Disease status

Gloeosporium and Colletotrichum spp. are important causes of rotting in stored apples in UK, Europe and other parts of the world where the summers are usually wet.

• In the UK these fungi were responsible for significant losses (30-50%) in Cox in the 1960s and 70s but declined in importance with advent of CA storage and better knowledge on the nutrition of apples for storage.

In the UK three fungal species are responsible:

            Gloeosporium album (Pezicula alba)

            Gloeosporium perennans (Pezicula malicorticis)

            Colletotrichum spp. (formerly Gloeosporium fructigenum) (Glomerella cingulata)

• Two species of Colletotrichum may be responsible – C. gloeosporiodes and C. acutatum. The relative importance of these two species on apple has not been investigated in the UK.
• G. album and G. perennans are most frequent on Cox. Colletotrichum spp. are usually more associated with poor quality or over-stored fruit.
• Often tests on young apple fruitlets in June indicate a high incidence of Colletotrichum infection.
• However, the rot fails to develop in store by the time the fruit is marketed.

Other hosts

• Gloeosporium perennans and G. album are more restricted in host range being mainly found in apple, pear and some ornamentals.
• G. album can also be found on weeds. Colletotrichum gloeosporiodes and C. acutatum are much more variable fungi with a wide host range worldwide, being reported causing rots on most fruit crops including apple, pear, cherry, strawberry, tomato, many tropical fruit: banana, cacao, mango, citrus and many ornamental plants: camellia, lupin and many other cultivated and wild plants.

Varietal susceptibility

Apple varieties do vary in susceptibility to Gloeosporium spp.

• These rots are commonly found on stored Cox, Golden Delicious and occasionally on Gala and Egremont Russet, but rare on Bramley and Jonagold.
• However, Colletotrichum spp. often occur on Bramley as both a cheek rot and a core rot.

Distribution

• G. album is widespread in the UK, France, Germany, Eire and other parts of Europe and also Australia, New Zealand, Canada
• G. perennans widespread in UK, Eire and other parts of Europe, Australia, New Zealand, USA and Canada
• Colletotrichum spp. (Glomerella cingulata) have worldwide distribution.

Symptoms and recognition

Fruit rots

Symptoms caused by the three fungal species on fruit are similar and symptoms are similar on all cultivars.

• The fungus usually enters via a lenticel producing a cheek rot, but it may also occur around the stalk or calyx where it enters via a wound or small crack.
• The rot is mid-brown (possibly darker on Egremont Russet), circular, moderately firm and frequently, but not always, forms concentric zones of different colours as the tissue is invaded.
• Lesions usually have yellow centres on Cox, progressing to dark brown at the interface between healthy and infected tissue.
• On Egremont Russet the rots are often uniform brown.
• Cream-coloured slimy pustules may be produced during storage on rots caused by Gloeosporium spp.
• Pink/orange slimy pustules may be present on rots caused by Colletotrichum spp., particularly after they have been in the rot bin for a day or two.
• The rot is rarely seen in the orchard.

Cankers

Cankers caused by Gloeosporium spp. (perennial canker) resemble those caused by Nectria galligena.

• They are elliptical, sunken and, in older cankers, the bark sloughs off.
• Black pinhead-sized fruiting bodies (acervuli) may be present which ooze opaque slimy spore masses in wet weather in the summer.

Other problems that may be confused with Gloeosporium

• Gloeosporium and Colletotrichum rots can be easily confused with those caused by Nectria galligena which also occur in the cheek, eye and stalk end of the apple.
• The presence of slimy spore masses usually distinguishes it from Nectria rot, but where these are absent, the two rots can only surely be distinguished by laboratory examination.
• Gloeosporium rot may also be confused with Penicillium rot, but the latter is usually squashier and readily distinguishable if the brilliant white/turquoise spore masses typical of Penicillium are present.

Disease cycle and epidemiology

All three fungi overwinter in the orchard on cankers (G. perennans), dead twigs, diseased bark, dead leaves or mummified fruit.

• Cankers result from fungal spores invading wounds, frost cracks, etc, in trees.
• During wet weather in the growing seasons spores (conidia) produced on fruiting bodies (acervuli) on the cankers and other inoculum sources, are splashed dispersed to infect fruit.
• Fruit infection can occur at any time during the growing season from blossom to harvest, when the weather is wet and is mainly through lenticels.
• G. album and G. perennans are favoured by cool humid weather, whereas Colletotrichum spp. are favoured by warmer temperatures.
• Gloeosporium and Colletotrichum rots are rarely seen in the orchard and infections remain latent for some time and start to appear in store from December onwards.

Disease monitoring and forecasting

Gloeosporium and Colletotrichum rots are difficult to monitor in the orchard, since the rots are rarely seen, the cankers not easily distinguishable from those caused by Nectria and the fungi not readily distinguishable on other inoculum sources such as weeds.

• An estimate of the incidence of Gloeosporium and Colletotrichum rots in the orchard can be obtained by examination of about 100 rotted fruit from the rot bin, when the fruit from that orchard is graded and identifying the rots present.
• Disease forecasting systems have not been developed.
• During the six years in which rot risk assessment was developed, the incidence of Gloeosporium rot in the Cox orchards used in the study was very low and sporadic.
• Consequently it was difficult to get any clear correlations between Gloeosporium incidence and other factors.

The following criteria are suggestions based on the limited data available.

Assessment of the risk of rotting in store

 The assessment for rots due to Gloeosporium and Colletotrichum spp. On Cox is based on:

• Orchard rot history (from packhouse records).
• Crop load – light crops with larger fruit size are more prone to Gloeosporium and Colletotrichum rots because of lower calcium levels.
• Rainfall in the month prior to harvest.  Above average rainfall increases risk of Gloeosporium and Colletotrichum rots.
• Fruit mineral composition – low calcium high potassium levels giving a K/Ca ratio of  >30.
• While on Cox fruit with low calcium levels are more prone to Gloeosporium rotting, the rot may occasionally occur at high incidence in store on fruit of good mineral composition.
• Reasons for the occurrence of Gloeosporium rots in these instances are not clear but must be assumed to be associated with high inoculum in the orchard and favourable weather prior to harvest.

The rot may occasionally occur at high incidence in store on Gala, a variety which is usually of high calcium content and does not merit pre-harvest calcium sprays.

• Reasons for the occurrence of Gloeosporium rots in these instances are not clear but must be assumed to be associated with high inoculum in the orchard and favourable weather prior to harvest.
• Gloeosporium and Colletotrichum rots often occur in orchards with a high incidence of Nectria canker.

Decisions on Gloeosporium and Colletotrichum rot risk

• Cox apples of low calcium status and where K/Ca ratio is >30 should not be stored long term.
• For Cox apples suitable for long-term storage but with a history of Gloeosporium and Colletotrichum  rots, monitor the rainfall in the month prior to harvest.
• Where rainfall is >than average then schedule fruit for marketing before January if fungicide sprays have not been applied.
• At present the incidence of Gloeosporium and Colletotrichum rots in Gala and Bramley is low (except where fruit has deteriorated prematurely due to a physiological disorder) and does not merit special action.

Cultural control

• Remove possible source of inoculum, such as cankers, die-back, pruning snags, mummified fruits during winter pruning.
• Remove from the orchard and burn or macerate up in the alleyway to encourage rotting.
• Summer prune at the correct time to avoid die-back of pruned shoots, which provide ideal sites for Gloeosporium and Colletotrichum to colonise.
• Apply calcium sprays to Cox and Egremont Russet in June.  Harvest to ensure good mineral composition for storage.

Physical control

• Research has shown that dipping the fruit in water heated to 48-50ºC for three minutes is sufficient to control Gloeosporium and Colletotrichum rot.
• However, such treatment is unlikely to be suitable for Cox since the margin between the temperature for effective control and fruit damage is small.

Biological control

• Biological control of Gloeosporium and Colletotrichum rots on tropical fruit has been well researched.
• In Germany trials have shown that a commercial product Boni-Protect, based on the yeast Aureobasidium pullulans, gave comaparable control of Gloeosporium rot in store  to fungicide standards.
• This product is not registered in the UK.

Chemical control

Pre-harvest sprays

• Orchards with a history of Gloeosporium or Colletotrichum rot should be sprayed pre-harvest with sprays of captan or Switch (cyprodinil + fludioxonil) or Bellis (pyraclostrobin + boscalid) in July and August.

• Captan or Switch (cyprodinil + fludioxonil) or Bellis (pyraclostrobin + boscalid) applied in early spring and summer may also reduce Gloeosporium and Colletotrichum rot.

Avoiding fungicide resistance

• Resistance of Gloeosporium and Colletotrichum isolates to benzimidazole fungicides is common in the rest of Europe.
• In the UK the incidence of resistance has increased in the last five years such that about 30-40% of isolates are resistant.
• Therefore pre-harvest sprays should be based on sprays of two different products rather than two sprays of the same product.