American gooseberry mildew is a common disease most notably of gooseberries, caused by the fungus Podosphaera mors-uvae.

Gooseberries

From early summer, symptoms appear on gooseberry bushes as:

• Powdery grey-white fungal patches on the leaves, which later turn brown
• Mildew on the fruits turns brown as it ages
• Young shoots are stunted and twisted at the tops, and die back
• Tiny fruiting bodies (just visible as black dots) may develop within the mildew growth in late summer and autumn

Eating affected fruit

The fungal growth on the berries can be rubbed off, and the gooseberries are edible, so there is no need to waste the crop if many of the fruits are affected. However, even after this laborious process the diseased berries will turn brown when cooked.

Blackcurrants

In recent years this disease has frequently appeared late in the season on blackcurrant bushes. Treatment is as for gooseberries.

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Non-chemical control

A number of measures will help reduce the occurrence of the disease;

• Gooseberries are best pruned to an open form to aid picking;  this also discourages stagnant air amongst the branches (which is favourable for the disease) and promotes the circulation of air
• Avoid planting in low-lying or enclosed areas or with poor air flow
• Opening up bushes will also greatly ease picking and control of gooseberry sawfly
• Avoid applying excessive quantities of nitrogen-rich fertilisers (dried poultry manure pellets for example) or mulching with manure, because the resulting soft growth is more readily attacked
• A dressing of a balanced fertiliser, such as Growmore, is more suitable – use 50g per sq m (1½oz per sq yd)
• The fungus overwinters within the buds and on the shoots, and (with blackcurrants in particular) on fallen leaves. It is therefore essential to ‘tip’ prune the bushes by removing and disposing of (bonfire or shredding and composting) the ends of affected shoots as soon as they are seen, as well as disposing of all fallen leaves in autumn

Resistant cultivars

Resistant cultivars are available and offer a useful level of resistance to this disease:

Gooseberry cultivars:
‘Captivator’: Resistant 
'Greenfinch' AGM: Resistant 
‘Hinnomaki Gold’: Slightly Resistant 
‘Hinnomaki Red’: Resistant
‘Hinnomaki Green’: Resistant
'Invincta' AGM: Resistant (Although young plants of this variety show moderate attacks of mildew the shoots of mature plants and fruits are highly resistant)   
‘Martlet’: Resistant
‘Pax’: Slightly Resistant  
‘Remarka’: Resistant 
‘Rokula’: Resistant

Blackcurrant cultivars:
‘Ben Alder’: Slightly Resistant
‘Ben Connan’ AGM: Resistant
‘Ben Gairn’: Resistant (Also resistant to reversion virus)
‘Ben Hope’: Resistant  (Also resistant to gall mite)
‘Ben Lomond’ AGM: Slightly Resistant
‘Ben More’: Resistant
‘Ben Sarek’ AGM: Resistant
‘Ben Tirran’: Resistant
‘Big Ben’: Resistant

To source plants use: RHS Find a Plant

Fungicides

The RHS recommends that you don't use fungicides. Fungicides (including organic types) may reduce biodiversity, impact soil health and have wider adverse environmental effects. If you do intend to use a fungicide, please read the information given in the link and download below to ensure that use, storage and disposal of the product is done in a responsible and legally compliant manner.
  
The products listed in the ‘Fungicides for gardeners’ document below are legally available for use by home gardeners in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally. Homemade products are not recommended as they are unregulated and usually untested.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Link

Chemicals: storing and disposing safely

As its name suggests, this disease originated in North America, appearing in Europe at the beginning of the 20th century.

The vast majority of the growth of the fungus is present on the surface of the shoots or fruits, but it also penetrates into the cells immediately below, producing specialised feeding structures through which it obtains food from the plant.

Large numbers of microscopic spores are produced within the mildew growth; these are dispersed on air currents and in water splash. Unlike many fungal diseases, powdery mildews do not require extended periods of leaf wetness to infect the plant. The disease can therefore be a problem in relatively dry summers, although high humidity is required.

On gooseberry, the fungus overwinters within the buds and on the shoots. On blackcurrants, most of the infections in spring are thought to arise from another type of spore, released from fruiting bodies of the fungus present on fallen leaves at the base of the plant.