Aug 14, 2019
Address American brown rot in sweet cherry orchard systems

American brown rot, Monilinia fructicola, caused significant economic damage in sweet cherry orchards in Michigan this season. While we spend time and effort every season discussing fungicide strategies, the difficult reality is that the American brown rot pathogen is aggressive and fast-growing to a degree that makes controlling it during a stretch of conducive weather extremely difficult, even with the best fungicides available. Under conducive environmental conditions, American brown rot-infected fruits can produce epidemic levels of inoculum in as little as 24-48 hours. As a result, American brown rot problems go downhill for the grower quickly in the orchard. Once significant fruit with active sporulation is present, the inoculum load is so high that infection of new fruits within the orchard is likely.

In general, it is particularly difficult to manage American brown rot in sweet cherries on standard Mazzard rootstock due to the large size of trees grown in this system. These trees can reach heights of 25-30 feet or more, making adequate spray coverage difficult to achieve. Unfortunately, brown rot spores originating from infected fruit in trees can easily find adjacent healthy fruit, rapidly driving disease epidemics. Furthermore, the large canopy size contributes to high humidity and prolonged fruit wetness after a rain event. This furthers the chances of secondary infections that occur as the fruit begin to ripen.

Solid horticultural practices to combat American brown rot in the orchard are a critical part of any comprehensive plan to manage this disease, but are particularly important in these large tree systems. American brown rot thrives under humid, wet conditions. As a result, there are several important techniques that can be deployed in a sweet cherry orchard to help reduce the suitability of the environment to this disease and improve management.

Reduce the density of the canopy by thinning out shaded wood. Cutting out shaded limbs to open up the canopy will help increase air flow and speed up drying time of fruit following a rain while decreasing humidity within the canopy, which will help reduce the favorability of the environment for the development of brown rot. This will also help to improve fungicide coverage.

Take the height of large trees down a few feet in the summer months. This is a valuable option for any grower of standard sized sweet cherry trees for two reasons. First, taking the height down will help reduce the size and density of the canopy, leading to better air flow in the orchard. This means lower canopy humidity and faster drying times after a rain event. Second, it will improve spray coverage. Most processing sweet cherry trees are sprayed with air blast sprayers without a tower, so getting good coverage 25-30 feet in the air can be extremely challenging. Brown rot is almost always the worst at the tops of trees in an older orchard, and this is no coincidence. Much like cherry leaf spot, this is evidence that spray coverage can be an issue in a tree this size.

Large cuts to the scaffold of the tree are best done in the hot, dry summer months when bacterial canker is not a concern. July and August are good months for this because the hot dry weather will allow the wounds to scar over quickly, and the hot temperatures are not conducive for the activity of the bacterial canker pathogen.

Remove pendant wood. Wood that hangs down towards the ground, particularly on the interior of the canopy, is problematic because water runs down the length of the limb towards the ground following a rain and prolongs wetness of fruit. Rather than drying quickly, these limbs can stay damp after a rain event for several hours, providing an ideal environment for American brown rot to develop. Encourage fruitful, lateral wood that will dry quickly after a rain event to avoid this issue.

Consider shaking fruit on to the ground if an orchard is abandoned due to American brown rot. Consider shaking any fruit you have walked away from onto the ground to help it break down before it can mummify on the tree fully. Brown rot mummies in the canopy of a tree are a potent source of inoculum the following year and should be avoided and removed. Mummies on the ground also contribute inoculum the following year, but many of the cherries shaken onto the ground during the early stages of brown rot infection or pre-infection will not mummify properly because they are broken down quickly by other secondary fungi and insects once they hit the ground. Brown rotted fruit left in the canopy will dry down quickly in the summer heat, stick tight to limbs and provide a healthy source of inoculum next season.

Pruning crews should also be on the lookout for mummies in the canopy during the pruning season. Cutting mummies out during pruning and flail chopping them in the row will help to reduce the inoculum load the following season.

For fresh market growers, consider new plantings trained in fruiting walls using Gisela rootstocks. This series of dwarfing rootstocks lend themselves extremely well for training sweet cherries in a fruiting wall, and this encourages thin canopies that dry quickly and get good air flow, making them far less suitable for American brown rot development. These rootstocks are not a practical option for growers of dark sweet cherries for processing or brine sweet cherries, but for growers expanding fresh market acreage they are an outstanding option to improve fruit output, increase orchard mechanization and reduce favorability of the environment for disease. The thin canopy of these systems also makes getting adequate coverage easier, maximizing efficacy of fungicide applications.

 – , and , Michigan State University, Department of Plant, Soil and Microbial Sciences

American brown rot infection on ripening sweet cherries. Photo: George Sundin/ MSU





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