Blossom wilt is a fungal disease of trees, especially fruit trees, caused by the fungi Monilinia laxa and M. fructigena. The two fungi are very closely related and indistinguishable to the naked eye. M. laxa is the most common cause of blossom wilt on pears and stone fruit, whilst a specific form, M. laxa f. sp. mali is restricted to apples. Whilst occasionally causing blossom wilt, M. fructigena more commonly causes the disease known as brown rot in the fruit.

Many tree fruit are affected, including apples, pears, plums, cherries, nectarines, peaches, apricots, and ornamental cultivars.

The damage begins at flowering time in mid-spring, but becomes more obvious as shoots die back in late spring and early summer.

You may see the following symptoms:

• Brown and shrivelled blossoms and leaves on the fruiting spurs at flowering time
• Small, buff-coloured pustules of the causal fungi on dead tissues. Usually seen under wet conditions
• Severity varies greatly from year to year, depending on weather conditions at the time of flowering
• Dieback due to blossom wilt rarely spreads back more than 20-30 centimetres along an affected shoot

Non chemical control

• Minimise carry-over of the pathogens by removing all brown, rotted fruit promptly. To dispose of fruit, you can bury them at least 30cm (1ft) below the soil surface, or put them in the local council green waste (although check first as some councils will not accept large volumes of rotting fruit). Do not allow rotted fruit to remain on the tree
• Brown rot infects through wounds, especially those caused by birds so, if possible, net to reduce bird damage
• If practical, prune out and burn infected spurs and blossoms to reduce the amount of fungus available to infect fruit
• Choose resistant cultivars: apricots ‘Monique’ and ‘Moorpark’; plums 'Jefferson', ‘Marjorie’s Seedling’, ‘President’ and ‘Reine Claude Violette’ are resistant. The apple cultivars 'James Grieve' and ‘Lord Derby’ are very susceptible

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.

There are no fungicides labelled specifically to control blossom wilt. On ornamental trees only, fungicides applied for other purposes, such as powdery mildew or rust control, may give some incidental control though this is not claimed by the manufacturers. These products can't be used on trees from which the fruit will be consumed.

Download

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

Link

Chemicals: using safely and effectively

Fruit becomes infected through wounds, particularly bird damage. Affected fruits exhibit brown rot, mummify and remain hanging on the tree and, where they touch the bark, cause small infections (cankers). The fungus remains in the dead fruit and cankers over winter and releases spores in the spring to cause the blossom wilt phase of the disease. These infections in turn release spores to infect wounded fruit.