Unfortunately, pests and pathogens are simply a fact of life for apple farmers and homeowners who like to grow these pretty fruit-bearing trees. From apple maggots and coddling moths to rusts and fire blight, your beloved trees live in a state of nearly constant assault.
But while these bugs and bacteria vary in prevalence and geography, there is one particularly common and insidious threat that all apple tree aficionados must be prepared to face: apple scab. Though rarely fatal, apple scab can cause problems for just about anyone growing apple trees, whether they do so for fun or profit.
We’ll explain everything you need to know about apple scab below, including the best way to identify it, some of the available treatments and a way to completely avoid having to worry about the issue at all.
What Is Apple Scab?
Apple scab is a disease caused by Venturia inaequalis – a sac fungus that is thought to be historically native to Central Asia but now found nearly everywhere apple trees grow. The fungus can affect a wide variety of plants and trees, but it seems most comfortable living on apple trees, including cultivated apples as well as wild-type crabapples.
Apple scab rarely causes the outright death of the trees it afflicts, but it can severely weaken afflicted trees, making them vulnerable to secondary infections and other environmental stressors. Accordingly, treatment and mitigation strategies are imperative for those who discover the fungus on their trees.
Apple scab can cause a number of troubling symptoms, including premature leaf drop and significant crop losses. In some cases, three-quarters of the fruit on a given tree may fall prematurely or become otherwise unfit for consumption.
Unfortunately, apple scab is often difficult to eradicate once established. It often overwinters in the dead apple tree leaves laying on the ground, until spring rains allow the fungus to become active and infect new tree growth. It typically begins by attacking new leaves, but it can spread to infect the fruit and older leaves too.
Wet weather is vital to the spread of the pathogen, so – while the fungus is found just about everywhere – it causes the most problems in areas with especially wet spring weather.
Identifying the Pathogen: What Does Apple Scab Look Like?
Apple scab is generally not too difficult to identify, but there are a few other diseases and environmental problems that may look similar. Accordingly, you’ll want to consult with a tree-care professional to obtain a positive identification before you begin trying to treat it.
Apple scab usually first appears as a series of irregular, brown to black spots on the tree’s leaves. Characteristically, these lesions begin on the undersides of the tree’s leaves, but with time, they’ll begin appearing on the upper surfaces of the leaves too.
As the season progresses, you may notice that the afflicted leaves begin dropping early – sometimes as soon as early summer. By mid-summer, you may see that your tree has become completely defoliated. Fruit damage provides another clue to the presence of apple scab, as afflicted trees often produce fruit with scabby brown or black areas. Some of the fruit may even crack or become soft in places.
How to Get Rid of Apple Scab
Successfully treating apple scab usually requires a multi-pronged approach, which includes management strategies and the application of anti-fungal agents.
Step 1: Pick Up Leaves
From a management perspective, the most important thing you can do is remove the dead apple tree leaves on the ground near your existing tree. These dead leaves will often harbor the fungus and serve as the source for successive rounds of infection. Try not to break apart the leaves while raking them up and dispose of them in a place far away from your trees.
Step 2: Prune
Also, because apple scab relies on moisture to develop and spread, you’ll want to perform a careful pruning of any afflicted tree. Obviously, you’ll want to remove any seriously damaged branches, but you should also attempt to thin out dense interior segments of the tree, to help improve sun exposure and airflow.
Step 3: Apply Fungicide
In addition to these management strategies, you’ll often find it necessary to treat afflicted apple trees with a fungicide. There are a few different options registered for use against apple scab, but multi-site inhibitors like copper- and sulfur-based antifungals are common and effective, as well as single-site inhibitors, such as Topsin-M.
However, it is important to note that there is some evidence that apple scab is developing resistance to Topsin-M and other benzimidazoles. Accordingly, many authorities recommend periodically using other antifungals to help avoid resistance. Typically, you’ll want to apply anti-fungals early in the season for the best results, although multiple applications may be necessary.
Note that many fungicides are quite toxic and require great care to use safely. You’ll need to prepare, mix and spray different fungicides in different ways, so always be sure to follow the label instructions to the letter (or, opt for professional help and avoid the danger completely). Additionally, always wear appropriate PPE – including safety goggles and gloves – when applying fungicides.
The Silver Bullet: Avoid Apple Scab with Resistant Cultivars
If battling apple scab sounds challenging and frustrating that’s because it is. Results are never guaranteed, and you may find that you spend a significant amount of time, money and effort only to watch your trees decline and crop to spoil. This is part of the reason apple farmers and universities have studied the pathogen so thoroughly.
But there is one easy and effective way to avoid apple scab completely: Simply install resistant apple cultivars. The product of nearly 100 years of work, apple-scab-resistant cultivars carry one of about a dozen different genes that make the trees immune to the fungus.
Fortunately, there are plenty of options to choose from. Researchers have created 1500-odd resistant cultivars in laboratory settings, of which about 15 are available in the marketplace. A few of the most common examples include ‘Prima,’ ‘Jonafree,’ and ‘Goldrush,’ but there are others, including:
- ‘Baujade Macfree’
- ‘Florina Querina’
Apple scab is certainly a frustrating foe for those who grow apple trees, but it isn’t an invincible threat. Just be sure to keep your eyes open for the first signs of an infestation and employ the management and treatment strategies discussed above. Alternatively, to avoid the entire problem, you can just grow resistant apple cultivars from the outset.
Just pick the path that works best for you, your trees and your goals, and you’ll likely find that you can beat apple scab and enjoy pretty trees and tasty fruit.