American gooseberry mildew

Symptoms

On the young shoots, leaves and fruit of gooseberry and currant, especially blackcurrant, a white, mealy, delicate coating of fungal mycelium first appears. Over time the coating becomes felted and brown, forming a rust-brown, hard-to-rub-off layer on the fruit and shoot tips. Infected shoot tips become deformed, grow poorly and die back, while the fruit becomes unsightly, cracks and loses its market value. The coating appears first on the youngest, apical parts of the new growth and on the upper side of young leaves, from where it spreads to the developing fruit. The distinguishing feature of American gooseberry mildew - setting it apart from the mild leaf mildews - is precisely this dense, felt-like crust, which turns from white to brown felt and adheres firmly to the fruit skin and the bark of the shoots; embedded in the brown layer are the overwintering fruiting bodies of the fungus (cleistothecia), visible as fine, dark dots. The affected growing points become twisted and stunted, the leaves at the tips are sometimes crinkled and curled under, and the bush forms clustered, deformed growth. The fruit beneath the coating can still be used once the layer is wiped off, but it loses its appearance and market usefulness; under heavy infection the shoot tips dry out completely. The symptoms build up from late spring and intensify in a warm, humid summer.

Treatment

Cutting out and destroying the infected shoot tips in spring and after harvest reduces the source of infection. In cases of heavy infection, fungicide sprays or biological preparations are applied from leaf emergence onwards; take care with sulphur-based products, as sensitive gooseberry varieties may react to them phytotoxically. The cut is made a few centimetres below the visible coating, into healthy wood, and the removed material is burned or taken off the plot (not composted). Sprays are carried out preventively or as an intervention from leaf emergence and are repeated in warm, muggy weather when disease pressure rises. Systemic fungicides (e.g. from the triazole/DMI or strobilurin/QoI groups) should be used in rotation between groups with different modes of action, so as not to let the fungus develop resistance, and the pre-harvest intervals before picking the fruit must be strictly observed. Among the gentler methods, products based on potassium bicarbonate (a contact action, mainly preventive) and bacterial biopreparations help. Sulphur is not applied in hot weather or on gooseberry varieties known to be sensitive, as it risks scorching the leaves and fruit.

Prevention

Planting resistant varieties, a sunny and airy site, and moderate nitrogen fertilisation, an excess of which favours the disease. It helps to avoid overcrowding the bushes and to thin the crowns regularly for better ventilation. The basis is to break the pathogen's overwintering cycle: raking up and removing fallen leaves in autumn and cutting out the infected, felt-covered shoot tips, because the fungus survives the winter on the shoots and in the fallen leaves as cleistothecia. The bushes are grown at a loose spacing, with a thinned, open crown, on a sunny and airy site, so that the shoots and fruit dry off quickly. It is worth balancing fertilisation towards potassium and avoiding an excess of nitrogen, which stimulates lush, susceptible growth. Mulching and keeping the ground beneath the bushes clean also help, as does watering at the base of the plant instead of wetting the leaves in the evening. The most effective, lasting prevention, however, remains the choice of modern gooseberry and currant varieties with increased resistance to this disease.

How to tell it apart from look-alikes

Control — organic and chemical methods

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