General Information.Powdery mildew of cherry is the most destructive disease in eastern Washington and Oregon cherry production areas. Mildew infestation of fruit can be financially disastrous.The disease is caused by a fungus, Podosphaera clandestina. Isolated severe outbreaks occur annually in Washington. Industry-wide epidemics transpired in the 1980's and 1990's. The disease is common on cherry foliage, but can also attack developing fruit if rains occur late in fruit development. Although all commonly grown sweet cherry cultivars are susceptible to powdery mildew, in Washington the disease is most severe on Bing, Rainier, and Sweetheart cherries. The disease also affects tart cherries. Severe epidemics on fruit typically occur during years with periods of excessive early-season rainfall, in orchards where spray intervals have been excessively extended, when spray coverage and penetration was poor, or when substandard compounds are used under conditions of high disease pressure.

Symptoms. Foliage, shoots, and fruit are susceptible to infection. The initial foliar symptoms frequently observed are light colored, roughly circular lesions on the surfaces of leaves in the inner portions of the tree. As the infections progress, sporulating mildew colonies appear on the leaf surfaces; the colonies can remain individual or coalesce, giving the leaves a white, mealy cast. Severely infected leaves are frequently blistered and puckered. As the mildew colonies age, numerous cleistothecia form. Severely infected shoots are stunted and assume a contorted and/or blighted appearance. On ripe fruit, powdery mildew appears as a roughly circular, slightly depressed area on the fruit surface containing hyphae of the fungus. The fungus can be present in small restricted areas, or can cover the entire fruit. There is little fruit-to-fruit spread in storage or transit. Heavy infestation of fruit pedicels can also occur. When this occurs, a thin film of white spores may line the fruit "bowls". The fungus produces numerous cleistothecia on infected foliage. Each cleistothecium contains 8 spores known as ascospores. Cleistothecia remain on foliage until a wetting event occurs in early autumn and are then blown or splashed into bark fissures and tree crotches.

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Symptoms of powdery mildew on a cluster of 'Bing' cherries. Note light dusting of spores on fruit surface

Closeup of symptoms of fruit infection. Note fungal growth on fruit surface and scarring.

Ascospores (released from overwintered cleistothecium of P. clandestina) on the surface of a cherry leaf. Ascospores are the primary inocula, i.e. the spores that originate from overwintering structures (cleistothecia) and initiate epidemics in spring

Numerous cleistothecia of Podosphaera clandestina on the surface of severely infested leaves of 'Montmorency' tart cherry.

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Epidemiology and Forecasting.The causal organism overwinters as cleistothecia on the orchard floor, cleistothecia trapped detritus in tree crotches, or as cleistothecia trapped in bark fissures. Ascospore release from cleistothecia requires free water and can occur at wetness durations as short as one hour. However, appreciable spore releases require at least 4 hours of continuous wetting. The optimum temperature for ascospore release is about 15 C (62 F). The latent period is 5-7 days at 20 C (70 F). Primary mildew colonies are first encountered on leaves of trunk sprouts, or on leaves originating from and positioned close to the main scaffold branches, or positioned immediately above tree crotches. The mildew colonies evident on infected foliage produce millions of a second spore type known as conidia. Secondary foliar infections progress throughout the fruit development period and after harvest. The foliar phase of the disease may become so severe that nearly all terminal shoots may be infected several weeks after harvest. Conidial germination on fruit and foliage is favored by high relative humidity, but can occur in the presence of free water. The optimum temperature for germination on foliage is 20 C (70 F). Fruit infections appear when rains occur near harvest. It is unclear whether the resultant moisture promotes fruit infection or profuse sporulation of preexisting colonies. Conidial germination is highest on green fruit and decreases in response to increasing fruit sugar concentrations. Germination is negligible at fruit sugar concentrations of 15% or higher. In Washington, conidia are present in the orchard air from early May until early- to mid-autumn with concentrations peaking immediately after harvest. Conidial dispersal is diurnal with concentrations peaking in early- to late- afternoon. Concentrations in the orchard air are positively correlated with temperature and wind speed, and negatively correlated with relative humidity and leaf wetness.

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Management. (also available in mp3 audio and Quicktime or mp4 video formats ). Successful, sustained powdery mildew management is a season-long proposition. Early season disease control is essential for successful season-long disease management. There is no totally effective prevention or cure, but a number of precautions can be taken to reduce disease pressure in order to avoid destructive epidemics. Prune trees to promote good air circulation and light penetration. Remove sucker shoots as early in the growing season as possible. In orchards where undertree impact sprinklers are used, delay the initial irrigation set as long as possible. Research at Washington State University indicates that following winters with normal precipitation the initial irrigation set can be delayed several weeks without damaging fruit or trees. Keep water from sprinkler systems off of developing fruit.

An effective fungicide program is usually the most important factor contributing to successful disease management. The grower must strive for good spray coverage using efficacioius materials at appropriate times and spray intervals during fruit development. Numerous fungicides are now available for the control of cherry powdery mildew (Table 1). There are several spray approaches that can be utilized to promote good disease management. Growers have traditionally commenced spraying for powdery mildew at shuck fall because this is the first time during the growing season that developing fruit are exposed to the inoculum potentially present in the orchard air. This approach makes practical sense. Fungicide applications made after the initial application are typically made at 7- to 14- day intervals depending upon the fungicide. The shorter intervals are necessary with less efficacious compounds such as the carbonates and flowable sulfurs. Spray intervals can be extended when using DMI, strobilurin, oil, and/or quinoline fungicides, but under high disease pressures even these compounds must be applied at shorter intervals (eg. 10 days).

With the exception of the petroleum spray oils, the more efficacious powdery mildew fungicides are prone to the development of mildew strains resistant to them. Resistance management guidelines should be followed in order to delay the development of fungus strains resistant to these fungicides. Good resistance management includes the inclusion of several fungicide classes in management programs, limiting the the number of applications of any single mode of action to no more than 3 per season, using resistance-prone compounds protectively (rather than as eradicants), and including compounds with lower resistance risk in the fungicide rotation. Sample fungicide programs that conform to these guidelines are presented in Table 2. Note that there are many iterations of combinations of these compounds that will provide adequate and sustainable mildew control.

Cornell plant pathologists recently reported an almost complete collapse of the strobilurin fungicides (e.g. Abound, Sovran, Flint) in selected New York vineyards during the 2003 growing season. Unfortunately for some New York grape growers, they have resistance to both DMI and strobilurin fungicides, leaving them very few synthetic options for powdery mildew management in a region known for its mildew pressure. The disturbing aspect of strobilurin resistance was that it developed much more rapidly than expected and in a fashion different from DMI fungicides. Whereas resistance to DMI compounds has historically manifested itself as a gradual loss of control, resistance to the strobilurins occurred as a sudden and complete control failure. Resistance developed in vineyards where 15-20 strobilurin sprays (since the introduction of the compounds) had been made. It is important to note that when resistance (either strobilurin or DMI) first occurred in other regions, it was dependent on the fungicide use history and pattern in particular vineyards and not region-wide. In other words, in some New York vineyards the strobilurins still perform superbly. Fortunately, strobilurin resistance does not appear to have developed in Washington vineyards and orchards yet. To avoid it, growers should consider the vineyard or orchard spray history and rotate out of the strobilurin class if the total number of applications is approaching 15. If the grower has been regularly using Flint (trifloxystrobin), Sovran (kresoxim-methyl; grapes only), Cabrio (pyraclostrobin; cherries only) or Abound (azoxystrobin) they should determine how many applications have been made during recent years. If that number is 15 or greater, it is prudent to rotate out of the strobilurin fungicide class. If the number is less, the grower should be actively using other fungicide modes of action in the spray rotation. The inclusion of a second mode-of-action (boscalid) to the strobilurin pyrasclostrobin to create the new fungicide Pristine should probably make lower the resistance risk of the compound.

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General Resistance Management Guidelines (for DMI, strobilurin, and quinoline fungicides)
1. Do not apply more than two consecutive sprays of a DMI (Rally, Rubigan, Orbit, Elite, Procure) fungicide.
2. Do not apply more than three sprays of a DMI fungicide per season.
3. Do not use DMI’s curatively.
4. Do not apply more than three sprays of a strobilurin fungicide(Cabrio, Pristine, Flint, Abound) per season.
5. If two or three consecutive applications of strobilurins are used then, they must be followed by at
least the same number of applications of fungicide(s) from different groups before a strobilurin
can be used again, either in the current or following season.
6. Do not apply more than two consecutive sprays of a quinoline (Quintec) fungicide.
7. Do not apply more than three quinoline sprays per year.
8. If affordable, tank mix resistance-prone compounds with other chemistries (oils, carbonates, or sulfur)
9. Use resistance-prone compounds PROTECTIVELY. Eradicative situations are best handled by using oils (early season) or carbonates.
An oil-based fungicide program has been used successfully in the region. This approach consists of 2-3 early petroleum oil (eg. Stylet Oil) applications followed by a suitable rotation of DMI, strobilurin, and/or quinoline fungicides. Because of the potential for damage to fruit, oil should be used no later than pit hardening. This approach usually results in excellent disease control at a fraction of the cost of programs that rely on more intense usage of DMI fungicides.

There are two chemical techniques that can be used after harvest to reduce the amount of potential overwintering inoculum that in turn will delay epidemic onset and/or lower disease pressure in the subsequent growing season. The first method involves the use of a petroleum oil or calcium polysulfide (Sulforix) applied soon (within 14 days) after harvest. The second involves the autumn application of liquid lime sulfur. The latter is best applied just before leaf fall. Allow for thorough wetting of foliage and tree trunks.

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Table 1. Trade and common names, mildew efficacy, resistance risk, and beneficial insect
characteristics of fungicides registered for use on Washington cherries

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Table 2. Sample fungicide programs that conform to APS-FRAC fungicide
resistance management guidelines.

Online References

Teviotdale,B.L., and Gubler, W.D. 1997. Cherry Powdery Mildew. UC Pest Management Guidelines, University of California Statewide Integrated Pest Management Project.

Travis, J.W., Rytter, J.L., Yoder, K.S., and Biggs, A.R. 1997. Powdery Mildew, Podosphaera clandestina. West Virginia University Kearneysville Web Site.

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