This post
is in continuation to my previous post “Diseases of Apple: Apple Scab Description” here in this post I am discussing Apple Scab Disease Cycle.
Disease
Cycle:
The fungus Venturia inaequalis
causes apple scab, which is now endemic to all apple- growing countries of the
world, is particularly severe in areas having high relative humidity and
rainfall.
In winter, the scab fungus
over-winters in infected leaves fallen to the ground. During autumn, the
fungus begins to form tiny fruiting bodies, which are embedded in the leaves
near the surface. Sacs filled with the primary or spring spores of the
fungus start to develop within the pseudothecia by late winter or early
spring. The ascospores continue to develop and mature as spring
progresses. A few spores are usually mature at the time of bud break
(green tip), and maturity progresses slowly until about the tight cluster stage
of blossom development. After this time, the percentage of mature spores
begins to increase rapidly whenever temperatures are favourable for tree
growth. Most ascospores have matured by the end of bloom. Mature ascospores are discharged into the air during periods of
rain. In daylight, discharge usually begins within 30 minutes after the
start of the rain and is largely completed within 3 to 6 hours. When
rainfall begins at night, discharge is often delayed until daybreak, although
significant night discharge can occur under some conditions. The number
of spores discharged during any one rain is determined by both the size of the
potential ascospore "crop" for the season (how many leaves were
infected the
previous year) and
the percentage of these spores that have matured since the last
discharge. Ascospore discharge usually peaks in the time from pink
through bloom, and nearly all ascospores are discharged within 1 or 2 weeks
after petal fall.
Ascospores are blown to nearby trees
by wind currents, and they germinate in a film of water on the surface of
leaves and fruit. If surface wetness continues long enough at prevailing
temperatures, growth from the germinated spore penetrates and infects the
organs just beneath the outer cuticle. Typical lesions, each bearing tens
of thousands of secondary or summer spores, appear about 9 to 17 days later
depending on temperature, although long periods of low humidity can delay their
development. Conidia are dispersed by splashing rain throughout the rest
of the season and are capable of causing new infections. This is due to
numerous additional conidia are produced on each new lesion, repeated secondary
infections have snowball or epidemic effect on disease development.
Incidence of infection is affected by
the age of leaves and fruit; young leaves generally are most susceptible 1 to 5
days after unfolding and become completely resistant from the time they finish
expanding until shortly before leaf drop in the autumn. Fruit are highly
susceptible until about 3 to 4 weeks after petal fall, but much longer wetting
periods are required for infection to occur after this time. Precise
requirements for infection of mature fruit are not known, but limited data
indicate that wetting periods must last at least 48 hours for significant
infection to occur immediately before harvest.
This fungus over winters on infected
leaves on the floor of the orchard. Spores are produced on these dead leaves at
about the time new growth first appears in the spring. Spores are discharged
during rainy periods and lodged on young leaves. If the trees are wet long
enough for the spores to germinate, infection occurs. The infection originating
from the over wintering stage is called primary scab. Infection depends on air
temperature and continuous wetness of the plant surface. At a given
temperature, the surface must be wet for a given number of hours. The
average temperatures listed in the table are calculated by averaging the
hourly temperature during
the wet period.
Research has shown that ascospore discharge is significantly reduced in
darkness, i.e. only 5 per cent of the available ascospores will be discharged
when wetness periods begin at night. Whether this 5 percent ascospore
number is significant depends upon disease pressure in particular orchard. For
example if there is a large amount of over wintered inoculum, 5% of the total
ascospore dose could cause severe disease problems. However, under the
disease pressure normally prevalent, the number of ascospores released at
night can probably be disregarded. When trees are dry for less than
eight hours between showers, add together the periods they are wet in
calculating the infection period. Spores produced on a primary scab lesion are
responsible for secondary infections. Secondary infection can occur in 12 to 20
days. Several microcomputer based disease forecasting systems incorporate
the Mill's information. Apply sprays to control secondary scab if one or two
primary scab lesions per tree are present.
