Blossom wilt – additional information | Apple Best Practice Guide

Blossom wilt – additional information

Disease status

Blossom wilt is not usually one of the most important diseases of dessert and culinary apples but can be a significant problem on some varieties.

The disease is much more prevalent on cider apples where most commercially-grown varieties appear to be susceptible and control measures are more difficult to implement.
The fungus attacks flowers, and at high incidence, can reduce yields. If not controlled, the disease can rapidly build up to significant incidence over one or two seasons.
Under favourable conditions on very susceptible varieties as much as 50‑70% of flowering spurs can be killed.

Other hosts

Monilinia laxa f.sp. mali is specific to apple.
A closely related species, Monilinia laxa, causes blossom wilt and brown rot of stone fruit but very rarely attacks apple.

Varietal susceptibility

Apple varieties vary in susceptibility.

James Grieve, Cox and Lord Derby are most susceptible
Other varieties such as Bramley are almost resistant.
Some cider apple varieties such as Somerset Redstreak and Stembridge Clusters are very susceptible.

Distribution

Widespread and common in UK apple orchards where susceptible varieties are grown. Monilinia laxa f. sp. mali is restricted to Europe.

Symptoms and recognition

Enlarge

Blossom wilt canker with characteristic brown zones of infection (watermarking)

The fungus attacks flowers causing them to wilt turn brown and collapse.
The symptoms appear from petal fall onwards.
The outer bark of infected spurs is discoloured and the underlying tissues necrotic.
Grey pustules of mycelium and spores develop on the affected flower parts. Wilted blossoms have a distinctive fetid smell, similar to the scent of sweet chestnut flowers.
The fungus progresses from infected flowers down through the spur into branches and forms cankers characterised by brown zones of infection (water marking).
Similar grey pustules develop on cankers.
On cider apples often only the blossom is killed and the fungus does not progress much into the spur so the only symptom is dead blossoms.

Other problems that may be confused with blossom wilt

Blossom

Wilting blossoms may be caused by other diseases such as Neonectria canker, where canker further down the branch results in wilting blossoms above.

This may be distinguished by the absence of grey pustules, internal rotting of the blossom truss and the distinctive smell.
Similarly fireblight can also cause wilting/dead blossoms. In this case milky-coloured bacterial ooze is associated with the affected blossoms.
Fireblight is more likely to be confused with blossom wilt in cider apple varieties as these flower later than culinary and dessert varieties and are therefore more subject to fireblight attack.
Wilting blossoms can also result from attack by bud moth (Spilonota ocellana).
Careful examination of such affected blossoms should reveal evidence of mining by bud moth larvae.

Cankers

Blossom wilt cankers can be confused with other cankers such as those caused by Neonectria ditissima.

These can be distinguished by the presence of white/creamy pustules or the red pinhead-sized fruiting bodies (perithecia) of N. ditissima and absence of water marking.
Fireblight cankers at the base of a flower spur may also resemble those of blossom wilt.
Cankers caused by brown rot (Monilinia fructigena) are very similar to those of blossom wilt fungus and only differ in the colour of pustules: cream for brown rot and grey for blossom wilt. These are probably only distinguishable by an expert.

Disease cycle and epidemiology

The fungus overwinters in infected cankers or dead blossoms on branches.
The sexual state is rare and unimportant in the perennation of the disease.
Conidia produced on the cankers or dead blossoms in early spring are disseminated by wind and rain and infect new blossoms.
Fungal hyphae spread from infected blossoms to twigs/branches to form new cankers.
The disease is favoured by cool moist weather.

Disease monitoring and forecasting

In winter, look for distinct cankers or dead blossoms particularly on susceptible varieties.
Inspect orchards for wilting /dying blossoms from the end of blossom.
The presence of infected blossoms in one season indicates the need for fungicide treatment the following season.
Forecasting models have been developed in other countries for other Monilinia species (M. fructicola and M. laxa) on stone fruit, but not for apple blossom wilt.
The development of specific forecasting systems is probably not merited as control can be achieved by a few well-timed protectant sprays.

Cultural control methods

Remove infected blossoms and cankers. This is best done soon after blossom when the affected parts are easily visible rather than delaying until winter pruning.
Removal of affected blossoms can be important where the disease incidence is high but is labour intensive and therefore expensive and never fully effective and not practically possible in cider orchards.
However, it is an important part of integrated control and is the only method available in organic systems.

Biological control

This is not an option at present. Research in other countries suggests that fungal antagonists exist which may suppress canker development but utilisation of such an approach has not been developed.

Chemical control

Protectant sprays combined with cultural control are the main means of control in the UK.

In dessert and culinary orchards, the following season or following two seasons after detection apply a spray of boscalid + pyraclostrobin (Bellis) or cyprodinil + fludioxonil (Switch) at first flower and repeat 7-10 days later.
In cider orchards where the disease is present, or on very susceptible varieties (e.g. Somerset Redstreak) apply a spray of boscalid + pyraclostrobin (Bellis) or cyprodinil + fludioxonil (Switch) at first flower and repeat 7-10 days later.
Where the disease is present at high incidence a four spray programme, using different fungicide products, may be needed, starting at pink bud.
Spray cover is important. Sprays must be applied at >150 l/ha to be effective. Trials show that sprays applied at 50 l/ha were ineffective.
Usually where the disease occurs at low incidence in dessert and culinary orchards, protectant sprays applied the following season after detection give adequate control.
A further season’s sprays may be necessary where the disease incidence is high. In cider orchards routine sprays may be necessary.

Avoiding fungicide resistance

The risk of resistance is low as a range of fungicide products are used and intensive spray programmes are not used.

Apple Best Practice Guide