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Mouldy core – additional information

Disease status

Mouldy core is an internal dry rot of certain apple cultivars, especially Cameo and Bramley’s Seedling. The average incidence in 2007/8 of mouldy core in Cameo was around 10%. The main problem with mouldy core is that it develops inside the apple, often in store or as the fruit ripens during marketing and can remain undetected until the fruit is eaten. Discovery of core rots in this way can obviously affect consumer acceptability and rejection of consignments by the supermarkets. Losses in Cameo can be significant especially as the core rot appears to continue to develop in store and during marketing.

Other hosts

Mouldy core can be caused by a range of different fungi including Alternaria, Stemphylium, Cladosporium, Epicoccum and Fusarium. These are commonly occurring saprophytic fungi that readily colonise decaying plant material. Their distribution is ubiquitous.

Varietal susceptibility

Apple cultivars susceptible to mouldy core generally have an open sinus extending from the calyx into the core that allows saprophytic fungi colonising senescing flower parts to enter the core region. Many cultivars of apples are affected worldwide especially Red Delicious and Red Delicious types. In the UK the problem is mainly associated with the cultivar Cameo and Bramley’s Seedling but is also found in Ida red, Braeburn, Gloster and certain cider apple cultivars.

Distribution

Widespread and common in UK apple orchards where susceptible varieties are grown. The problem has been recorded in the USA, Australia, New Zealand, Canada, South Africa and The Netherlands.

Symptoms and recognition

 

Mouldy core

Mouldy core is an internal dry rot of certain apple cultivars, External symptoms are rare but fruit may colour and fall prematurely. The problem is characterised by the growth of fungus mycelium within the apple core, initially without invading the apple flesh. The fungi may invade the flesh leading to a slow, dry rot confined to the apple centre.  The core rot may also continue to develop in store and may then appear at the cheek, eye or stalk end of the fruit. This is often true where the core rot is caused by Fusarium spp.

Some apple varieties, especially Bramley, can be affected by a wet core rot. This appears after harvest or post storage, usually as a soft, internal rot that eventually rots the whole fruit. Wet core rots are caused by a range of fungi including Fusarium, Mucor and Penicillium. Wet core rots generally arise from fungi that enter the core when the fruit is drenched post-harvest in anti scald agents.

Other problems that may be confused with mouldy core

The only visible symptom in the orchard is fruit may colour and fall prematurely. Such symptoms may also be caused by physical damage to the fruit such as bird pecks and also insect damage particularly that caused by codling moth.

Disease cycle and epidemiology

Apple cultivars susceptible to mouldy core generally have an open sinus extending from the calyx into the core. Flower parts are rapidly colonised by a range of saprophytic fungi as they start to senesce and the open sinus allows these fungi to enter the core region. Several factors can affect the open sinus including weather conditions, irregular fruit growth and use of growth regulators. Wet weather during blossom encourages colonisation of flower parts by fungi and can increase the risk of mouldy core. Mouldy core can be caused by a range of different fungi including Alternaria, Stemphylium, Cladosporium, Epicoccum and Fusarium. In an AHDB Horticulture funded study in the UK in 2008 and 2009 (Project TF 184), the main fungi isolated from the rotted cores were at least three different species of Fusarium, as well as Alternaria and Penicillium. In other countries Alternaria is generally the most important cause of core rots. Limited investigations in the UK on Cameo and Bramley fruits indicate that Fusarium may be important. Identification of the main fungi responsible is important as this can affect fungicide efficacy.

Wet core rots are caused by a range of fungi including Fusarium, Mucor and Penicillium. Wet core rots generally arise from fungi that enter the core when the fruit is drenched post-harvest in anti scald agents contaminated with fungal spores.

Disease monitoring and forecasting

As there are generally no external symptoms, orchard monitoring to assist in decisions on fungicide timing are not practical. Wet weather during blossom encourages colonisation of flower parts by fungi and can increase the risk of mouldy core. Assessing the incidence of premature ripened fruit near harvest may give an indication of the incidence of mouldy core in the fruit.

Cultural control

Orchard training and pruning should allow good airflow and light penentration and allow trees to dry rapidly and reduce the risk of mouldy core. At harvest apples that are riper than usual should not be picked as these have a high chance of having mouldy core.

Wet core can be solved by only drenching post-harvest if necessary and adopting strict hygiene measures in the drenching operation to prevent the build up of mud, fungal spores and other debris in the drenching solution.

Biological control

Studies on biocontrol of mouldy core are limited. In China certain strains of Bacillus subtilis were shown to give some control of mouldy core caused by Alternaria, Tricothecium and Fusarium. Bacillus subtilis as Serenade is registered in the UK for disease control on protected strawberries. It also has a Sola (0246/2009) for use on top fruit. Efficacy against mouldy core is not known.

Chemical control

There have been many studies conducted in other countries on control of mouldy core, particularly on the variety Red Delicious where Alternaria is generally the main cause of the problem. Application of fungicides between first flower and petal fall reduced the incidence of mouldy core in most experiments. Not all fungicide trials were successful and this may be related to the fungi responsible for mouldy core. Treatments found to reduce the incidence of mouldy core in overseas trials included carbendazim, mancozeb (Karamate), various DMI fungicides (e.g. myclobutanil), vinclozolin (Ronilan), strobylurine fungicides (e.g. azoxystrobin) and potassium phosphite (see tables below).

In a more recent AHDB Horticulture UK funded study (Project TF 184) in 2009, fungicide trials conducted in two commercial orchards found that sprays of either Bellis (pyraclostrobin + boscalid) or Switch (cyprodinil + fludioxonil) at full bloom and 10-14 days later, had some effect in reducing mouldy core compared to where no treatments were used.

Choice of fungicide that may control mouldy core – efficacy factors

Active Ingredient TradeName FungicideGroup Typh Safety Other DiseasescontrolledPartly Controlled 
cyprodinil + fludioxonil Switch anilinopyrimidine + cyanopyrrol safe Scab, storage rots, canker
fenbuconazole Indar triazole (DMI) safe Scab, reduces mildew
mancozeb Karamate Dry FloPenncozeb WDG  dithiocarbamate harmful frequent use suppresses mites
myclobutanil Robut 20Systhane 20 EW Triazole (DMI) safe Scab, mildew
pyraclostrobin + boscalid Bellis Strobylurine (QoI) + anilide safe Scab, mildew, storage rots, canker

 

 

Choice of fungicides that may control mouldy core– safety factors

Activeingredient Hazards HarvestInterval(days) MaxNo.sprays Buffer zone
Human Fish aquatic life  Bees Width(m)
cyprodinil + fludioxonil a, c d u 3 3 30 m
fenbuconazole ir t u 28 10 (when applied at 1.4l/ha) sm
mancozeb ir t u 28 4 40 m
myclobutanil h d u 14 10 (when applied at 0.45l/ha) sm
pyraclostrobin + boscalid h d u 7 4 40 m

d = dangerous; h = harmful; ir = irritating, a = may cause allergic reaction, t = toxic

PH = post harvest; Pre bb = pre-bud burst, sm=statutory minimum of 5 m for broadcast air assisted sprayers

u=uncategorised/unclassified/unspecified, c=closed cab required for air assisted sprayers

Avoiding fungicide resistance

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

Control in organic orchards

Orchard training and pruning should allow good airflow and light penentration and allow trees to dry rapidly and reduce the risk of mouldy core. At harvest apples that are riper than usual should not be picked as these have a high chance of having mouldy core. Serenade (Bacillus subtilis) (Sola 0246/2009) or potassium phosphite may give some control of the problem.