Excerpt from Publication 310, Integrated Pest Management
Table of Contents
Apple scab, caused by the fungus Venturia inaequalis, is a serious fungal disease of apple in Ontario infecting foliage, blossoms and fruits. Severely infected trees become defoliated and infected fruit are not marketable. Apple scab also attacks flowering crab apples, mountain ash and firethorn.
Infection and subsequent lesion development occur on both sides of leaves, usually developing first on the lower side. On leaves, young lesions are velvety brown to olive green with indistinct margins (Figure 4-116) and are often not noticeable until after petal fall. With time, olive green lesions turn dark brown to black (Figure 4-117). The number of lesions on leaves varies depending on the season and variety susceptibility. Lesions on older leaves are typically raised, dark green to gray brown with distinct margins, and cause cupping on the underside of the leaf, (Figure 4-118). Leaves that are heavily infected with scab will curl, shrivel and fall from the tree.
Figure 4-116. Young apple scab lesions appear velvety brown to olive green with indistinct margins
Figure 4-117. The olive green apple scab lesions eventually turn dark brown to black
Figure 4-118. Lesions on older leaves are raised and appear dark green to gray brown with distinct margins
Small black spots develop on fruit, enlarging more slowly than on leaves (Figure 4-119). As these spots grow and become older, the centre loses the velvety appearance and becomes brown, corky and scabby (Figure 4-120). Heavily infected fruit becomes deformed and cracked when infected at an immature stage (Figure 4-121). Fruit infections occurring late in the summer may not be visible at harvest to the naked eye, but enlarge in storage to pin-head size. These lesions are "pin point scab" (Figure 4-122). The entire fruit is susceptible to scab, but lesions tend to cluster around the calyx end of the fruit.
Figure 4-119. A small scab lesion on a fruit
Figure 4-120. Older scab lesion on fruit becomes brown, corky and scabby
Figure 4-121. Scab lesions cause fruit to become deformed and cracked when infected at an immature stage
Figure 4-122. Fruit infections late in the summer may not be visible at harvest but enlarge to form "pin point scab" lesions in storage
Apple scab overwinters in infected apple leaves on the orchard
floor. During the winter and early spring, small black pseudothecia
develop in the infected leaves on the orchard floor. By early spring,
ascospores which serve as the primary inoculum for early season
infections are formed inside the pseudothecia.
Mature ascospores are discharged from the pseudothecia by rain and carried up to emerging green tissue in the trees by wind currents. Moisture - dew or rain - is necessary for ascospore discharge and germination, as well as subsequent infection of apple tissue. Olive green, velvety lesions appear 10-28 days after infection by an ascospore. The lesions initiated by ascospores result in primary infections, and in turn, produce spores called conidia.
Conidia are spread from primary lesions by splashing rain drops and wind, and initiate further infections when the combination of temperature and leaf wetness enables them to germinate and become established. These are called secondary infections, and generally occur within a tree or between adjacent trees rather than at a long distance.
The secondary cycle can be repeated many times during the growing season. With frequent rainfall, the control of apple scab becomes extremely difficult, particularly if the disease becomes established from primary infections in the spring.
Leaves are most susceptible to infection until they are fully expanded. Old leaves may again become susceptible to the fungus in late season, and previously inhibited mycelia inside the leaf tissues may resume growth, resulting in new visible lesions. This phase of epidemics in autumn has significant implications for disease management because it provides additional primary (ascospores) inoculum next spring.
Accurate weather data provides daily maximum and minimum temperatures, a necessary component in using the degree day model for assessing ascospore maturity and primary infection (described below). Reliable weather monitoring equipment in the orchard is a good investment. Various computerized weather instruments and software are commercially available. These measure temperature, relative humidity, leaf wetness to indicate ascospore maturity and when infection periods have occurred.
The incidence of fruit scab at harvest does not necessarily reflect the disease pressure in the orchard, since foliar infections continue to occur after harvest. To predict the amount of inoculum in the orchard in spring, assess the leaf scab present in the fall (Potential Ascospore Dose, or PAD). In late September or early October, before leaf fall, assess for foliar scab using the following technique.
Select a sampling scheme to sample every 10-30 trees, making sure trees are selected throughout the block. Be sure the total number of shoots examined equals 600. Here are three sampling schemes:
Use the following procedure to sample.
Do not use PAD under the following conditions.
Primary infection periods
Mature ascospores are always present in infected overwintering leaves at bud break. The number of ascospores present depends on the amount of inoculum in the overwintering leaves. The rate ascospores mature in the overwintering leaves on the orchard floor is mainly determined by temperature.
Researchers at Cornell University have developed a model relating temperature to ascospore maturation. The model is expressed in degree days Celsius (or DDC) and predicts the percentage of the season's ascospores that have matured. The model is useful for describing the beginning, peak and end of ascospore maturation in general terms - but unusual weather conditions may contribute to significant ascospore discharges earlier or later than the model predicts.
For further information, see Figure 4-123 Cumulative percentage of ascospores matured at various degree day accumulations. Daily accumulated degree days are calculated as follows:
DDC = (maximum °C + minimum °C) ÷2) - 0°C
For example, on a day with a high of 10°C and low of 2°C, accumulated degree days are 6 DDC. If either of the daily temperatures are below 0°C (i.e. a negative number) use a value of zero for that temperature. Begin degree day accumulations at bud break - the day when at least 50% of fruit buds on McIntosh are between silver tip and green tip.
Use the degree day accumulations to estimate percentage
of mature ascospores from the central curve in Figure 4-123. The
upper and lower curves are the upper and lower 90% confidence limits
around the estimates on the graph. The estimate must fall within
this range 90% of the time.
Figure 4-123. Cumulative percentage
of ascospores matured at various degree day accumulations
There are two points of particular interest on this graph.
Ascospores are released when there is rain to wet the pseudothecia
in the dead leaves. Most of the available mature spores are discharged
within two hours after the start of rain. Ascospore release is light
dependent and only a small percentage of available ascospores are
released at night from 7 p.m. to 8 a.m. Eastern Daylight Savings
Time (EDST). Under most circumstances, more than 95% of available
ascospores are not released until after sunrise when rain begins
after sunset. It is safe to assume that in low-inoculum orchards
(as defined above), primary infection periods begin at dawn when
rain begins at night.
In a low-inoculum orchard, calculate the length of the wetting period using the following method:
For a high-inoculum orchard, calculate the length of the wetting period from the start of the rain until the leaves are dry, regardless of the time of day.
Rainfall is needed for the release of ascospores, and leaves and
fruit must be wet for infection to occur. The length of the wetting
period required for infection varies with temperature. This relationship
between hours of wetting and temperature is outlined in Table 4-4
Relationship of temperature and moisture to apple scab infection.
Adapted from: Stensvand, A., Gadoury, D. M., Amundsen, T., Semb, L., and Seem, R. C. 1997. Ascospore release and infection of apple leaves by conidia and ascospores of Venturia inaequalis at low temperatures. Phytopathology 87:1046-1053.
Once the length of the wetting period is determined, calculate the average temperature during this interval and check Table 4-4 to see if leaves were wet long enough for an infection to occur.
Secondary infection periods
Secondary scab infections occur when splashing rain spreads conidia,
developed from scab lesions on leaves. As with primary infections,
secondary infections occur only if moisture from rain is present
long enough at a given temperature. Since secondary infections occur
day or night, calculate from the beginning of the wetting period,
regardless of time of day. Scab infection periods continue throughout
the summer from lesions caused by conidia.
Adapted from: Schwabe, W.F.S., Jones, A.L., and Jonker, J.P. 1984. Changes in Susceptibility of Developing Apple Fruit to Venturia inaequalis. Phytopathology 74:118-121. Cultivars tested were: Golden Delicious, Starking Delicious, Starkrimson Delicious, and White Winter Pearmain.
Periods of dew or high humidity (more than 90%) contribute to a wetting period, but are usually significant only if preceded by rain. Add together wet periods caused by intermittent rain to determine the length of an infection period, unless wet periods are separated by 10 hours or more of dry, sunny weather.
As fruit matures, it takes a longer wetting period for infection
by apple scab to occur. Table 4-5. Relationship of temperature,
hours of wetting and weeks after full bloom to secondary apple scab
infection of fruit, outlines this relationship.
Fungicides are used to control scab in most commercial orchards. See OMAFRA Publication 360, Fruit Production Recommendations for information about the activity of different fungicides available for apple scab.
Residues inactivate and kill fungal spores and prevent infection. These products are formulated to stick to foliage if it rains after the spray has dried. If it rains, a small proportion of residue goes back into solution and redistributes on tissue that was sprayed, but does not redistribute to unsprayed leaves or fruit. Coverage with these products must be complete, on both upper and lower leaf surfaces and the entire surface of fruit. Apply fungicides to expanding foliage when infections are imminent.
An after-infection spray is applied within a specific time after
the start of an infection period. Time available to apply the fungicide
effectively depends on the product used and average temperature.
To calculate after-infection activity, count the time interval from
the beginning of the infection period. For example, at 6°C apply
Captan within 48 hours from the beginning of the infection period.
Nova 40 W and Nustar are members of demethylation inhibitors (DMIs)
- a group of fungicides known as sterol inhibitors. These products
are used differently than protectant fungicides such as captan or
Sovran and Flint 50 WG are members of the family of fungicides
called strobilurins (or quinone outside inhibitors, QoIs). These
products have good protectant and post-infection activity on apple
scab and powdery mildew. Do not use Sovran or Flint 50 WG earlier
than tight cluster, and apply in blocks of two applications, 10
days apart. Due to the mode of action of this fungicide group, it
is important to carefully follow resistance management strategies.
Use another chemical family for two applications before returning
to either Sovran or Flint 50 WG. After an application of Sovran
or Flint 50 WG, tissue is protected from infection for five to eight
days. In conditions of rapid growth, shorten intervals between applications.
Vangard and Scala are members of the anilinopyrimidine family.
These products provide good protectant activity against apple scab,
but do not control other foliar or fruit diseases such as powdery
mildew or cedar-apple rust. To manage apple scab, make no more than
two applications per season and only at pre-bloom. Scala is registered
for pre-harvest control of some post-harvest diseases.
Several consecutive days of continuous wet conditions and mild temperatures are common in the spring - conditions that encourage apple scab infection. It is often difficult to maintain sufficient fungicide coverage on trees to provide protection for rapidly expanding foliage and fruit under such conditions.
If concerned about adequate protection, it is better to apply a fungicide with protectant activity during a break in the rain than to not spray at all. Use a fungicide with good retention properties - products with mancozeb such as Dithane, Manzate and Penncozeb have good retention activity.
After infection sprays are required when protectant fungicides become too diluted due to high rainfall to prevent the apple scab fungus from establishing itself on the foliage or fruit.
If an infection occurred, scab lesions become visible on the foliage and fruit in about nine days after the infection when the average temperature is 20°C - or in about 14-21 days if average temperatures are 12°C or lower. When scab lesions are present in the orchard, avoid using fungicides with high potential for resistance (DMIs, strobilurins) and focus on protecting uninfected tissue from further infections.
Strong winds hamper spray operations by causing pesticides to drift away from the intended target. Spraying during the evening or early morning when winds are light helps reduce spray drift. Lowering the spray trajectory into the wind also minimizes drift, but take extra care to ensure treetops get good spray coverage.
The end of primary season is determined through the degree day model. At 418 DDC, more than 95% of ascospores are mature and after a significant rain most of the ascospores have been discharged for the season. See Figure 4-123. After this point, wait two weeks before checking trees for scab. It may take up to 14 days for lesions to appear after a scab infection period. Be sure to check the tops of standard trees, the centre of a thick tree canopy or anywhere else spray coverage may not have been adequate.
If primary scab was controlled, lengthen the interval between sprays for the remainder of the growing season and reduce the rates of fungicides (with labeled rate range) to the lower rate. Some growers successfully eliminate all use of fungicides during the summer months where primary scab is controlled. If scab lesions are found, maintain a fungicide program for the remainder of the season.
Several fungicide families available to apple growers are prone to the development of resistance, so fungicide programs must be developed to minimize this risk. Implement these strategies to avoid or manage the development of fungicide resistant scab populations in apple orchards.
Do not apply DMI (sterol-inhibiting) or Qol (strobilurin) fungicides when scab lesions are present as this encourages resistance to develop.
The amount of inoculum in overwintering leaves is reduced by the following practices.
For more information: