Apple scab and its control
Controlling apple scab is the most important activity undertaken by top fruit growers during the year. It is essential that orchards are kept clean from the start of the growing season. Given the poor financial returns from apples over the last three years, it is important to use a value for money spray programme without compromising disease control.
Leaf litter
As a consequence of recent mild winters leaves have been remaining on the trees for longer than usual, resulting in a marked reduction in leaf degradation on the orchard floor prior to bud burst. Whilst mowing will improve leaf decomposition, this will not happen quickly enough now to eliminate new scab spores being produced. Therefore if there is a lot of leaf litter present protectant fungicides should be used right from bud burst.
- Apple scab is caused by the fungus Venturia inaequalis.
- V. inaequalis over-winters in fallen leaves on the orchard floor.
- In the spring V. inaequalis in the rotting leaves releases spores known as ascospores into the air.
- These ascospores cause the initial scab infection on newly developing leaves and fruitlets.
- Ascospore release can begin as early as late March and may continue over several weeks.
- Ascospore release is affected by weather, particularly rain and rising temperatures and most ascospores are released during daylight.
- If there was a lot of scab on leaves in the orchard at the end of 2001, there is a high risk of early infections in spring 2002 and early fungicide application is essential.
- Before bud-burst, the orchard should be inspected to estimate the amount of leaf litter which has survived over the winter. Where significant leaf litter still remains it is beneficial to mow to encourage rapid breakdown. The first fungicide spray must be applied promptly at bud-burst in all orchards.
Fig 1 Life cycle of Venturia inaequalis in an apple orchard
The leaf scab lesions initiated by ascospores then start to produce a different type of spore called conidia. Conidia are spread from tree to tree in rain-splash and by wind. If the weather conditions are suitable then the fungus spreads very rapidly and there can be a dramatic increase in the incidence of scab.
When a conidium lands on a fresh leaf it germinates and penetrates the leaf causing infection. This only happens if there is moisture on the leaf surface, so in dry conditions little scab will develop. It is very important to protect freshly developing leaves from infection. V. inaequalis will also infect fruitlets and fruit at any stage of development. Infections of fruitlets are particularly damaging, leading to significant yield losses. Fruit can also be infected at any time during the growing season resulting in blemishes, which may even continue to develop during storage.
Spray Programme 2002
When planning a fungicide programme for the control of apple scab great care must be taken in selecting which to include.
- ·protectant fungicides must be applied before infection. When the weather is damp there is a high risk of scab development and fungicide should be applied to protect foliage and fruit.
- ·eradicant fungicide can be applied for some time after infection has occurred. The time delay (kick-back) varies from chemical to chemical (see Table).
- ·Bud burst to pre-blossom
- At the start of the growing season, fungicides which are protectant in their action are most useful to keep leaves clean e.g. Dithianon, alternatives are Stroby or Scala. Strobilurin fungicides such as Stroby have long protectant activity and are most effective at this stage of the programme. Due to the risk of resistance, no more than two consecutive strobilurin treatments should be applied and no more than three strobilurin treatments made in any one season.
- ·Blossom
- Avoid products containing fungicides which may damage pollen e.g. captan. DMI fungicides such as Dorado, Indar or Systhane can be used.
- ·Post-blossom
- Fungicides with eradicant activity such as DMIs should be supplemented with a protectant to reduce the risk of fruit scab e.g. Topas C or Indar + Karamate. During periods of rapid shoot, leaf and fruit development the spraying interval should be shortened. This usually occurs one to two weeks after blossom.
Types of fungicides approved for scab control
It is essential to follow the instructions on the approved label before handling, storing or using any crop protection product.
| Fungicide group/Active ingredient(s) | Product name (examples) | Duration of protectant activity (days) + | Duration of eradient activity (hours) | Resistance risk |
|---|---|---|---|---|
| DMI | ||||
| fenbuconazole | Indar | 6-10 | 96 | ++ |
| myclobutnil | Systhane | 6-9 | 96 | ++ |
| penconazole | Topas | 7-14 | 96 | ++ |
| pyrifenox | Dorado | 6-10 | 96 | ++ |
| phtalimide | ||||
| captan | Alpha Captan | 10 | 0 | - |
| guanidine | ||||
| dodine | Radspor | 10 | 36 | ++ |
| dicarbonitrile | ||||
| dithianon | Dithianon | 10 | 48 | - |
| dithiocarbamate | ||||
| mancozeb | Karamate | 10 | 0 | - |
| strobilurn | ||||
| Kresozium-methyl | Stroby | 10 | not claimed | * |
| anilinopyrimidine | ||||
| pyrimethanil | Scala | 7-10 | 48-72 | * |
+ derived from manufacturers' data
++ resistant strains have occurred in the UK and may affect performance
- resistant strains have never been reported
* resistant strains have been reported in other pathogens, but not in apple scab in the UK
** benzimidazole fungicides are a useful addition to the spring-summer programme where canker is a problem, but should not be relied on for scab control due to the risk of resistance
In general, fungicides with at least two days’ eradicant activity are more expensive than the protectants.
Where protectants are used, it is vital to apply the next spray before the cover provided by the previous one is exhausted. Where this has not been achieved, and weather conditions favour scab, a fungicide with eradicant activity must be used.
Local Environmental Risk Assessments For Pesticides (LERAPS)
Certain pesticides have a buffer zone requirement which are needed to afford protection for aquatic life against pesticide spray drift.
For those pesticides with a buffer zone requirement there is a legal obligation to carry out and record the results of a Local Risk Assessment For Pesticides (LERAPS). Different product carry different buffer zones – check the label.
The scheme has been in force for ground crop sprayers since 1999 and is now extended to broadcast air assisted (orchard) sprayers.
LERAPS are only necessary when pesticides are applied next to watercourses such as streams and ponds. You must record your LERAP
Carrying out a LERAP
Step 1
1. Decide whether it is necessary to use a pesticides – consider if there is an alternative form of control
2. Decide whether you wish to retain the buffer zone or whether to explore the potential for a reduced buffer zone.
You can simply comply with the standard buffer zone requirement.(found on the label) – remember that this decision needs to be recorded.
Step 2 – If you wish to use a reduced buffer zone
1. Check whether the sprayer you are using qualifies as a LERAP "low drift"sprayer and what star rating it has.
Manufacturers of sprayers are able to apply to have their equipment officially recognised as having "LERAP low drift" status. Spray equipment is graded by the allocation of star ratings. Standard equipment with no specific ability to prevent drift fallout will not obtain a star rating. Equipment that offers the greatest reduction in spray drift gets a 3 star rating. the star rating of the equipment should be recorded as part of the LERAP . (see tables one and two)
2. Decide whether it is appropriate to apply the product at a reduced dose.
You should always apply the lowest dose that is appropriate to control the target weed, pest or disease. If the dose you intend to use is less than the maximum permitted dose for a single application then you may be able to reduce the buffer zone. (see tables one and 2two) If you are applying more than one product in a tank mix the product with the widest buffer zone requirement determines the buffer zone of the tank mix.
3. Will you be spraying near a watercourse shielded by a living wind break
You can reduce the buffer zone if a living windbreak meets the following conditions:-
- ·it is formed from broad leaved trees or shrubs
- ·it is managed to protect the crop from the effects of wind or to minimise spray drift
- ·it is a least two metres higher than the crop to be sprayed
- ·it extends for the full length of the boundary between the treated crop and the watercourse
- ·it has no gaps over this length including those resulting from stripping of lower branches
- ·leaves are visible over its entire length.
4. Identify the appropriate table to consult
Table 1 applies if there is no windbreak., Table 2 if there is a windbreak
a. Read down the left hand column of the table to select the row that matches your sprayer type.
b. Read across the top of the table to select the column which matches the dose rate you want to apply.
c. The box at which the row and column intersect gives the amount in metres by which the buffer zone shown on the label can be reduced
d. There are minimum permitted zone distances which cannot be further reduced
e. even if a smaller buffer zone is suggested by using the tables below – the buffer zone cannot be less than the minimum of five metres if you only reduce the dose but do not use a low drift sprayer or an windbreak the minimum buffer zone will be greater then five metres.
Table 1 without windbreak
| Applied dose Sprayer type |
Full rate (75.1 - 100%) |
3/4 rate (50.1 - 75%) |
1/2 rate (25.1 - 50%) |
1/4 rate (1 - 25%) |
|---|---|---|---|---|
| Standard | no reduction | reduce buffer zone by 3 metres | reduce buffer zone by 6 metres | reduce buffer zone by 12 metres |
| LERAP -low drift 1 star | reduce buffer zone by 3 metres | reduce buffer zone by 6 metres | reduce buffer zone by 9 metres | reduce buffer zone by 15 metres |
| LERAP - low drift 2 star | reduce buffer zone by 6 metres | reduce buffer zone by 9 metres | reduce buffer zone by 12 metres | reduce buffer zone by 18 metres |
| LERAP - low drift 3 star | reduce buffer zone by 9 metres | reduce buffer zone by 12 metres | reduce buffer zone by 15 metres | reduce buffer zone by 21 metres |
Table 2 With windbreak
| Applied Dose Sprayer type |
Ful rate (75.1 - 100%) |
3/4 rate (50.1 - 75%) |
1/2 rate (25.1 - 50%) |
1/4 rate (1 - 25%) |
|---|---|---|---|---|
| Standard | reduce buffer zone by 6 metres | reduce buffer zone by 9 metres | reduce buffer zone by 12 metres | reduce buffer zone by 18 metres |
| LERAP - low drift 1 star | reduce buffer zone by 9 metres | reduce buffer zone by 12 metres | reduce buffer zone by 15 metres | reduce buffer zone by 21 metres |
| LERAP - low drift 2 star | reduce buffer zone by 12 metres | reduce buffer zone by 15 metres | reduce buffer zone by 18metres | reduce buffer zone by 24 metres |
| LERAP - low drift 3 star | reduce buffer zone by 15 metres | reduce buffer zone by 18 metres | reduce buffer zone by 21 metres | reduce buffer zone by 27 metres |
If the watercourse is a dry ditch you can record an unsprayed buffer zone of 5 metres for all pesticides covered by LERAP regardless of the rating of the sprayer.
Contributions from:
Alistair McCracken, Louise Cooke and Seán MacAntSaoir
Applied Plant Science Division
Joan Hamilton, Greenmount College
Edited by W Simpson