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).