Thursday, 8 May, 2014: The fungi causing brown rot blossom blight invade stone fruit flowers during wet weather when stone fruits are in bloom. A modest fungicide program is usually sufficient to protect stone fruit flowers, but under unusually favorable conditions, trees may require several fungicide applications over the 7-14 days when stone fruits are blooming, including sprays at petal fall and/or shuck split.
Most of the brown rot in eastern United States is caused by Monolinia fructicola and causes the disease known as American brown rot (ABR). However, Monolinia laxa is a different, less-common brown rot pathogen that has recently become more prevalent in some cherry orchards, especially in Michigan where it caused major losses in some tart cherry blocks in 2013 (see MI story). The brown rot disease caused by M. laxa is called European brown rot (EBR). Differentiating between EBR and ABR in the field is almost impossible because the symptoms are very similar. However, EBR often causes spur and twig die-back in addition to blossom blight, whereas that phenomenon is less common with ABR.
Several factors have favored the appearance of M. laxa in recent years. First, European red-fleshed tart cherry cultivars such as Balaton and Danube appear to be especially susceptible to M. laxa. Where these cultivars are included in cherry plantings, they may play a role in harboring inoculum that can then spread to other cultivars. Second, chlorothalonil (Bravo and generics) is reasonably effective for protecting against blossom blight caused by ABR under modest pressure, but it is NOT effective for protecting flowers from blossom blight caused by EBR. Third, EBR can infect flowers at somewhat lower temperatures than those needed for significant levels of ABR infection.
Cherry growers whose plantings include European tart cherry cultivars may want to take a pro-active approach toward EBR by including Rovral in at least one fungicide spray during bloom. (Efficacy of Rovral against EBR has not yet been verified, but I am assuming that it will work because of its broad activity against this kind of fungi, and it will certainly provide better control of ABR than what is provided by chlorothalonil.) Rovral can be applied only twice per year, and it cannot be applied after petal fall. It is the preferred alternative to chlorothalonil in blossom blight sprays because it is a totally different chemistry than the DMI, QoI (strobe), and SDHI fungicides that are important products for preharvest brown rot control. Thus, using Rovral during bloom will not generate any selection pressure for resistance to products such as Orbit, Indar, Gem, and Pristine that really should be reserved for use as preharvest brown rot sprays.
EBR seems to be a problem primarily on tart cherries, so Rovral will be most useful on cherry blocks, although it would also be an excellent choice for peaches, nectarines and apricots when conditions favor severe blossom blight infection. Chlorothalonil is still the preferred fungicide for plums/prunes during bloom because it is the only product that is highly effective in preventing black knot. Rovral will NOT protect against black knot.