germination in vitro closely paralleled the influence of wetting duration and blossom blight. temperature on the rate of increase of disease incidence. When potted Additional keywords: Prunus cerasus. Brown rot is a ubiquitous disease of stone fruits in New York major sources of infection before harvest (i.e., aborted or thinned State. Although both the "European" (causal agent Monilinia fruitlets) was not investigated in these studies. laxa (Aderh. & Ruhl) Honey) and "American" (causal agent Blossom blight occurs only infrequently on sour cherries in New M. fructicola (Wint.) Honey) forms have been reported in the York State, yet many growers have been hesitant to alter their region, M. fructicola is considered to be the far more common traditional protective fungicide programs in the absence of pathogen (6). supportive epidemiological or biological data. Zehr (19) has M. fructicola can cause both blossom blight and fruit rot on recently outlined a disease management program for peaches in susceptible hosts (4), yet the relative importance and relationship South Carolina in which blossom sprays are reduced or eliminated of these two phases of brown rot have not been documented on depending primarily on the perceived level of overwintering sour cherry (Prunus cerasus L.), the most widely planted stone fruit inoculum sources within or immediately external to the orchard. species in the Great Lakes region. Blossom blight may cause severe Climatological factors are deemphasized under this program, due losses on peach by reducing the number of flowers capable of to the generally warm, humid environment favoring epidemic developing into fruits (11) or by reducing the amount of bearing development of brown rot in the southeastern United States. surface as a result of subtending twig cankers (19); however, in the However, the cool climate that often prevails during bloom in the eastern United States blossom blight is considered to more sour cherry growing districts adjacent to the Great Lakes, and frequently cause economic damage on peach by providing a source evidence that this species is relatively less susceptible to blossom of inoculum for subsequent rot of the ripening fruit (11). Wilson blight than peach (14), collectively suggest that an independent set and Ogawa (18) also have noted the general primacy of blighted of parameters might be developed to assess the need for blossom blossoms as a source of inoculum for rot of stone fruits in blight sprays on sour cherry under New York conditions. AccordCalifornia, as has Kable (7) for apricots in New South Wales, ingly, this study was initiated to determine the influence of Australia. temperature and wetness duration on disease incidence; the Nevertheless, the perennial importance of blossom blight is interaction of these environmental variables with inoculum subject to question. In reviewing peach brown rot, Roberts and density; and the seasonal production of conidia of M. fructicola Dunnegan (12) concluded that control measures for the blossom within selected orchards. A brief portion of this work has been blight phase are needed only in certain seasons, even on relatively published previously (17). susceptible varieties. Anderson (2) noted that blossom blight is relatively rare in the midwestern United States, although he MATERIALS AND METHODS recognized that sporulating blossoms can represent a significant threat to ripening fruit in regions where blossom blight is common. Controlled environment studies. Plant material used in all tests Recent epidemiological studies of peach brown rot in South consisted of 3-10-yr-old sour cherry trees (Montmorency cultivar) Carolina (8) and the Niagara Peninsula of Ontario, Canada (3), on Mahaleb rootstock grown in 30-cm-diameter clay pots. Trees suggest that blighted blossoms are a relatively minor source of were maintained outdoors during the growing season, stored inoculum during the preharvest period, although their importance indoors at 1 C during the winter, and induced to bloom in the as a bridge (7) between overwintering inoculum sources and the spring by repotting and transfer to a greenhouse. Immediately before inoculation, unopened blossom buds were removed from ___________________________________________ the trees, leaving about 100-200 fully opened blossoms per tree. © 1989 The American Phytopathological Society Inoculum was prepared by growing a benomyl-sensitive isolate