This disease is associated with wet and humid conditions. It attacks woody plants in the Rosaceae family, which includes apples and pears, their ornamental equivalents plus others including amelanchier, aronia, Chaenomeles, cotoneaster, Crataegus, pyracantha and Sorbus.
How to recognise it
Venturia inaequalis and V. pirina are the two key scab fungi. Grey/black blotches appear on infected leaves and fruit. Spots on the underside of leaves sometimes look velvety due to fungal growth.
Affected leaves might twist or pucker. In minor cases this will affect only a few, irregularly scattered leaves, but if severe all foliage could show symptoms. Severely affected leaves often turn yellow and drop. Severe infections can lead to stem canker formation and defoliation on some ornamental specimens.
Other Venturia species include V. saliciperda, which can attack Salix vitellina, Salix 'Tortuosa' and Sorbus fragilis. V. inaequalis f. sp. pyracanthae attacks pyracantha as well as Eriobotrya and medlar.
Fungi overwinter on fallen leaves or on stem lesions and will reinfect vulnerable plants via water splash from overhead irrigation or during long periods of wet weather. Spore cases (perithecia) develop during the spring months and, when the weather is favourable, ascospores are ejected from these casings onto newly emerging foliage.
Infection occurs most rapidly at 12-23 degsC. Leaves or fruit must remain continuously wet for a minimum of nine hours for initial infection to occur at these temperatures. As the disease develops, it produces a summer spore (conidia) that helps to maintain infections through summer and early autumn.
When scab affects flower stems it can cause flowers to drop. Scabby spots can appear on fruit later in the season. These begin as velvety or sooty gray/black - sometimes greasy looking - lesions that sometimes have a red halo. The lesions later become sunken and can have areas of olive-coloured spores around their margins.
Severely infected fruit becomes distorted and usually drops from the tree. Fruit can also crack, which allows entry of secondary organisms. Stems may produce lesions where spores can overwinter for early reinfection the following spring.
Treatment: biological control
- Make sure plants are stress-free.
- Harden up foliage to reduce the amount of soft growth that can be vulnerable to infection.
- Avoid overhead irrigation wherever possible.
- Irrigate in the early morning to allow foliage to dry off.
Treatment: cultural control
Sweep up and destroy fallen leaves and fruit in the autumn to prevent reinfection in the spring. Work carried out by AHDB Horticulture with the help of the fruit industry has produced detailed information on scab along with PC-based prediction software. Levy payers can download detailed information from the AHDB Horticulture website (see http://apples.hdc.org.uk/Apple-Scab.asp).
Treatment: chemical control
Close monitoring of weather conditions with reference to Mills Period or Smith Period Tables (see www.dovebugs.co.uk/scab risk periods.pdf) will provide a guide as to when periods of scab infection are either imminent or have already occurred and hence increased spray frequencies are required.
On ornamental specimens, start a protectant fungicide programme just before new foliage emerges in the spring.
Nursery stock growers should note that an "edible" spray programme must be used if apples and pears are to be sold with their fruit on. All harvest intervals must be completed before despatch. If a "non-edible" spray programme is used then fruit must be removed before despatch.
Active ingredient Captan
FRAC code M4
Formulation Alpha Captan 80 WDG (Adama)
Action(s) Protectant fungicide. Avoid use on certain apple varieties (see label). Selected RPE required when handling concentrate.
Active ingredients Boscalid and pyraclostrobin
FRAC codes 7 + 11
Formulation Bellis (BASF)
Action(s) Protectant and systemic fungicide. Best results obtained as a protectant spray on apples and pears from bud burst.
Active ingredient Dithianon
FRAC code M9
Formulation Dithianon WG (BASF)
Action(s) Protectant and eradicant fungicide. Apply at bud burst. If applied at high rate within 48 hours of a Mills period, it will prevent new infection. Do not treat Golden Delicious after green cluster stage.
Active ingredient Dodine
FRAC code M7
Formulation Radspor 400 (Agriphar)
Action(s) Protectant and eradicant fungicide. Apply protective spray at bud burst and at 10- to 14-day intervals until late June to early July. Apply post-infection spray within 36 hours of rain. Product can prevent spore production if scab is present. Can cause russeting on some pear varieties under certain conditions.
Active ingredient Fenbuconazole
FRAC code 3
Formulation Indar 5EW (Landseer)
Action(s) Protectant, curative, systemic fungicide. Effective if used within a routine preventive spray programme from bud burst to petal fall. Four-week harvest interval for fruit.
Active ingredient Kresoxim-methyl
FRAC code 11
Formulation Stroby WG (BASF)
Action(s) Protectant fungicide. Apply before infection occurs. Product must be rotated with other modes of action and should not be used as the final spray of the season on apples.
Active ingredient Mancozeb
FRAC code M3
Formulation Various including Karamate Dry Flo Neotec (Landseer)
Action(s) Protectant fungicide.
Active ingredient Myclobutanil
FRAC code 3
Formulation Various including Systhane 20EW (Landseer)
Action(s) Protectant, curative, systemic fungicide. Best results as a tank mix with captan or mancozeb.
Active ingredient Penconazole
FRAC code 3
Formulation Topas (Syngenta)
Action(s) Protectant fungicide. Treat at earliest signs of disease.
Active ingredient Pyrimethanil
FRAC code 9
Formulation Scala (BASF)
Action(s) Protectant fungicide. Early-season control of scab. Apples can only be treated from bud burst until the end of flowering.
Active ingredient Sulphur
FRAC code M2
Formulation Various including Sulphur Flowable (United Phosphorus)
Action(s) Protectant fungicide. Off-label approval required for quince. Treat at earliest signs of disease. Some apple varieties are very sensitive to sulphur. Other label restrictions apply.
Fully updated by Dove Associates.
Use plant protection products safely. Always read the label and product information before use.
Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.