Disease cycle and epidemiology

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 Neonectria 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 1-2^oC (e.g. Gala) is reduced compared to that in fruit stored at 3.5-4^oC (e.g. Cox and Bramley).

Symptoms of Neonectria 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 sporing 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, whereas those in higher oxygen storage tend to be brown with white/creamy sporing pustules.

Other problems that may be confused with Neonectria fruit rot

Neonectria fruit rot can be confused with rots caused by Gloeosporium spp or Colletotrichum 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 turquoisey-green spore pustules present.
• Rots caused by Gloeosporium or Colletotrichum species may only be distinguishable by microscopic examination of spores, if present, or culturing the fungus on to agar media.

Disease monitoring – 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
• The rainfall between blossom and harvest.

Inspect orchards in 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:

 

Guidelines for decisions based on orchard canker risk

Orchard canker risk	RainBlossom-harvest	Action
High	Low	Market pre-Christmas if no sprays applied in blossom
	Average
	High
Moderate	Low	Low risk no special action  needed
	Average	Market pre-Christmas if no sprays applied in blossom
	High
Low	Low	Low risk no special action  needed
	Average
	High	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 – rainfall, surface wetness duration, ambient temperature and ambient relative humidity; these are recorded on a logger and downloaded to the PC.
• 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 fruit rot

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

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
• Unmacerated or unpulverised 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
• Remove prunings from the orchard and burn. Alternatively throw in alleyway and macerate up to encourage breakdown
• 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

Chemical control

Cankers

Fungicides with good activity against Neonectria are limited.

• Copper fungicides e.g. Cuprokylt FL 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 Bellis (pyraclostrobin + boscalid) and Switch (cyprodinil + fludioxonil) are active against scab and storage rots and will also give some control of canker and Neonectria rot.

Control of Neoctria 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 cyprodinil + 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

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 cyprodinil + 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. Orchard trials are planned to investigate this.
• 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.