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Apple Scab Management in British ColumbiaApril 2006Table of Contents
Apple scab, caused by the fungus Venturia inaequalis is a serious disease of apple in British Columbia.
It is an annual threat in the wetter interior fruit growing districts (Creston and Kelowna
to Salmon Arm) and is even more severe in the coastal areas of B.C. and the Fraser Valley. Apple scab has also caused severe losses in the drier areas of the Okanagan and Similkameen Valleys in years with higher than normal rainfall.
Fruit lesions are circular and brown to black in colour. The skin ruptures around the lesions revealing a dark velvety layer of spores. Older lesions become corky in appearance. The fruit is most susceptible when young. Early infections often result in large fruit lesions and severely cracked and malformed fruit.
Life CyclePrimary Infection:Apple scab overwinters in infected leaves on the orchard floor. Spores (ascospores) are produced in these dead leaves just about the time that new growth appears in the spring (green tip stage). They are then discharged during rainy periods, and can be carried long distances in air currents. Ascospores lodge on new leaves, blossoms and fruit, and if the leaves or fruit are wet long enough for the spores to germinate, primary infection of fruit and foliage occurs.The primary scab season lasts until all the ascospores from last year's leaves have been released. This varies from year to year but typically ends in late June.
Secondary Infection:Once primary infections have taken place, lesions develop on the leaves and fruit which produce secondary spores (conidia). Conidia are spread short distances mainly by splashing rain, and cause additional spread and build-up of the disease. The time needed for a new infection to begin producing a new crop of spores (incubation period) varies from 9 to 17 days, depending on the temperature.Conidia require a similar or slightly longer period of foliage wetting as needed for a primary spring infection. However, unlike ascospores, they do not require an initial period of rainfall to initiate spore release. Early morning dew will therefore be more important in initiating wet periods that may lead to infection, particularly if followed by showers. Secondary scab lesions can remain active for about a month, producing large numbers of conidia. Fruit becomes more resistant to apple scab as it matures, requiring increasingly longer wet periods for infection (see Table 3). With prolonged rainy weather, fruit may be infected right up until harvest.
Monitoring Primary Infection Periods:The length of time required for infection depends upon the temperature during the time period when the leaves or fruit are wet. The Mills' chart below illustrates the relationship between leaf wetness duration and average temperature, and can be used to determine whether or not an infection period has occurred. The average temperature may be calculated by taking the average of the high and low temperatures during the wet period. When trees are dry less than 10 hours between showers, the periods the leaves are wet should be added together in calculating the infection period.
Compendium of Apple and Pear Diseases, APS Press. MacHardy and Gadoury Model for Apple ScabMost apple scab forecasting in the Okanagan has been done using a modification of the Mills' model, shown above.A new model was developed in 1989 by MacHardy & Gadoury (sometimes called the MacHardy model). This model is based on the fact that ascospore release is strongly influenced by light, and that only a small percentage of ascospores are released at night. Thus, if leaves on the orchard floor are wetted at night by rainfall, the majority of the spores will not be released until the following morning. The Mills' model assumes that ascospores are released at the beginning of rain. It could therefore overpredict the number of infection periods, assuming that at least some rainy periods every year will start at night. For example, if it started to rain at 2 a.m. and the leaves dried at 11 a.m., the leaves were wet for 9 hours, long enough for a Mills infection period. However, according to the MacHardy model, the length of the infection period was only 3 hours (8 a.m. to 11 a.m.), not long enough to cause an apple scab infection. The MacHardy model uses a different chart (see Table 2) for predicting primary infection. (Do not use the Mills chart if following the MacHardy model). Shorter time periods for primary infection are used in the MacHardy model, and these have been validated by research.
Table 2. MacHardy & Gadoury Model for Apple Scab.
Which model should be used?The modified Mills' model has been used successfully in the Okanagan for many years, and continues to be used by most field advisors. There are advantages and disadvantages to both models, and which one is used should be a matter of educated choice.The Mills' model may miss some infection periods that have occurred in shorter wet periods than the chart indicates, and it may overpredict infection periods that start at night. The MacHardy model may not account for rare, massive spore releases that occur at night. (Night-time spore release is more likely to be high following a long dry period, which allows a build-up of mature spores waiting to be released by rain.) Also, in a high inoculum orchard (lots of scab the previous year), the small percentage of spores that are released at night could be enough to cause an unacceptable level of damage. The MacHardy model is recommended in some areas for orchards that have low levels of overwintering apple scab. This may help to save one or more sprays, depending on the season and its weather patterns. If scab was not well controlled last year, it is probably safer to use the Mills model. Comparing the results of both models can be useful, particularly for borderline infection periods.
The suggested threshold used in Ontario for a low ascospore inoculum orchard is less than 0.7%
leaves with scab. This assessment should be done in the fall for the following season. Examine
the leaves of 600 shoots for scab lesions. (e.g.. 20 shoots on 30 trees or 10 shoots on 60 trees)
and record the number of leaves with one or more lesions. Then determine the average number
of leaves per shoot. If less than 0.7% of the leaves examined have scab lesions, the orchard can
be considered "low inoculum". Be aware that scab may be difficult to identify in the fall, especially
when there is leafminer injury present.
Table 3 shows the length of wet period required for secondary apple scab infection on the fruit at various intervals after full bloom. Fruit of several varieties was evaluated, and all reacted similarly.
Table 3. Hours of wetting for 2% fruit infection.
Table 4. Length of wet period required for light,
Apple Scab ControlResistant VarietiesCommercial apple cultivars vary in their susceptibility to scab infections. Cultivars such as Red Delicious, Golden Delicious, Spartan, Jonagold and Fuji are susceptible, whereas McIntosh, Empire, Mutsu and Gala are rated as very susceptible.A number of apple cultivars are available which are resistant to the apple scab fungus. However, few of these varieties are suitable for long-term storage, or for the commercial fresh fruit market at the present time. Liberty is perhaps the most widely grown scab resistant variety. The resistant variety Goldrush is considered to have commercial potential in some areas. There are many promising resistant varieties under development. Many resistant varieties are suitable for backyard gardeners, and will eliminate the need for fungicide sprays for apple scab control. Resistant varieties may also be of interest to organic growers or direct farm marketers. Most currently available resistant varieties have a single gene that governs apple scab resistance. As a result, it is not difficult for the apple scab fungus to overcome the resistance (similar to fungicide resistance). Failure of resistant varieties has already occurred in at least one B.C. orchard in the lower mainland, and at other locations in Europe and the United States.
If you are planning to use scab resistant varieties in a commercial setting, it is important to isolate the resistant trees from sources of apple scab inoculum. Constant exposure to apple scab will eventually result in a breakdown of resistance. Consideration should also be given to using fungicide sprays on resistant trees that are exposed to inoculum, at least through the peak period of scab susceptibility in the spring, or at least until scab levels have been brought under control in the orchard. Other cultural control methods, such as leaf removal or leaf shredding will also help to reduce inoculum pressure. Relying solely on apple scab resistance is not a sustainable IPM practice.
Water managementDo not run over-tree sprinklers longer than the time needed to cause an infection period. Infection can occur with as little as 6 to 9 hours of leaf wetness at optimum temperatures.The use of over-tree sprinklers can also prolong leaf wetness periods initiated by rain or dew, creating an infection period. When rain ends, shut off over-tree sprinklers until trees have been dry for at least 10 to 12 hours. Do not start the sprinklers when wet weather is predicted.
Orchard design & pruningGood air movement can reduce the number of infection periods because the foliage dries more quickly. An open canopy also improves spray coverage and light penetration. To improve air movement in the orchard, avoid overcrowding and keep trees well pruned. Reducing primary inoculumSeveral strategies will help to reduce the amount of overwintering apple scab. Primary inoculum is produced by infected leaf litter on the orchard floor. Any practices that reduce the amount of leaf litter will help to reduce the amount of inoculum.
Removing fallen leaves by raking, vacuuming, or using leaf blowers is effective but may not be practical except for small plantings. Home gardeners who remove all fallen leaves from under their apple trees should experience good control of apple scab. Leaves may be safely composted. Applying nitrogen fertilizers, such as foliar application of urea in autumn, will speed natural decomposition of leaves and help to deactivate the scab fungus. However, the rates of nitrogen required and potential adverse effects of fall nitrogen application on tree vigour and fruit quality have not been fully evaluated. In England, a spray of 5% urea to trees about one week before leaf fall was found to be effective in limiting spore development and hastening leaf decomposition. Research in the United States has also shown large reductions in spore production, often as high as 80 to 90% following application of 5% urea. Care should be taken that urea is not applied too early in the fall. A 5% urea solution may be equally effective when applied to leaves on the orchard floor after they have dropped in the fall. Spring application to the leaf litter has also proved effective in some areas, but it is less likely to have the same impact as fall application. The effectiveness of urea may vary from year to year. Fall application of dolomitic lime to leaf litter on the orchard floor has been shown to have similar effects to urea application in Oregon, however urea is considered to be more effective. Chopping or shredding the leaf litter in the spring or fall using a flail mower can also dramatically reduce the amount of scab inoculum. Chopped leaves will decompose much faster. Leaf chopping can also be done in combination with urea application.
Note that these control methods do not replace the need for fungicides in commercial orchards.
Reducing the initial inoculum will reduce the disease pressure in the orchard, making apple scab
easier to control. It will not, however, eliminate the disease, and fungicidal protection will continue to
be necessary. Low apple scab pressure is also valuable in helping to prevent or delay the
development of fungicide resistance.
Fungicide Resistance ManagementThe fungicides Nova, Nustar, Sovran, Flint, Vangard and Scala are at risk for resistance. Note, Nova and Nustar belong to the same class (sterol inhibitors), Sovran and Flint belong to the same class (strobilurins), and Vangard and Scala belong to the same class (anilinopyrimidine). To help prevent resistance from developing:
Apple scab resistance to Senator (formerly known as Easout) and Equal is widespread in the central and north Okanagan, and has been found in other interior fruit growing districts. Because of the extent of resistance, these chemicals are not recommended for scab control. Growers in areas where resistance is known to be low (e.g. south Okanagan and Similkameen), may choose to use these chemicals with caution. Discontinue use if resistance is suspected. Resistance to Senator develops more quickly and easily than resistance to Equal. Protectant SpraysProtectant sprays (see table) are applied to protect new growth during prolonged rainy periods or in anticipation of a rainy period. The materials provide a film of fungicide which prevents spores from germinating. Protectant sprays are usually applied on a calendar basis, at approximately 7 day intervals. Fungicides with eradicant properties also have protectant properties, and are generally more effective when used in protective programs.Eradicant SpraysEradicant fungicides (see table) prevent or reduce further growth of infections that have already started, if the sprays are applied soon after the start of a rainy period. The advantage of eradicant sprays is that they are applied only when necessary; i.e. when infection periods occur. In most seasons this will reduce the number of sprays required. However, growers must have spray machinery available to cover the trees quickly after an infection period, and winds must be low enough to permit good coverage. Nova, Nustar, Flint and Sovran may be applied up to 96 hours after the start of a rainy period, while Vangard and Scala must be applied within 48 hours after the start of a rainy period for eradicant activity to be effective.
Spray TimingFirst Spray: The risk of apple scab infection may begin as early as the green tip stage. If using a protectant program, and scab was a severe problem the previous year, the first spray should be applied at the green tip stage. If scab was very well controlled last year, it may be safe to delay the first spray until the tight cluster stage. Sprays may also be delayed if there is no possibility of rain in the forecast.If using an eradicant program, the first spray should be applied immediately after the first infection period. Begin monitoring infection periods at the green tip to tight cluster stage, depending on severity of scab the previous year. To successfully use an eradicant schedule, the grower must be able to determine whether or not an infection period has occurred after every rainfall. Many packinghouses issue scab warnings using code-a-phones. Alternatively, growers could install weather monitoring equipment in their own orchards, or hire orchard consultants who are monitoring infection periods. Scheduling: Scheduling: Sprays may be applied either on a protectant or an eradicant schedule. For a protectant schedule, fungicides should be applied every 7 to 14 days during the primary scab season. A 7 day interval should be used from the pink to calyx stage of tree development. Following the calyx stage, protectants such as Kumulus are best used on a 7 day interval, while Polyram, mancozeb, or captan may be used at 10 to 14 day intervals during most years. This interval should be shortened to 7 days following heavy rain. With an eradicant schedule, fungicides are applied shortly after an infection period has occurred. Often protectant and eradicant schedules are combined. For example, eradicant schedules may be backed up with protectant sprays during the critical pink to petal fall period. Refer to the Fungicide Notes section for information on individual products. A protectant schedule is highly recommended during the critical pink to petal fall period. During this time there is abundant scab inoculum, the weather is often rainy, and tree growth is rapid and highly susceptible. Protectant schedules are also recommended for orchards with high levels of overwintering apple scab inoculum. Fungicide NotesNova is an eradicant fungicide with 4 days of kick back, or post-infection activity. For bloom and post bloom sprays, tank mix the full rate of Nova with the half rate of a suitable protectant such as Dithane, Polyram or captan. Tank mixing Nova with a protectant will improve scab control on the fruit and will also help to prevent resistance problems from developing. Nova + half rate protectant is recommended for use on a 10-day protectant schedule. If the first application of Nova was applied as an eradicant (post infection), then a second application of Nova or Nova plus protectant should be applied in 7 days. This will help to ensure complete eradication, particularly under severe scab conditions. If switching from Nova to a straight protectant program (e.g. full rate of captan, Dithane or Polyram), the full rate of protectant should be applied no later than 4 to 5 days after the last Nova spray. This reduced spray interval is necessary because the half rate of protectant used with Nova may not last beyond 4 to 5 days. Control failures have occurred in the Okanagan where this spray interval was stretched beyond 5 days. Nustar belongs to the same class of fungicides as Nova, and also provides 4 days of kick-back. Nustar may be used on a protectant schedule at 7 to 10 day intervals. Use the shorter interval on susceptible varieties or when rainy weather results in frequent infection periods. Nustar should be used in a tank mix with Manzate 200 or captan, as per label rates. Use the high rate of Nustar if powdery mildew control is also needed. Do not apply more than 4 times per season or within 77 days of harvest. Sovran: Apply as a protectant beginning at ˝ inch green or when conditions become favourable for primary infection. Continue on a 10-14 day interval, using a 10 day interval during periods of rapid shoot growth. Sovran provides 96 hours of “kick-back” for apple scab control. Use the high rate if applying Sovran post-infection or under high disease pressure. Do not apply more than 4 times per season or within 30 days of harvest. Alternate with fungicides of other classes for resistance management. Caution: Sovran drift may severely injure cherries, resulting in leaf burn and defoliation. Flint belongs to the same class of fungicide as Sovran, and also provides 96 hours of “kick back” for apple scab control. Flint may be used on a protectant schedule at 7-10 day intervals. Do not apply more than 4 times per season or within 14 days of harvest. Alternate with fungicides of other classes for resistance management. Vangard is best used as a protectant fungicide, however it does have 48 hours of “kick-back” for apple scab control. Protectant sprays should be applied at 7 to 10 day intervals. Vangard is recommended at the full label rate for pre-bloom sprays. Do not apply more than 2 applications of Vangard per season, and do not apply within 72 days of harvest. Tank mixes using a reduced rate of Vangard (see label) have not been fully evaluated in B.C. Caution: Vangard drift may injure some cherry varieties Scala is best used as an early season protectant fungicide, and has good activity during cool temperatures. Apply at 7-12 day intervals, from green tip to petal drop. Do not apply within 72 days of harvest. Scala provides better scab protection on leaves than on fruit, thus it is not recommended for scab control after bloom.. Secondary Scab SeasonIn late June growers should carefully check leaves and fruit for the presence of scab lesions. If primary scab has been well controlled, such that only a few lesions can be found per tree, the interval between sprays may be lengthened for the remainder of the growing season. If scab lesions are not found, the grower could decide to eliminate fungicide applications entirely after the end of the primary season. However, this should be done with caution because it takes only a small number of lesions to produce a major epidemic later in the season if leaves and fruit are not protected, and if an extended period of wet weather occurs. If sprays are suspended, the orchard should be periodically monitored for the development of scab lesions for the rest of the season. Pin-Point Scab and Storage ScabPin-point scab is simply another name for late season apple scab fruit infection. As fruit matures, it becomes increasingly resistant to scab. However, infection can occur at any time up until harvest during prolonged wet weather (see Tables 3 and 4). Late season infections may not become visible until the apples are in storage. They appear as either pin-point sized lesions, or jet black spots. Fortunately, scab will not spread from diseased to healthy apples in storage.
Pin-point or late season scab infections can develop only when there are visible
leaf or fruit infections in the tree. For this reason, it is important that
spray programs be continued throughout the summer if scab is present in the
orchard. In the Okanagan, spray programs are often relaxed during dry summer
weather. Be aware that this may result in fruit infection if wet weather occurs
while the trees are unprotected and scab lesions are present in the orchard.
Captan and ziram are used as late season protectants. Do not use eradicant
fungicides as protectants for pin-point scab, and observe pre-harvest intervals
for all late-season sprays. There is no valley-wide program at the current time for ascospore monitoring. A limited number of locations are monitored in some years by BCMAL and Agriculture & Agri-Food Canada for research purposes. In the absence of monitoring data, it is assumed that spore release begins at green tip, peaks around blossom, and finishes by mid to late June. Monitoring to date has indicated that these guidelines are very close to reality. Action Thresholds
Recently, there have been techniques developed that can assess the "potential ascospore dose",
or PAD in an orchard. If the PAD is determined to be low enough, it is deemed safe to delay the
beginning of the spray program until the pink fruit bud stage or until after 3 infection periods,
whichever comes first. To determine PAD it is necessary to do an assessment of scab levels on
leaves in the fall. The use of PAD as a threshold has not been evaluated in B.C.
Because microclimates differ between orchards, it is always preferable to collect weather data as close to your own orchard as possible. Growers with larger acreages of apples may consider purchasing their own weather stations. There are several on the market that include software for forecasting apple scab. For more information on weather stations, contact your field advisor.
Return to Tree Fruit Pests and Diseases
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