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.
Control strategies
Four control strategies can be considered:
Curative strategy
- 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.