Q What is gooseberry mildew?
A There are two forms of this fungal disease. American gooseberry mildew (Sphaerotheca mors-uvae), which was introduced to Britain around 1900, is common and debilitating. European gooseberry mildew (Microsphaera grossulariae) is less common and much less severe in its effects.
Caption: American gooseberry mildew makes the fruit brown and felty
Q How can I recognise the disease?
A American gooseberry mildew appears as a white powder on young shoots and rapidly spreads to older leaves and shoots. The plant can turn greyish-white all over, with distorted shoot tips that may also die back. It affects the fruit, rendering them brown and felty with age. The mildew can be scraped off affected fruit; they are still edible, though are often small and tasteless.
European mildew produces a light powdering and may well go unnoticed in summer, but is normally visible as dark spots on fallen leaves in winter.
Q Is gooseberry mildew found on other plants?
A It affects some blackcurrants, but other plants can be affected by other types of mildew.
Q How serious is American gooseberry mildew?
A It is rarely fatal, but it does weaken the plant and spoil the fruit.
Q What’s the best way to control it?
A Prune out badly affected shoots and fruit.
Q Can any preventive measures be taken?
A Do not feed unless the bushes crop poorly, and use sulphate of potash (70g per sq m) rather than growmore. Prune the bushes to thin growth in the centre and remove overcrowded shoots to help prevent the humid air conditions in which the disease thrives.
If blackcurrants are affected, collect up and burn fallen leaves, as the spores overwinter on them. This is not worthwhile with gooseberries as the spores overwinter in the buds.
When planting new bushes, choose an open, sunny spot and do not overcrowd them.
Q Are there any resistant varieties?
A 'Greenfinch', 'Invicta' and the red varieties 'Pax' and 'Rokula' have some resistance to American gooseberry mildew.