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Apple canker – additional information

Disease status

Apple canker is an economically important disease of apple and is one of the most important diseases in the UK and recognised as a serious problem as early as 1710.

  • The fungus attacks twigs and branches, causing cankers and dieback in mature trees, and often death of young trees.
  • It also attacks fruit causing rots both in the orchard and in store.
  • Losses due to canker are difficult to estimate, but those of 10% or more in young trees in newly planted orchards are typical and, in seasons favourable to the disease, losses due to rots in stored fruit can be as high as 30%.
  • The fungus is not specific to apple and attacks pear and quince and several forest and hedgerow trees including beech (Fagus), poplar (Populus), hawthorn (Crataegus) and Acer.
  • N ditissima on ash (Fraxinus) is thought to be a separate strain  formae speciale
  • Alder (Alnus) is susceptible to artificial inoculation but no natural infections have so far been observed.
  • These other susceptible species could therefore act as a source of Neonectria inoculum. In practice only poplar has been implicated in canker outbreaks in apple orchards.
  • The disease is present in virtually all the apple producing areas of the world except Australia.
  • Its prevalence as canker or fruit rot is dependent on seasonal rainfall patterns.

 

Symptoms and recognition

Cankers

  • These initially appear as sunken areas of bark around buds, leaf scars, shoot bases or open wounds.
  • As the canker develops the centre dies and bark flakes off.
  • Old lesions show as flaky dark brown strips of bark surrounded by swollen wound tissue.
  • Red or white fruiting bodies may be present.
  • Young cankers, particularly those on young shoots, tend to have white fruiting bodies (conidial spore masses – asexual state).
  • White fruiting bodies tend to be present in the summer and early autumn, whereas red fruiting bodies or perithecia (sexual state) are present in autumn, winter and spring.
  • Shoot dieback due to canker is common in canker prone orchards in early summer.
  • Cankers on wood may result in wilting and/or browning of leaves and blossoms on the branch above the canker, which may occur even before the branch is girdled.
  • Trees infected with canker show brown staining in the wood when cut which can usually be traced back to a canker.
  • Both the leaf symptoms and wood staining are thought to be due to the production of toxin by the N. ditissima fungus.
  • Similarly, blossoms wilting as a result of Neonectria canker located further down the branch can be confused with blossoms wilting or dying due to blossom wilt, fireblight or bud moth.

Diagnosis of cankers

Disease/problem Canker description Fruiting bodies Canker location
Blossom wilt(Monilinia laxa f.sp. mali) Brown/cracked, distinct light/dark zones of infection Grey pustules in spring Fruiting spur, base of fruiting spur, branch
Brown rot(Monilinia fructigena) Brown/cracked, distinct light/dark zones of infection Buff pustules in summer Fruiting spur, base of fruiting spur, branch
Apple canker(Neonectria ditissima) Distinct cankered areas. Initially sunken areas around bud, leaf scar, shoot base, or wound. Older cankers, flaky brown bark strips surrounded by swollen tissue. Sometimes papery bark on cankered young shoots. White/creamy pustules especially on young cankers in summer. Red pin-head sized fruiting bodies in autumn and winter, which can be confused with eggs of fruit tree red spider mite. Young shoots causing shoot dieback, shoot bases, branches of all ages, tree crotches, branch angles, main trunk especially of young trees, rootstock.
Fireblight(Erwinia amylovora) Cankers indistinct, associated with dieback. Cankered area slightly sunken and darker than healthy tissue with separating crack. Internal tissue water-soaked with red/brown streaks. Milky bacterial ooze. Shoot dieback. Disease progression to branch
Coral spot(Nectria cinnabarina) Cankers indistinct, associated with twig or branch dieback Pinkish pustules in summer. Pinhead –sized red fruiting bodies in winter Shoot /branch dieback. Often associated with pruning snags.
Papery bark(physiological) Initially pale blister-like swellings which eventually develop into peeling papery bark.   On young shoots and older branches. Often associated with excessive soil moisture.
Silver leaf(Chondrostereum purpureum) Associated with pruning wound. Blistering and papery bark near wound. Affected wood if cut is discoloured. Foliage on tree or tree part is silvered. Small bracket fruiting bodies (creamy-coloured above and purple below) arise on affected tree parts once they die. On large branches, associated with pruning wounds especially major tree restructuring.
Perennial (Neofabraea – formerly Gloeosporium) canker (Pezicula malicorticis) Distinct cankers. Initially small circular brownish / purplish spots that develop into elliptical cankers separated from healthy tissue by crack. Bark in affected area sloughs off Cream-coloured fruiting bodies develop on the cankers. Associated with wounds, either pruning, frost cracks etc.

Diagnosis of wilting dying blossoms

Disease/Problem Blossom symptom Fruiting bodies Smell Othersymptoms
Blossom wilt(Monilinia laxa f.sp. mali) Wilting/brown,Internal browning/necrosis Grey pustules on infected parts Fetid smell, similar to scent of sweet chestnut flowers Disease progression into spur and branch forming cankers
Apple canker(Neonectria ditissima) Wilting/brown, no internal browning None None Nectria canker somewhere on branch with wilting blossoms
Fireblight(Erwinia amylovora) Wilting/brown, internal browning/necrosis Milky bacterial ooze on infected flower parts None Disease progression into spur and branch, possible further ooze
Bud moth(Spilonota ocellana) Wilting/brown blossom. Hollow None None Evidence of internal mining, caterpillar and frass

Fruit rot

  • The fruit rot occurs on the eye, the stalk end or on the cheek.
  • The rots are soft, slightly sunken, with the rotted part easily scooped out from the sound flesh.
  • Eye rots are visible in the orchard from early summer as well as in store. They are usually brown in colour with white/creamy cobwebby sporulating pustules visible on mature rots.
  • Cheek and stalk-end rots only appear in store and are circular, brown with pale brown centres.
  • Neonectria rots appear in cold-stored fruit from late December onwards and increase in incidence the longer the fruit is stored.
  • The rot colour depends on variety and storage conditions.
  • Rots on fruit stored in low oxygen tend to be green in colour with very little sporulation.
  • Those in higher oxygen storage tend to be brown with white/creamy sporing pustules.

Other problems that may be confused with apple canker

Cankers

Many other fungi cause cankers on apple trees.

  • The most common are blossom wilt, brown rot, perennial (Neofabraea – formerly Gloeosporium) canker.
  • Neonectria cankers can usually be readily distinguished from these because they are distinct cankers, rather than die back, and because of the presence of white or red fruiting bodies.

Fruit rots

Neonectria fruit rot can be confused with rots caused by Neofabraea – formerly Gloeosporium spp. or Penicillium spp. These rots similarly occur at the stalk, cheek and calyx end of the fruit.

  • Those caused by Penicillium spp. are usually squashier, paler green in colour with pure white or turquoise-green spore pustules present.
  • Rots caused by Neofabraea – formerly Gloeosporium species may only be distinguishable by microscopic examination of spores, if present, or culturing the fungus on to agar media.

Disease cycle and epidemiology

Canker phase

Neonectria can only infect through wounds such as those caused by pruning, mechanical damage, manganese toxicity, frost damage, woolly aphid, wood scab etc. or through natural openings such as bud scale, fruitlet scars, fruit scars and leaf scars.

  • Newly exposed leaf scars are extremely susceptible to infection and spores landing on scars within 24 hours of leaf fall are readily drawn into the tracheids or water-conducting vessels of the shoot.
  • Leaf scars become increasingly resistant to infection 48 hours after leaf fall in autumn.
  • In early spring or summer (June or earlier if very wet) white pustules containing conidia are produced on cankers, especially young cankers.
  • Conidia are short lived, are spread short distances by rain splash and are mainly responsible for fruit infection, which can be considerable in wet summers.
  • In late summer/autumn red fruiting bodies (perithecia) are produced on older cankers.
  • Spores (ascospores) are released from these during rain in autumn, winter and spring and spread by wind and rain to new infection sites.
  • Ascospores are responsible for long distance spread of the disease.

The time of maximum ascospore release varies considerably between region, country and season. Neonectria spores and potential infection sites are available all year round, but there is considerable variation between countries with regard to the season considered the most important for infection.

  • In England, usually the most important time for infection is during autumn leaf fall. Wet weather during leaf fall usually results in a high incidence of cankers on young shoots causing dieback the following spring/summer.
  • In contrast in Northern Ireland, approximately 75% of infections occur in spring.

Such patterns of infection however, are not rigid and are very much dependent on seasonal rainfall patterns. What is most important is an understanding of when apple trees are most susceptible. These timings are listed below:

  • Bud burst
  • Bud scale scars
  • Summer leaf fall (especially on Cox)
  • Fruitlet drop, usually June
  • Summer pruning
  • Fruit harvest
  • Autumn leaf fall
  • Winter pruning
  • Frost cracking

Although spores can germinate at 0oC and orchard infection can occur between 5-16oC, most infection occurs between 10 and 16oC.

  • Rainfall is essential for spore production and dispersal.
  • A minimum of six hours surface wetness is required for infection of fresh leaf scars.
  • The actual hours of surface wetness necessary for infection will depend on temperature.
  • After infection the fungus can remain latent for some time before external symptoms become visible.
  • In young nursery trees, inoculation experiments have shown that the fungus may remain symptomless up to three years or more after infection.

Fruit rot

Fruit infection occurs on the tree through the calyx, lenticel or stalk end and takes place between blossom and harvest.

  • Recent inoculation experiments have shown that fruit is most susceptible to infection at blossom and petal fall. Fruit susceptibility then declines in summer with a small increase in susceptibility before harvest.
  • Fruit infection that occurs in late bloom may develop into visible eye rot in the orchard or remain latent and develop in cold store.
  • The factors that determine whether infection develops into eye rot or remains latent are not clear.
  • Infected apples in the orchard rot and mummify. These mummies can then act as a source of inoculum.
  • The resistance of young Bramley apples to Neonectria is thought to be related to the presence of benzoic acid in apples, the toxicity of which decreases as the fruit matures.
  • Controlled atmosphere storage also influences the development of Neonctria rot in store.
  • In Bramley, concentrations of CO2>5% v/v in the fruit store atmosphere progressively inhibit the production of benzoic acid and hence increase rotting due to Neonectria.
  • Storage under ultra low oxygen regimes also increases the incidence of Neonectria rot.
  • Hence storage regimes for Bramley of 5% CO2 and 1% O2, used as an alternative method to post-harvest treatment with DPA for control of superficial scald, will increase the incidence of rotting due to Neonectria in stored fruit from cankered orchards.
  • Rot development in fruit stored at 1-2oC (e.g. Gala) is reduced compared to that in fruit stored at 3.5-4oC (e.g. Cox and Bramley).

Factors affecting tree susceptibility to N. ditissima and canker development

Many factors affect the susceptibilty of the tree to canker. These include climate, variety, rootstock, soil type, water content, pruning and fertilizer regime.

  • Applications of excess nitrogen fertilizer, especially farm yard manure, increase the canker risk.
  • A high water table and heavy clay acid soil will also increase the incidence of canker.
  • Factors that put the tree under stress, including water stress on light sandy or thin chalky soils, also appear to increase the canker risk.
  • Shoot growth also appears to influence canker development, such that when shoot growth is rapid canker development is increased.

Varietal susceptibility

All varieties are apparently susceptible to canker to some degree and a variety can vary in its degree of resistance between localities.  Rootstock can also influence the susceptibility of the scion variety.

  • Trees on less vigorous rootstocks such as M.9 tend to be more canker susceptible.
  • Cameo, Gala, Spartan, Fiesta, Discovery, Mackintosh, Ida Red, Red Delicious are considered very susceptible.
  • Cox, Bramley, Jonagold are moderately susceptible and Grenadier has low susceptibility.
  • Varietal susceptibility to N. ditissima may be linked to the size of the xylem or water conducting vessels in the wood.
  • More susceptible varieties such as Spartan tend to have larger vessels.

Susceptibility of some apple varieties to fungal diseases

Variety Scab Powdery mildew Nectria canker Blossom wilt
Braeburn m m m ?
Bramley h m m vl
Boskoop m m ? ?
Cameo h m h ?
Cox h h m m
Crispin vh h h vl
Ceeval m m ? ?
Discovery vl (pg) vl (pg) h vl
Early Victoria l l ? ?
Edward VII l vl ? ?
Egremont Russet m m m vl
Elstar h m m ?
Falstaff m l l vl
Fiesta h m h vl
Florina l (vf) l ? ?
Gala vh m h vl
Greensleeves m l ? ?
Grenadier vl (pg) vl (pg) vl vl
Golden Delicious h h m vl
Idared h h m vl
James Grieve m m m vh
Jonagold h m m vl
Judeline l l ? ?
Kent h m h vl
Liberty m (vf) m ? ?
Lord Derby l m m vh
Lord Lambourne l vl ? l
Laxton Superb m m m vl
Red Charles Ross m m ? ?
Saturn vl (vf) m ? ?
Spartan m m vh vl
St. Edmunds Pippin m m m m
Topaz vl (vf) m l ?
Vanda vl (vf) h l ?
Winston h m ? ?

vl = very low, pg = polygenic resistance, vf = major gene resistance

l = low susceptibility, m = moderate, h = high, vh = very high

Canker in new apple orchards

Canker on trees in newly planted apple orchards can arise from two sources. Either from the nursery of tree origin as symptomless infection, which can take up to 2 or 3 years to express itself or spread in from existing canker in an adjacent orchard.

  • The relative importance of these inoculum sources varies according to site.
  • A new theory on canker in young orchards questioned the accepted view, but recent research using DNA fingerprinting techniques has confirmed that canker in new orchards may have spread in from adjacent cankered orchards and/or be introduced from the nursery as symptomless infection on young trees.

Disease monitoring

Canker

  • Inspect orchards in late June for shoot dieback, and during winter pruning for larger cankers.
  • Assessment of Neonectria rots (i.e. eye, stalk and cheek rots), during grading, in fruit stored post Christmas, will also give an estimate of the canker problem in the orchard.

Fruit rot

The risk of Neonectria fruit rot in store can be estimated pre-harvest, based on the incidence of cankered trees in the orchard, the rot history taken from packhouse records and the rainfall between blossom and harvest.

Inspect orchards in the spring for cankered trees and estimate the % cankered trees.

Canker incidence                  Risk

(%cankered trees)

No canker                                no risk

< 5%                                       low

5-25%                                      moderate

>25%                                       high

  • In orchards with more than 5% of trees with canker, where long term storage of the fruit is planned, apply fungicide sprays for control of Neonectria fruit rot during blossom and at petal fall.
  • Monitor rainfall from blossom to harvest
  • Decisions on the need for early marketing of fruit can be made as follows:

            Orchard canker risk

Orchard canker risk RainBlossom-harvest Action
High < average Market fruit pre-Christmas if no sprays applied in blossom
average
> average
Moderate < average Low risk No special action needed
average Market pre-Christmas if no sprays applied in blossom
> average
Low < average Low risk No special action needed
average
> average Market pre-Christmas if no sprays applied in blossom

Disease forecasting

The ADEM system is a PC-based system and contains a disease forecasting model for Neonectria canker and fruit rot. The disease models are driven by the following weather variables recorded on a logger and downloaded to a PC:

  • Rainfall
  • Surface wetness duration
  • Ambient temperature
  • Ambient relative humidity

The models use the weather data to determine the favourability of the weather for Neonectria infection of fresh leaf scars and near-mature fruits and indicate the incidence of disease likely to occur at these two infection sites.

Forecast of cankers

The model assumes that the inoculum level is high and varietal susceptibility to Neonectria is high.

  • In addition to weather, the system requires inputs on the age of leaf scars based on inputs on date for leaf fall – 10%, 50%. 90% leaf fall.
  • Currently the model for leaf scar infection is not used in practice since it records infection that has already occurred and as there are no fungicides with eradicant action against N. ditissima and so it is difficult to make use of the information.

Forecast of fruit rot

  • The model will be revised to include new data on fruit susceptibility to Neonectria rot from recent inoculation experiments.

 

Chemical control

Cankers – paints

Wounds do provide entry points for Neonectria and if used correctly paints can provide protection.  Canker paints contain chemicals active against Neonectria and are applied to pruning wounds or pared back cankers to provide temporary protection against infection while the tree develops its own protective callus layer. The value of treating pruning wounds with paints is often questioned, for if they are not treated rapidly the paint can act as a seal on fungal infection that has already occurred.

  • Treat pruning wounds on trees, especially those in cankered orchards and in young orchards where it is essential that the scaffold branches remain canker free.
  • Paints must be applied to the wound or pared back canker immediately, and at the very least within one hour of pruning.
  • Apply to larger cankers after paring back, especially those on the trunk or scaffold branches.
  • Time of application is not specified but should be applied as soon as possible after the pruning or paring wound is made.

Canker – sprays

Fungicides with good activity against Neonectria are limited.

  • Copper fungicides (e.g. Cuprokylt) give good prolonged protection against Neonectria, but are phytotoxic and can only be used post-harvest and pre-bud burst.
  • Fungicides that are mainly active against apple scab such as dodine, dithianon and captan, also have some protectant action against Neonectria.
  • Similarly pyraclostrobin + boscalid (Bellis) and cyprodonil + fludioxonil (Switch) are active against scab and storage rots and will also give some control of canker and Neonectria rot.

Control of Neonectria in orchards presents a particular challenge. Entry points for infection are available all year round, inoculum (either conidia or ascospores) is available all year round and the rain, essential for Neonectria sporulation and infection, often makes timely spraying impossible. Therefore the strategy for control, especially in cankered orchards, must be to protect at key times to limit infection.

  • In canker risk orchards, apply a spray of a copper fungicide before bud burst.
  • Apply dodine or dithianon spray at bud burst and mouse ear to provide protection on bud scale scars.
  • Thereafter use dithianon or captan as part of the scab control programme. These products will give some protection against canker.
  • Dithianon + pyraclostrobin (Maccani) or pyraclostrobin + boscalid (Bellis) or cyprodonil + fludioxonil (Switch) will also give some control.
  • In orchards with low canker incidence at autumn leaf fall, apply a spray of a copper fungicide at 10% leaf fall and repeat at 50% leaf fall. Copper also speeds up leaf fall and reduces the time when trees are susceptible to infection.
  • In orchards with moderate to high canker incidence, apply a spray of tebuconazole (Fathom) before the end of leaf fall, a spray of a copper fungicide at 10% leaf fall, then a spray of tebuconazole (Fathom) at 50% leaf fall with a second copper spray at 90% leaf fall.
  • Tebuconazole applied post-harvest but before leaf fall is reported to harden the wood of apple shoots and reduce their susceptibility to Neonectria infection.

Fruit rot – sprays

Recent inoculation experiments have shown that fruit is most susceptible to infection at blossom and petal fall. Therefore it is important to apply protectant sprays at this time.

  • Apply sprays of captan or pyraclostrobin + boscalid (Bellis) or cyprodonil + fludioxonil (Switch) during blossom and at petal fall.
  • These will give fruit some protection against Neonectria rot and in orchards with a high canker incidence, are essential if fruit is to be stored without significant losses beyond Christmas.
  • The same treatments can be applied pre-harvest in late July and August.

The recent inoculation studies indicate a slight increase in fruit susceptibility to Neonectria ditissima pre-harvest, but it is not known whether there is any benefit from additional sprays at this time and there is the risk of fungicide residues in fruit from the late applications.

  • In orchards where a high canker risk has been identified, the best option may be to avoid chemical treatment and schedule the fruit for early marketing before Christmas to minimise losses.

Avoiding fungicide resistance

  • The risk of resistance developing to fungicides is minimal as either the fungicides are multisite compounds, such as captan, or they are rarely sprayed intensively.

Cultural control

  • Remove Neonectria cankers from orchards during winter pruning. Smaller cankers can be pruned out completely. Larger cankers on the trunk or scaffold branches can be pared back to healthy tissue and treated with a suitable wound protectant paint immediately after.
  • Cut out shoot dieback due to canker in the spring.
  • Avoid pruning in wet conditions.
  • Un-macerated (un-pulverised) cankered prunings left in the tree row can continue to produce spores (ascospores) for at least 1-2 years and therefore are a canker risk.
  • In areas where conditions favour canker, remove prunings from the orchard and burn.
  • If this is considered too costly, alternatively dump all prunings, including young shoots, in the grass alleyway and pulverise to ensure rapid decay.
  • Repeated mowing of grass and prunings will increase the speed of breakdown.
  • Avoid dumping young cankered shoots in the tree row as these can generate more inoculum.
  • Remove mummified fruit from trees and from under trees and either remove from orchard or throw into alleyway to be macerated.
  • Prune trees to open and encourage air circulation to improve tree drying out and reduce surface moisture and conditions favourable for canker.
  • Avoid use of high nitrogen, especially farmyard manure as that will encourage canker.

Biological control

Research has indicated that biological control of leaf scar infection is at least feasible. The tissues of leaf scars become invaded by various host-specific fungi, common primary saprophytes and bacteria.

  • Experimentally an isolate of Bacillus subtilis was found to be highly antagonistic to N. ditissima when applied to leaf scars immediately after leaf removal.
  • Other potential antagonistic fungi included Alternaria spp. and Trichoderma viride.
  • So far no biocontrol agents for Neonectria have been developed commercially.

Best practice compared to standard farm practice

Given how widespread canker is in UK apple orchards and the significant tree and fruit losses it causes every year, AHDB Horticulture funded Project TF 167 to compare the level of control achieved by a best practice control programme compared to a standard programme used by commercial apple growers. The project, led by East Malling Research used two commercial orchard sites, both planted with Gala on M9 rootstock. The effect of a full integrated progamme for canker control on the incidence of new cankers, Neonectria fruit rot and shoot growth, was compared with that of a standard fungicide programme with no addditional specific measures for control of canker.

Standard fungicide programme

  • Cuprokylt applied at 10% and 50% leaf fall

Fully integrated programme

Standard fungicide programme + additional sprays for canker at key times as follows:

  • 2-3 at bud burst- bloom (protection of bud scale scars and late frost damage)
  • Petal fall
  • Petal fall + 2-4 weeks (summer leaf fall)
  • June/July (fruit thinning)
  • August (summer pruning)
  • Sprays for fruit rot at bloom and pre-harvest
  • Tebuconazole (Fathom) applied pre-leaf fall (October), Cuprokylt at 10% and 90% leaf fall and Fathom at 50% leaf fall

The results of the three year study clearly showed that application of an intensive integrated programme for canker control resulted in a significant reduction in numbers of new cankers and Neonectria fruit rot. However, such an intensive programme is costly and can result in detectable residues in fruit at harvest.

Further Reading

Swinburne T R 1975. European apple canker. Review of Plant Pathology, 54, 787-799. Compendium of Apple and Pear Diseases edited Jones and Aldwinkle. APS Press 1990.