Disease cycle and epidemiology (Apple scab)

The scab fungus overwinters in several ways:

• As the sexual state (pseudothecia) on overwintering leaves on the orchard floor or on trees as mycelium
• On wood scab lesions on shoots
• On bud scales
• On leaves remaining on shoots
• On unsealed buds or shoot tips
• As conidia in buds

The relevant importance of these overwintering states depends on the variety, the season and the amount of scab.

Scab overwintering on leaves on the orchard floor is by far the most important form of overwintering and, in cold winters, is probably the only form of overwintering.  In milder winters, scab overwintering on the tree as mycelium can also be important.  The amount of inoculum arising from overwintered leaves depends on various factors:

• Variety
• Time of leaf fall
• Leaf decomposition
• Autumn and winter weather.

In the UK, Cox leaf fall occurs fairly rapidly in autumn, whereas Bramley, Gala and Jonagold leaves may still remain on trees in mid-late December.  In addition Cox, Fiesta and Gala leaves rot down and are removed by earthworms fairly rapidly, whereas the tougher, larger leaves of Bramley and Jonagold rot more slowly and appear less palatable to earthworms.

• The later leaves fall, the greater the amount of scab fruiting bodies (pseudothecia).  Formation of pseudothecia only occurs on leaves once they have fallen.
• Once leaves have fallen pseudothecia initials form within four weeks after leaf fall.
• Then following a distinct rest or dormant period (if temperatures are above O^oC) the pseudothecia continue to mature.
• Moisture is needed for pseudothecia development.
• Optimum temperatures for pseudothecia development are 8-12^oC.
• Optimum temperatures for ascospore development are 16-18^oC.
• The factors that affect development of ascospores are similar to those that influence bud development on the apple trees, such that at bud burst there are usually mature ascospores ready to infect if weather conditions permit.

In spring when overwintered leaves on the orchard floor become wet, ascospores are forcibly released and disseminated by wind to initiate the primary infections on new growth. In areas where the inoculum source at the start of the season is almost exclusively ascospores from overwintered leaves it is possible to rationalise the early season fungicide programme based on an assessment of the likely ascospores available for infection or potential ascospore dose.

• Ascospores continue to mature and are discharged over a period of 5-9 weeks or more.
• The peak period of ascospore discharge usually occurs between pink bud and full bloom growth stages.
• Ascospores are responsible for long distance scab spread to around 100 metres from the inoculum source.

Scab overwintering as mycelium on the tree produce asexual spores (conidia) in spring which are splash-dispersed short distances (localised spread) and infect newly developing leaves on the tree.

• Wood scab pustules can continue to produce conidia throughout the season.

Scab spores germinate when they land on susceptible leaf or fruit surfaces in moisture.

• Free moisture is essential for initiating germination, but once initiated, germination will proceed as long as relative humidity is greater than 95%.
• The time required for leaf infection is dependent on hours of leaf wetness and temperature.
• The duration of the wet period required for fruit infection increases with fruit age.
• Infection takes place from 1-26^oC.  Infection is rare above 26^oC.
• After germination the fungus penetrates the cuticle and establishes itself between the cuticle and epidermis.
• Eventually conidiophores and conidia are produced.  Lesions with conidia become visible 9-17 days after infection depending on temperature.
• Conidia produced are dispersed by rain splash and wind within the tree to infect leaves and fruit.
• Several secondary cycles of scab infection may occur through the season depending on host susceptibility and scab periods.
• Late scab on mature leaves in late summer and autumn contributes most to the scab overwintering on fallen leaves.
• Young shoot tissue becomes infected in summer, visible as small blisters on the shoots.  These will mature and rupture to release conidia the following spring.

Approximate wetting period required for primary apple scab infection at different air temperatures and time required for development of conidia^a

TABLE

^aAdapted from Mills, 1994, as modified by A.L. Jones

^bThe infection period is considered to start at the beginning of the rain

^cApproximate number of days required for conidial development after the start of the infection period (i.e. appearance of scab lesions)