Apple scab

Symptoms

Olive-green, velvety, blurred spots appear on the leaves and gradually darken and turn brown; heavily infected leaves turn yellow and drop prematurely. Dark, corky spots form on the fruit and crack as the fruit grows, producing the characteristic fissures and deformations. Infected fruit stay small, store poorly and lose market value. The first spots are usually seen on the youngest, developing leaves and on the sepals of the flowers; at first they are poorly delimited, later taking on a distinctly dark-olive to blackish-brown colour with a velvety sporulating coating. The primary spring infections are caused by ascospores discharged during rain from the fruiting bodies overwintering in the fallen leaves; the resulting spots in turn produce conidia responsible for numerous secondary infections through the summer. Early infection of the fruitlets leads to one-sided growth and severe fruit deformation, and the cracks in the corky tissue become entry points for rot fungi and storage rots. On especially susceptible cultivars the disease also produces crusty lesions and cracks on the young shoots and twigs, which become an additional source of spores the following season. A separate form is the so-called storage scab - small, pinpoint spots that only show up in cold storage when the fruit was infected just before harvest.

Treatment

Fungicide sprays are applied preventively during the primary spring infections (according to the moisture and temperature thresholds in the Mills tables); in home gardens, sulphur-based preparations can help. Raking up and removing or composting the fallen leaves breaks the disease cycle, because the spores overwinter in the leaves on the ground. Treatments are targeted at the most vulnerable phenophases - from the green-tip and pink-bud stage, through flowering, to petal fall and the first weeks after flowering, that is the period of ascospore release. Effective protection combines contact fungicides (sulphur, phthalimides) that form a protective layer on the surface and act only preventively, with products that act translaminarly or systemically (triazoles from the group of sterol biosynthesis inhibitors, strobilurins, anilinopyrimidines), which can act curatively shortly after infection. To prevent the fungus from developing resistance, the modes of action must be rotated and the same group must not be used several times in a row. Spraying the fallen leaves with a urea solution in autumn speeds up their decomposition and reduces the spring spore pool. On cultivars with scab on the shoots, cutting out the infected twigs during pruning helps.

Prevention

Growing resistant or less-susceptible cultivars (e.g. with the Vf gene), keeping the crowns airy and thinned out by pruning, avoiding leaf wetting, raking up leaves in autumn and applying balanced, moderate nitrogen feeding. The foundation is limiting the source of infection and the conditions that favour the fungus. It is worth choosing sunny, airy sites, planting the trees at a spacing that lets the leaves dry quickly, and forming a loose, well-thinned crown. Autumn leaf removal can be aided by shredding the leaves with a mower or by speeding up their decomposition (urea, earthworm activity). Nitrogen feeding should be kept moderate, because an excess stimulates lush growth that is prone to infection. It helps to carry out treatments according to forecasting and the Mills tables (leaf wetness duration and temperature) rather than routinely by the calendar. It should be remembered that cultivar resistance based on a single gene (Vf/Rvi6) can be broken by new races of the pathogen, so even resistant cultivars may in time need limited protection and basic orchard hygiene.

Control — organic and chemical methods

Organic and biological methods

Method	How it works	When to use
Removing fallen leaves (breaking the cycle)	Rake up, compost or shred the leaves with a mower; an autumn urea spray speeds up their decomposition and cuts the pool of overwintering ascospores.	Autumn and early spring, before the spring release of spores.
Resistant cultivars and an airy site	Choosing cultivars with a resistance gene (Vf/Rvi6), a sunny and airy spot and a loose crown help the leaves dry and limit infections.	When establishing the orchard and during formative pruning.
Monitoring and forecasting (Mills tables)	Predicting infection periods from leaf wetness duration and temperature lets treatments be targeted instead of routine spraying - a supporting method.	The whole growing season, especially in spring.

Chemical methods

Method	How it works	When to use
Contact fungicides (sulphur, phthalimides)	They form a protective layer on the leaves and fruit and act only preventively (before rain); sulphur is also permitted in organic growing.	During the primary infections, according to the Mills tables.
Systemic/translaminar fungicides (triazoles, strobilurins, anilinopyrimidines)	They penetrate the tissue and can act curatively shortly after infection; the mode-of-action groups must be rotated to avoid the fungus developing resistance.	Spring, during the key periods of spore flights.

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.