Chemical control (Apple scab)

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:

Protectants

• Chemical activity that prevents infection.
• Fungicide has to be present on the leaf surface before the spore lands.
• Examples are sulphur, copper fungicide (check status of current authorisation), 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).

• The DMI (triazoles) fungicides are usually considered to have the greatest kick-back action of 4 days.
• Dodine, 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

Efficacy

• Effective control of scab.

Mode of action

• Protectant, curative or conidia suppression depending on needs.

Fungicide resistance

• Ensuring fungicides with differing modes of action are used to reduce the risk of resistance.

Phytotoxicity

• 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.

Cost

• 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 (check status of current authorisation) may give some control of scab inoculum overwintering on the tree.
• Research has shown that post-harvest application of a DMI fungicide e.g. penconazole (Topas/Topenco) or difenoconazole (Difference/Change) or tebuconazole (Fathom), 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.
• BASIS qualified fruit agronomists are available to guide growers on the optimum spray programme to follow during the growing season.

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.