Apple scab disease is a serious disease of apples and ornamental crabapples.
It is important to note that apple scab disease, Venturia inaequalis tends to attack both leaves and fruits of the plant.
It also produces lesions that you can observe on leaves, as well as blossoms, sepals, pedicels, petioles, and even the fruit itself.
Moreover, the fungal disease tends to first appear as pale yellow or olive green spots on the upper surface of the leaves.
Dark, velvety spots may also appear on the lower surface of young leaves as they are exposed to the infection.
These lesions tend to have poorly formed borders and for this reason, in some cases, you can mistake them for sooty mold or even leaf fuzz.
It is important to note that severely infected leaves and tissues tend to become twisted and puckered, turn yellow, and may drop early in the summer.
2 to 3 years of consecutive defoliation in such a way can weaken the tree significantly making it even more vulnerable to other diseases, insects, and stresses like freeze damage.
Keep on reading.
Apple Scab Disease
Apple scab is a common fungal disease that tends to affect the leaves and fruit of the plant to the point where the tree loses its leaves.
As a result, the apples are so blemished that they tend to become unfit for eating.
It is important to note that the apple scab fungus tends to thrive in areas where there is lots of rain and high humidity.
Moreover, they also thrive in warm, wet springs.
As there is no treatment for infected trees, early identification and prevention are crucial for its control.
Identifying Apple Scab
It is important to note that trees can become infected with this disease at any time during the growing season.
Depending on when your tree gets infected, the symptoms tend to be different.
The first signs of apple scab will occur on the undersides of the leaves, dull, olive-green round spots, often along the leaf veins, and on the leaves that are closest to the bulbs.
Moreover, these spots tend to gradually grow into brown or black large circles with a velvet-like surface up to half an inch across.
As this disease progress, the leaves of the plant may also become dwarfed or twisted, they will then turn yellow before falling prematurely in mid-summer.
The first signs of this fungus on your tree are olive-green spots that will gradually turn into brown or black lesions with a rough, wart-like, or corky surface and cracks in the fruit.
It is important to note that the immature fruit may also drop.
Furthermore, infections late in the season can lead to black spots on the apples.
In case the infection occurs just before harvest, these symptoms will only show when you store the apples, hence the term ‘storage scab’.
Causes of Apple Scab
The fungus that causes apple scabs is Venturia inaequalis.
It tends to overwinter in diseased leaves on the ground.
In the spring, the fungus in the leaves tends to produce spores and then release them into the air.
Moreover, the wind, rain, or irrigation then carries the spores to growing tips, leaves, flowers, or developing fruit on the same tree or of nearby trees.
When these spores tend to land on a wet surface and the temperature is about 70 degrees Fahrenheit, they will then germinate and start a new, secondary infection.
However, it is important to note that the spores will need a number of hours of moisture to do this.
A secondary infection can also generate new spores within 9 to 17 days.
And this disease cycle can also repeat itself many times during the growing season in warm, wet weather.
What are the Host Plants for this Disease?
The fungus Venturia inaequalis causes apple scab and tends to affect
- crab apples, Malus spp,
- mountain ash, Sorbus spp,
- Hawthorn, Crataegus spp,
- loquat, Eriobotrya japonica,
- firethorn, Pyracantha spp.
However, it is important to note that not all scab fungi are the same.
There are different, host-specific strains and the apple scab strain that affects the apples and crabapples is the same.
This also means that an infected crabapple tree can spread this disease to a nearby apple tree and vice versa.
The strain that affects mountain ash, on the other hand, will not affect apples or crabapples.
It is important to note that not apple cultivars are equally susceptible to the fungus and a number of disease-resistant apple varieties have been bred.
Moreover, a related fungus Venturia pirina tends to infect pears, i.e. Pyrus spp., and causes the same symptoms.
The apple scab pathogen, on the other hand, will not infect pears and in the same way, the pear scab pathogen will not infect apples.
Damage from Apple Scab
It is important to note that trees that are infected with apple scab will be unsightly.
However, the damage tends to be far-reaching.
If your tree loses it leaves prematurely and if that happens for a number of years in a row, the tree will become weaker.
Moreover, its growth will be affected and it will bloom less, will be prone to winter injury, and will be more susceptible to other diseases and pests.
The apples from the infected tree can also become unappealing and deformed so you may not be able to use them for apple cider at best, or they may be unfit for eating altogether.
Preventing Apple Scab
The best way to prevent apple scab is to choose disease-resistant varieties.
Their resistance often ranges from moderately resistant to very resistant.
In case you have your mind set on a variety that is susceptible to apple scab, you can plant a dwarf or semi-dwarf tree.
This will allow you to apply a preventive fungicide on the entire tree more about the below.
Moreover, proper sanitation is key to preventing the spread of the fungus.
Make sure to thoroughly remove fallen trees, not only in the fall but also during the growing season.
However, if apple scab is present, do not throw those leaves in the compost, however, make sure to dispose of them in the garbage.
On the other hand, pruning is another important measure you can take to control apple scab.
In a tree with an open canopy where the leaves can dry quickly after a spring shower, the spores are less likely to germinate.
Furthermore, spraying an infected tree with a fungicide is only an option if the tree has lost most of its leaves for three or more consecutive years.
Apple Scab Fungicide Treatment
It is important to note that fungicides against apple scab can only protect healthy trees from getting infected, they will not help get rid of the fungus on the infected tree.
Once the tree begins to show symptoms of the disease during the growing season, spraying a fungicide will not have any effect.
All that a fungicide tends to do is create a protective coating so that the spores that land on the leaves or fruits cannot become active.
Thereby breaking the disease cycle on the tree.
Make sure to avoid applying indiscriminately a fungicide as a preventive measure on your apple or crabapple trees.
However, only spray your tree that has been infected by the fungus.
Again the rule of thumb is if your tree has lost its leaves prematurely for three consecutive years.
Moreover, it is important to note that timing is key.
Make sure to spray when the bud grows behind and repeat with the frequency and in the interval specified on the fungicide label.
When and how often to apply a fungicide will also depend on the weather conditions in your area and the severity of the infection.
Before you spray again, make sure to always thoroughly inspect the leave for symptoms to decide whether another application of the fungicide is indeed essential.
It is important to note that there are different types of fungicides for eating apples and crab apples.
Thus, make sure that the fungicide you are using is suitable for eating apples.
Fungicides that are available to homeowners are products with captan, lime-sulfur, and dusting wettable sulfur as active ingredients.