Cherry Programs

Bacterial canker or gummosis (Pseudomonas syringae) is a serious disease of cherry in the Pacific Northwest. It is particularly damaging to young trees and can result in replanting issues if un-managed. Spread of the pathogen is favored by cool, moist weather. Optimum timing for control of bacterial gummosis is in late winter before trees break of dormancy and spring frost, and wet weather occur. In the fall, apply most materials before autumn rains or after October 1.

Brown rot is a serious disease of stone fruit when wet conditions occur in the orchard. The disease is caused by the fungus Monilinia fructicola although other species (i.e. M. laxa and M. fructigena) have been reported in other regions. There are both floral and fruit phases of the disease. Brown rot is explosive and highly favored by rain events during bloom (blossom infection) and immediately prior to harvest (fruit infection). Many fungicide materials are effective on both brown rot and powdery mildew. Use the products list on the bloom table for brown rot, as they are effective, and mildew sprays are not recommended at this stage of tree growth. Neither iprodione nor fenbuconazole are first-rate powdery mildew materials. Always follow fungicide resistance management guidelines. Current resistance management guidelines are available at https://www.frac.info

Powdery mildew, caused by the fungus Podosphaera cerasi, is one of the most serious disease of cherries in the Pacific Northwest. The fungus attacks both foliage and (less commonly) fruit. Most cultivars are susceptible to the disease but it is particularly severe on the cultivar ‘Sweetheart’. Management of the foliar phase is important because spores that infect fruit are produced on infected leaves. The fungus survives winter as chasmothecia (the sexual fruiting body); epidemics are initiated in spring when moisture results in ascospore release from the overwintering propagules. Ascospore release requires free moisture at 50F or greater. Ascospores serve as primary inoculum and give rise to powdery mildew colonies that continually produce millions of asexual spores (conidia). Conidia serve to spread and intensify the epidemic on both foliage and fruit. The disease is favored by moderate temperatures and high humidity, particularly during the two weeks before harvest. Management of powdery mildew in sweet cherries involves intensive and expensive fungicide application programs. Resistance to Group 3 (DMI) and Group 11 (QoI) fungicides has been documented in the cherry powdery mildew pathosystem. Resistance to Group 7 (SDHI) fungicides has been documented but is of quite limited distribution. Current resistance management guidelines are available at https://www.frac.info

Coryneum blight or shothole, caused by Wilsonomcyes carpophilus, is a fungal disease of minor importance in the Pacific Northwest. The fungus overwinters in twig cankers. Spores are produced on canker surfaces during early spring rains (or over-the-canopy irrigation) and are splashed to foliage and fruit where they germinate, infect, and cause small lesions. The lesions are small and circular. Necrotic lesion centers may drop giving heavily infected leaves a “shothole” appearance. The disease is managed using fungicide programs early in the growing season.

Rain-Induced Cherry Cracking is often the greatest single cause of sweet cherry fruit cullage, even in the arid regions east of the Cascade Mountains in the Pacific Northwest. Cherry growers know how devastating a rain event can be if it occurs when cherries are nearing maturity. Cherry cracking has been studied for several decades, but the causes of cracking are still not fully understood.

Pre-bloom applications of pesticides can be effective and will also conserve natural enemies for leafroller and biological control agents of other pests, such as aphids. If treatments for leafrollers were applied at pink and/or bloom, sampling to determine the density of surviving leafrollers should be completed prior to deciding to apply additional controls at this timing. Most products listed act primarily as stomach poisons versus direct contact to residues, therefore, complete coverage is very important to achieve maximal control. Repeating an application of any product should be based on the leafroller population surviving previous treatments. Use the leafroller models on the WSU Decision Aid System (https://decisionaid.systems) for the optimum timing.

Good sanitation (removing large wood prunings, dead limbs, and woodpiles from the orchard) is the most effective management tactic. Insecticides are only effective against adults. Beetles begin flying in late April and are active through May. The second generation flight begins in late July or early August. Yellow sticky traps placed on orchard borders will detect adult beetle activity. Spraying the border trees (rows) with high water volumes will protect the remainder of the orchard in many situations where external sources are the primary problem.

Spotted-wing drosophila (SWD) is one of the newer invasive species from Asia, first detected in the continental US in 2008, and achieving pest status in eastern Washington in 2010. Among the tree fruits, only cherries are considered to be vulnerable to attack preharvest, although like any drosophilid, SWD will use injured or rotting fruit of any type to complete development. SWD can be controlled by several groups of insecticides, and rotation among MOAs is important for resistance management. Monitoring tools are available, and should be used to gain a general idea of pest pressure in a given year. Experience since 2010 indicates that cold winters, especially those with sudden and extreme cold snaps, will decimate overwintering populations, and result in low pressure the following growing season, with little need for spray coverage. Conversely, mild winters and early springs have preceded extremely high pest pressure, necessitating a full season spray program.

Western cherry fruit fly is the key direct pest of cherries, and quarantine regulations create a zero tolerance for this pest. Adult flies lay eggs in the fruit, and the larvae feed and develop inside the fruit until they are ready to pupate. A baited yellow sticky trap can be used for monitoring adult emergence of the single generation per year, and a degree-day model is available. Females have a 7-10 day pre-oviposition period, so sprays (either canopy or bait sprays) can start about a week after first fly detection or when the model predicts emergence. Fly emergence continues after harvest, so post-harvest clean up sprays will help prevent future problems, especially if unharvested fruit remains in the orchard. Many of the materials that kill western cherry fruit fly are also effective on spotted-wing drosophila; the neonicotinyls are a notable exception.

X-disease vector leafhoppers, primarily Colladonus montanus reductus and Colladonus geminatus vector X-disease phytoplasma, which causes fruit malformation in cherries, peaches, nectarines, and other stone fruit. Colladonus montanus reductus is currently the main vector and has three generations, with adults occurring in June, August, and September/October, with transmission occurring in the second and third generation. Removing the broadleaf weeds that host leafhoppers through the planting of competitive grasses and/or herbicides reduce vector abundance. Use yellow sticky traps to monitor leafhoppers and apply insecticides after harvest when vectors are present. To avoid resistance evolution, apply insecticides in at least two-week intervals and rotate insecticide classes.