Apple scab – additional information
Apple scab is the most economically important disease of apples worldwide.
- All parts of the tree are affected and crop losses can be severe (70% or more) when weather conditions are favourable in the early part of the season.
- Losses can result directly from fruit infection or indirectly by the effect of repeated defoliation of the tree over several seasons on subsequent growth and yield.
- Several factors influence the scab attack including varietal susceptibility, local topography, overwintering inoculum and weather conditions.
- The fungus V. inaequalis only attacks members of the Malus family.
- It also occurs on ornamental and wild crab apples, hawthorn (Crataegus spp.), Pyrus, Sorbus, Pyracantha, Cotoneaster, Viburnum, Sarcocephalus esculentus and loquat (Eriobotrya japonica).
Apple scab is widespread and common in all UK apple orchards, and occurs worldwide wherever apples are grown. It is less common in semi-arid regions in North America, Australia, New Zealand and South Africa.
- It is usually mainly a serious problem in temperate regions with cool moist springs and in the semi-arid regions probably goes undetected in most seasons.
Apple varieties differ in susceptibility to scab. Resistance to V. inaequalis is based mainly on Vf gene (major gene resistance). Varieties with polygenic resistance also are grown commercially.
- Most of the main culinary and dessert varieties grown in UK are susceptible to scab.
- Gala is very susceptible, Cox, Bramley, Jonagold, moderately susceptible; Discovery and Grenadier have a high degree of resistance (polygenic resistance).
- There are many varieties now available resistant to apple scab, based on Vf resistance, e.g. Saturn, Ecolette, Ahra, Topaz but few of these are commercially acceptable alternatives in the UK to the currently grown susceptible varieties.
The scab fungus overwinters in several ways:
- As the sexual state (pseudothecia) on overwintering leaves on the orchard floor or on trees as mycelium
- On wood scab lesions on shoots
- On bud scales
- On leaves remaining on shoots
- On unsealed buds or shoot tips
- As conidia in buds
The relevant importance of these overwintering states depends on the variety, the season and the amount of scab.
Scab overwintering on leaves on the orchard floor is by far the most important form of overwintering and, in cold winters, is probably the only form of overwintering. In milder winters, scab overwintering on the tree as mycelium can also be important. The amount of inoculum arising from overwintered leaves depends on various factors:
- Time of leaf fall
- Leaf decomposition
- Autumn and winter weather.
In the UK, Cox leaf fall occurs fairly rapidly in autumn, whereas Bramley, Gala and Jonagold leaves may still remain on trees in mid-late December. In addition Cox, Fiesta and Gala leaves rot down and are removed by earthworms fairly rapidly, whereas the tougher, larger leaves of Bramley and Jonagold rot more slowly and appear less palatable to earthworms.
- The later leaves fall, the greater the amount of scab fruiting bodies (pseudothecia). Formation of pseudothecia only occurs on leaves once they have fallen.
- Once leaves have fallen pseudothecia initials form within four weeks after leaf fall.
- Then following a distinct rest or dormant period (if temperatures are above OoC) the pseudothecia continue to mature.
- Moisture is needed for pseudothecia development.
- Optimum temperatures for pseudothecia development are 8-12oC.
- Optimum temperatures for ascospore development are 16-18oC.
- The factors that affect development of ascospores are similar to those that influence bud development on the apple trees, such that at bud burst there are usually mature ascospores ready to infect if weather conditions permit.
In spring when overwintered leaves on the orchard floor become wet, ascospores are forcibly released and disseminated by wind to initiate the primary infections on new growth. In areas where the inoculum source at the start of the season is almost exclusively ascospores from overwintered leaves it is possible to rationalise the early season fungicide programme based on an assessment of the likely ascospores available for infection or potential ascospore dose.
- Ascospores continue to mature and are discharged over a period of 5-9 weeks or more.
- The peak period of ascospore discharge usually occurs between pink bud and full bloom growth stages.
- Ascospores are responsible for long distance scab spread to around 100 metres from the inoculum source.
Scab overwintering as mycelium on the tree produce asexual spores (conidia) in spring which are splash-dispersed short distances (localised spread) and infect newly developing leaves on the tree.
- Wood scab pustules can continue to produce conidia throughout the season.
Scab spores germinate when they land on susceptible leaf or fruit surfaces in moisture.
- Free moisture is essential for initiating germination, but once initiated, germination will proceed as long as relative humidity is greater than 95%.
- The time required for leaf infection is dependent on hours of leaf wetness and temperature.
- The duration of the wet period required for fruit infection increases with fruit age.
- Infection takes place from 1-26oC. Infection is rare above 26oC.
- After germination the fungus penetrates the cuticle and establishes itself between the cuticle and epidermis.
- Eventually conidiophores and conidia are produced. Lesions with conidia become visible 9-17 days after infection depending on temperature.
- Conidia produced are dispersed by rain splash and wind within the tree to infect leaves and fruit.
- Several secondary cycles of scab infection may occur through the season depending on host susceptibility and scab periods.
- Late scab on mature leaves in late summer and autumn contributes most to the scab overwintering on fallen leaves.
- Young shoot tissue becomes infected in summer, visible as small blisters on the shoots. These will mature and rupture to release conidia the following spring.
Approximate wetting period required for primary apple scab infection at different air temperatures and time required for development of conidiaa
Wetting period (hr)b
|Average temperature(oC)||Light infection||Moderate infection||Heavy infection||Incubation Periodc (days)|
aAdapted from Mills, 1994, as modified by A.L. Jones
bThe infection period is considered to start at the beginning of the rain
cApproximate number of days required for conidial development after the start of the infection period (i.e. appearance of scab lesions)
Apple scab infects most parts of the tree including leaves, petioles, blossoms, sepals, fruits, pedicels, shoots, bud scales. Symptoms are most easily observed on leaves and fruit.
Late scab on leaf underside
- As leaves first emerge in spring, the lower surface is first exposed and scab lesions therefore are first found on the leaf underside.
- Later when leaves unfold both surfaces are exposed and lesions appear on both sides.
- Young lesions are velvety brown to olive green with feathery indistinct margins which become more diffuse with age. Infected leaves may become distorted.
- Leaves with numerous scab lesions may shrivel and fall prematurely.
- Leaves are only susceptible when young, however, once the leaves become old and the cuticle cracks, they again become susceptible to scab.
- This usually occurs in late summer/autumn and the scab that develops is most visible on the underside as diffuse olive green almost black lesions (late scab).
- It is this scab that contributes to the overwintering stage.
- The green tissue that first appears at bud burst later becomes the fruit sepals. Scab infection on these appears similar to that on leaves, eventually becoming shrivelled.
- Once the sepals are infected scab can readily infect fruit and scab lesions located around the fruit calyx usually indicate infection of the sepals at bud burst.
- Lesions on young fruit are similar to those on leaves, but as the fruit enlarge, the lesions become brown and corky.
- Early fruit infection can result in distorted, cracked fruits and premature fruit drop.
- Fruits are most susceptible to infection when young and susceptibility declines with age.
- Fruit infections that occur near harvest may not be immediately visible but develop in store as pin-prick or storage scab.
- These appear more as black, circular lesions ranging from 0.1‑4 mm diameter.
- They are very easy to see on varieties such as Bramley, Gala or Fiesta.
Wood scab and bud scale scab
Wood scab lesions
- Wood scab and bud scale scab are similar and appear as raised blisters which eventually rupture exposing olive-green fungal growth.
- Usually only scab lesions on one-year-old wood produce viable conidia.
Other problems that may be confused with scab
- Scab lesions on leaves, wood and fruit are usually easily distinguished as scab.
- Wood scab may sometimes be confused with Nectria canker. Wood scab is usually more superficial and associated with olive-green fungal growth.
- Late scab on leaves may be confused with sooty mould growth. The latter can easily be scraped off.
Scab monitoring and forecasting is an essential part of integrated disease management to rationalise fungicide use.
- However, it is not possible in the early part of the season to base decisions on fungicide use on assessment of visible scab because of the long time (up to three weeks at low temperatures) between infection and visible scab.
- Decisions on fungicide use are usually based on weather-related risks.
- Searches for and assessment of scab is still important, to check on inoculum levels at the start of the season and during the season; and to gauge the success of control measures such that any modification to spray decisions can be timely.
- Information on inoculum levels, classified as low, moderate, high is also an essential input for using ADEM.
- Visual monitoring of scab to meet the above criteria therefore should take place throughout the season.
Visual scab monitoring (20-50 trees/orchard)
|Time/growth stage(sampling unit)||Scab item||Threshold||Action|
|Dormant periodwhole tree
whole orchard (Feb)
overwintering leaf litter
|remove during pruning
macerate to encourage breakdown prior to bud burst
|Green cluster – pink budwhole tree
Early blossom – petal fall
10 rosettes on 4 branches per tree
|scab on rosette leaves
% infected trees
scab on rosette leaves, flowers, fruitlets
% infected trees
scab on leaves, flowers, fruitlets
% infected rosettes
presence ) ≤ 5 = low
}5-20 = moderate
} >20 = high
≤ 2.5 = low
2.5-9 = moderate
>9 = high
|Petal fall – harvest(every 2 weeks)
extension shoots 4 per tree
|scab on leaves/shoots
% infected shoots
|≤ 2.5 = low
2.5-9 = moderate
>9 = high
|Autumn – post harvest – before leaf fall10-20 leaves/tree||late scab on leaves
% scabbed leaves
≤ 3 = low
>3 = high
macerate leaves, 5% urea before leaf fall
To better target fungicide sprays to control scab, the concept of curative spraying was developed, based on the use of curative fungicides in relation to scab infection periods.
(1) Scab infection model – Mills
The first scab infection prediction schemes were based on the Mills infection criteria of temperature and hours of leaf wetness.
- These scab infection criteria have been studied by various researchers and modification applied according to local conditions, e.g., Mills/MacHardy model.
- In many countries, warning services have been established to monitor infection periods and advise growers on the need to spray.
- These are usually all based on the Mills or Mills/MacHardy scab infection models.
- Similarly, software provided by companies producing automatic weather stations for use in apple orchards, e.g., Metos, are also all based on Mills apple scab infection model.
- Warnings given by Mills models are based only on weather.
(2) Scab infection model – ADEM
ADEM is a computer program developed by East Malling Research which gives warnings of several diseases.
- The program contains models of apple scab on leaves and fruits, powdery mildew, Nectria canker and fruit rot and fireblight.
- The disease models are driven by the following weather variables: rainfall, surface wetness duration, ambient temperature and ambient relative humidity.
- These are recorded on a logger and downloaded to the PC.
- The leaf scab and fruit scab models in ADEM alert users to leaf scab infection periods and forecast the risk of scab on the leaves and fruits of named apple varieties in named blocks on named farms.
- In orchard tests over several years, the leaf scab model in ADEM has been more accurate than a scab warning system based on the Mills table for detecting leaf scab infection periods, particularly in the early part of the season.
- This more accurate performance is most likely because the leaf infection models in ADEM is built from biological knowledge discovered after publication of the Mills model.
- When the model is run for a period of time, ADEM first scans the weather data for that period downloaded to the PC from the data logger and identifies potential leaf scab infection periods based purely on weather.
- Then for each potential infection period ADEM next forecasts leaf and/or fruit scab for each apple variety in the orchard taking into account the effects of varietal susceptibility and fruit age (days from bloom) and scab inoculum level present.
- Spray decisions are then based on this information provided.
- Therefore, ADEM requires inputs of estimates of scab inoculum level for the orchard block.
- Assessments are made at frequent intervals during the season – Ascospore potential in autumn, early season scab on trees, leaf scab on rosette leaf clusters and leaf scab on extension shoot leaves.
- Scab incidence at these timings is assessed and input to ADEM as low, moderate or high.
Making use of scab warnings and monitoring
Apple scab is a serious disease of apples and failure to achieve control results in economic loss.
- Growers aim for zero fruit scab at harvest and levels below 1% fruit scab at harvest are acceptable.
- The most reliable and easiest to manage practical method to achieve 0-1% fruit scab is generally a routine fungicide programme.
- Scab warnings can improve scab control, reduce costs and fungicide inputs all of which are of increasing importance in modern fruit production.
- Usually apple scab warning systems are associated with curative spraying where fungicides with curative action are applied soon after infection periods are detected.
- Such an approach can achieve substantial reductions in fungicide inputs. However, in the UK, curative spraying is also the most risky strategy to adopt.
- Scab warnings have to be reliable, and the grower must have the capacity to respond rapidly to warnings.
Four control strategies can be considered:
- Curative treatments targeted according to scab forecasts.
Mixed keystage – curative strategy
- Routine treatments applied at key growth stages i.e. bud burst and petal fall
- Scab control (fungicide choice, dose) at other times is based on scab risk generated by ADEM (i.e., curative spraying) but taking into account other factors such as the need for pest or nutrient sprays, other diseases such as powdery mildew, and practical considerations such as weekends, holidays, etc. when there may not be personnel available to apply sprays.
Mixed scheduled curative strategy
- A routine protectant schedule is followed but occasional curative treatments are targeted to specific orchard blocks according to scab risks.
Flexible scheduled strategy
- A routine protectant schedule but the choice of chemical and dose, volume and spray interval is adjusted in response to scab warnings.
In trials over several years, the most practical approach has been the key stage – curative strategy.
- This system achieves similar or better scab control compared to the routine system at lower fungicide inputs.
- It can also be integrated with pest and nutrient sprays into the orchard protection programme.
While the above strategies can be applied to both Mills, Modified Mills systems and ADEM warnings, in practice it is recommended that ADEM warnings be used as they have been shown to be more accurate than the Mills-based warnings.
- Scab warnings are based on identifying weather suitable for scab infection either by ascospores or conidia.
Further refinements to spray decisions can be made where additional information on ascospore release using simulation models (e.g., RIMpro, which also includes Mills scab warnings) can be included.
- Studies have shown that not all scab infection periods correspond with large ascospore releases likely to result in scab infection.
- Therefore, if scab infection warnings are combined with ascospore simulation model information on spore release then theoretically it may be possible to ignore some scab infection warnings, because the predicted ascospore release is too low to merit treatment.
In practice such an approach may be very risky in the UK because:
- It is too complicated for practical use. Many growers are concerned about adopting scab warning systems into their scab management regime;
- It assumes that ascospores are the only inoculum source in spring which, from recent UK experience, may not be so;
- Ascospore simulation models have not always proved accurate compared to actual ascospore trap data, particularly where spring weather patterns have been unusual.
Likewise, basing spray decisions only on ascospore release models ignores conidial scab sources on the tree.
- It is therefore recommended that spray decisions be based on scab infection warnings such as those generated by ADEM or Mills-based systems.
- Ascospore monitoring may be useful in determining the end of the ascospore season.
In areas where the inoculum source at the start of the season is almost exclusively ascospores from overwintered leaves it is possible to rationalise the early season fungicide programme based on an assessment of the likely ascospores available for infection or potential ascospore dose (PAD).
- Where PAD is low, then the early season sprays can be delayed until an action threshold is reached based on the growth stage, the PAD for the orchard and scab infection periods.
- The PAD for an orchard is calculated from PAD = LD x PD x AD x LLD x n where LD = scab lesions per square metre of leaf tissue at leaf fall; PD = pseudothecia density or mature ascocarps per visible lesion; AD = asci per ascocarp; LLD = leaf litter density prior to bud burst; and n = number of ascospores per ascus.
- In practice only two measurements are made per orchard; scab incidence and severity are assessed just before leaf fall by recording the number of scab lesions on a random sample of 200 shoots per orchard; and leaf litter density assessed at bud break.
- The leaf litter density is assessed by selecting four trees in the orchard at random and running a measuring tape diagonally across the orchard from each tree and recording the presence or absence of leaves under the tape at 30 cm intervals.
- The LLD is then calculated from the percentage of points under which leaves were found.
- The information on scab incidence and leaf litter density is then input into a computer model which calculates the PAD for the orchard.
- This system is being developed for commercial use in the USA.
- In the UK, while the information collected could be used similarly, it assumes that the early season scab infection only arises from ascospores, which may not be true for UK orchards, particularly following wet autumns and mild winters.
- Therefore, basing decisions on early season sprays purely on PAD could result in a significant scab level on some trees in the orchard.
- However, at present PAD gives the best forecast of ascospore production in an orchard.
Predicting ascospore maturity and discharge
Ascospores are usually the main means by which the scab fungus can infect new growth in the spring.
- Therefore, the ability to predict when ascospores are mature and able to infect is an important part of scab management.
- In addition, prediction of ascospore discharge, the proportion of the season’s ascospores that are mature on a given date, and the date from which ascospore discharge is almost complete are also useful for managing apple scab in an orchard.
- Such information can be obtained manually based on using a simple spore trap placed over overwintered scab leaves and monitoring ascospore discharge.
- Such monitoring is usually not suitable for individual growers but can be provided centrally by consultants or research stations.
- In other countries, e.g., Belgium, such information is provided by research stations.
A dot-ELISA diagnostic kit has been developed in the USA that combines a spore trap with an antibody-based procedure that stains the mucilage (slime) that surrounds ascospores of scab.
- The kit has potential for use by growers or consultants because the staining procedure requires little time (30 mins), is easy to perform, and the red-stained dots are countable using a hand lens (30x).
- The kit could be used to determine the start of ascospore release and to monitor ascospores through the season.
Alternatively, the information can be obtained from computer simulation models developed from data on temperature and rainfall and ascospore maturation and discharge collected over many seasons.
- Several such models have been developed in the USA, Canada, South Africa and various parts of Europe.
- RIMpro is the model most commonly used in the UK.
- Such models are useful but do not always give accurate forecasts especially when rain follows long dry periods which result often in massive ascospore release.
- Prune trees to allow good air circulation and rapid drying of leaves to reduce scab risk.
- Remove wood scab during winter pruning.
- After harvest and before leaf fall, apply a spray of 5% urea to the trees. Urea encourages microflora that speed up leaf breakdown and increase the palatability of leaves to earthworms, which are mainly responsible for leaf litter disposal in orchards.
- Urea also directly affects the scab fungus by preventing formation of the sexual state.
- Earthworms remove leaf litter better from bare soil than from grass. Therefore, keep the grass well mown to encourage earthworm activity.
- Mow leaves in autumn to macerate for more rapid leaf decomposition. Maceration of leaves can decrease scab inoculum by up to 90%.
- Even where leaves are trapped in hedges maceration can reduce inoculum by 50-60%.
- Brushing machines can be used to sweep leaf litter from under trees into alley ways for maceration.
- Where leaf litter still remains in the spring or weather conditions have made action in the autumn impossible, mow to macerate leaves in spring.
- A spray of 5% urea to leaf litter in spring will also encourage leaf rotting and prevent ascospore release.
Apple scab control is based on use of fungicides in an integrated programme from bud burst to the end of scab risk.
Classification of fungicides
Fungicides recommended for control of apple scab are classified as:
- Chemical activity that prevents infection.
- Fungicide has to be present on the leaf surface before the spore lands.
- Examples are sulphur, copper fungicide, captan, mancozeb.
Curatives or fungicides with kick-back action
These include fungicides with:
- Post-infection action – fungicide activity that stops further development of the fungus after infection has been initiated, thereby preventing symptom development. After infection, activity is usually effective when applied within a few hours to 2-4 days after infection is initiated. Examples are dodine, DMI (triazoles) fungicides.
- Pre-symptom action – chemical action that effectively suppresses the fungus after infection has progressed for several days to just prior to appearance of symptoms. Lesions may develop but may be chlorotic or non-sporing. Examples are dodine, DMI (triazoles) fungicides.
- Post symptom action or chemical action that stops scab activity in scab lesions – burns out scab lesions. Examples are DMI (triazoles).
Post-infection action ) = curative or kick-back action
Pre-symptom action )
Post-symptom action = suppressing/preventing conidial production
Elimination of overwintering = eradication
- The DMI (triazoles) fungicides are usually considered to have the greatest kick-back action of 4 days.
- Dodine and dithianon and pyrimethanil also have kick-back actions.
- Protectant action varies and the DMI are usually considered to have limited protectant action – around 6 days depending on product, e.g. on the fungicide label for a DMI the 10 day programme generally refers to 6 days forward protection and 4 days kick-back.
- The protectant action of DMI fungicides on fruit is also poor and mixing with a suitable protectant such as captan, dithianon or mancozeb is usually recommended from pink bud or blossom.
Factors affecting fungicide choice
- Effective control of scab.
Mode of action
- Protectant, curative or conidia suppression depending on needs.
- Ensuring fungicides with differing modes of action are used to reduce the risk of resistance.
- Apple varieties differ in sensitivity to fungicides, e.g. Captan at the full label dose may cause leaf spotting on certain varieties including Bramley, Cameo, Spartan, Red Delicious.
Other diseases/pests controlled
- At bud burst, dithianon or dodine may be used because of protectant action against Neonectria canker.
- Similarly, at green cluster/pink bud a DMI fungicide is usually selected because of its dual action against scab and powdery mildew.
Safety to insects parasite/predators
- Integrated pest management in orchards is based on Typhlodromus pyri (typhs) to regulate populations of fruit tree red spider mite and rust mite. Certain fungicides e.g. mancozeb and sulphur, are harmful to typhs and repeated use can result in reduction of typhs and resurgence of the mite problem.
- Use of these fungicides may be an essential part of the scab management programme, but by restricting the number of successive treatments, such products can usually be safely used in the programme.
- More information on the safety of fungicides to beneficial insects can be found in the fungicide tables.
- Fungicides may be selected over others on the basis of price.
- Fungicide efficacy should not be sacrificed for savings in costs.
- This may prove to be a false economy.
Fungicides to control overwintering scab
- A pre-bud burst application of a copper fungicide, e.g. Cuprokylt (copper oxychloride) may give some control of scab inoculum overwintering on the tree.
- Research has shown that post-harvest application of a DMI fungicide e.g. myclobutanil (Systhane), resulted in the production of fewer fruiting bodies (pseudothecia) in overwintered leaves and fewer ascospores in developing pseudothecia.
- The use of such fungicide products post-harvest at present may not be a sensible option because of the risk of fungicide resistance.
- However, if strategies for control of apple diseases were adopted where fungicide use was restricted in the post-blossom period, then use of a DMI fungicide post-harvest may be more acceptable.
- At present the use of 5% urea spray post-harvest is the preferred option to using DMIs only.
- Five percent urea sprays similarly interfere with formation of the scab sexual state and also encourage leaf rotting.
- A combination of a DMI fungicide followed by 5% urea may be more effective.
Typical fungicide spray programme for scab control
- Fungicide choice, dose and spray interval will alter according to the management strategy employed.
- A routine programme would closely follow the defined schedule.
- A managed programme would adjust product choice, spray interval and fungicide dose according to the scab risk identified.
Typical fungicide spray programme for apple scab control on Cox
|Timing||Target diseases||Fungicide product||Comments|
|Pre-bud burst||overwintering scab on tree/canker||copper fungicide||e.g., Cuprokylt
|Bud burst||scab/canker||dodine or dithianon||If canker is not a problem pyrimethanil is an alternative|
|Mouse ear||scab/canker||dodine or dithianon|
|Green cluster||scab/mildew||myclobutanil + captan||Other DMI, e.g., penconazole + captan is an alternative|
|Pink bud||scab/mildew||myclobutanil + captan|
|Blossom||scab/mildew||myclobutanil + captan||Where mildew incidence is low then kresoxim-methyl (Stroby) is an alternative|
repeat 10 day intervals to late May/mid June
|scab/mildew||myclobutanil + captan|
|Fruitlet (mid June)||mildew||bupirimate or
boscalid + pyraclostrobin (if bupirimate not registered on apple)
|Include scab protectant fungicide only if scab present|
continued at 10 day intervals to August
|bupirimate or penconazole or sulphur (if bupirimate not registered on apple)||Include scab fungicide only if scab risk.NB Do not use triazoles on Cox after mid June if risk of Diffuse Browning Disorder|
|August||mildew/storage rots||boscalid + pyraclostrobin|
|Post-harvest||scab||urea||To encourage leaf rotting and elimination of overwintering scab|
Avoiding fungicide resistance
Resistance of apple scab to dodine and benzimidazole fungicides developed in commercial orchards in most apple growing areas in the 1970s.
- More recently failure to control scab by DMIs has been reported to be due to reduced sensitivity of scab isolates to DMI fungicides.
- In the UK both dodine and DMI fungicides remain effective in control of scab provided they are used as part of a mixed fungicide programme and are not used exclusively for scab control.
- Resistance to kresoxim-methyl has been reported in parts of Europe and Israel, although not so far in the UK.
- Isolates with reduced sensitivity to pyrimethanil have been detected in the UK.
In the UK most examples of poor scab control can be explained by poor spray timing, starting the spray programme too late, or gaps in the programme due to weather, rather than attributed directly to fungicide resistance.
- Fungicide programmes for scab control must be based on several fungicides with different modes of action to reduce the risk of fungicide resistance.
- Where a new orchard is planned, consideration should be given to choice of variety and if possible selecting a scab resistant or tolerant variety, or if this is not possible, then avoiding very susceptible varieties such as Gala.
- Select an orchard site with good air circulation and consider planting trees at wider spacing to encourage good air circulation so that trees dry more rapidly after rain.
- Similarly, consider tree pruning and training and possibly selecting tree shapes which promote good air circulation.
- Where existing orchards are converted to organic production, then careful consideration should be given to pruning and tree training to promote good air circulation to promote rapid drying of leaf and fruit surfaces and reduce the apple scab risk.
- Emphasis must also be placed on cultural approaches to control. This is mainly concerned with elimination of overwintering inoculum such as elimination of leaf litter and wood scab etc. as outlined previously.
- The use of urea is not permitted in organic production.
- Therefore, the emphasis must be on physical removal of leaves or on maceration of leaf litter to encourage rotting.
- The use of biocontrol fungi (A. bombacina and C. globosum), if they become available, would also be appropriate.
Fungicides acceptable in organic production may also be used in conjunction with the above.
- Such fungicides should be a last resort rather than the only method of control used.
- Currently in the UK, the use of sulphur fungicides is permitted for scab control. Sulphur is a protectant fungicide, only partially effective against scab, and therefore requires frequent applications to achieve control.
- Efficacy can be improved if applications are carefully timed with scab warnings.
- However, repeated applications of sulphur may initially interfere with management of phytophagous mites with T. pyri.
- The use of copper fungicides e.g., Cuprokylt (copper oxychloride) is at present permitted with restrictions in organic production, but may eventually be banned.
- Copper fungicide may be used prior to bud burst to control scab overwintering on the tree.
- Copper fungicides may be phytotoxic to young foliage and so should be used with caution.
In other countries, various ‘natural products’, either plant extracts or natural chemicals such as calcium hydroxide or kaolin (china clay) are reported to be effective alternatives for scab control.
- These products either act directly on the scab fungus in a similar way to conventional fungicides, or act on the plant to increase resistance to scab.
- Usually repeated applications are required to control scab.
- The scab control achieved has not always been successful.
Two fungi potentially useful as biological control agents are Chaetomium globosum and Athelia bombacina.
- C. globosum applied during the secondary scab season could be beneficial through its effectiveness in reducing the size and number of lesions, conidial density and spore germination.
- A. bombacina applied just before leaf fall could be beneficial through its effectivness in suppressing the production of ascospores and, to a lesser extent, to soften leaves to encourage rotting and earthworms.
- Cladosporum sp. may also be useful post-harvest to suppress the scab sexual state.
Biocontrol measures generally only reduce scab, rather than control it and are therefore probably not effective enough for use during the growing season.
- There may, however, be more scope for use post-harvest with urea to encourage leaf rotting and ascospore suppression.
- Although much research has been done on biocontrol of scab, at present there are no commercial products available for use.
MacHardy, W E (1996). Apple scab. Biology, Epidemiology and Management. APS, Minnesota, USA.