Venturia inaequalis
Apple maggots are irritating and powdery mildew is a pain, but when it comes to apple tree problems, scab is a big deal.
This disease costs commercial growers millions of dollars in damage every year and home growers endless heartache. In addition to apples, it devastates native crabapples as well.
Anywhere apples grow, scab is sure to be present, and while it isn’t a huge problem in hot regions, those growing in cool, moist climates might be in serious trouble.
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Even though this disease can be quite devastating, don’t give up on your dreams of growing apples or run for the chainsaw to take out your existing trees.
Since scab is such a big problem, experts have spent a lot of time figuring out how to deal with it, and there are some pretty good systems that you can implement.
We’ll explain the ins and outs of identifying, preventing, and dealing with apple scab in this guide. Here’s what you can expect:
What Is Apple Scab?
Apple scab is a fungal disease that attacks both apples and crabapples (Malus spp.).
Hawthorn (Crataegus spp.), mountain ash (Sorbus spp.), and cotoneaster (Cotoneaster spp.) can also be hosts.
The disease is caused by an ascomycete fungus called Venturia inaequalis, which has several different strains. Each strain is species-specific, so the strain that attacks mountain ash doesn’t infect apples.
The same strains infect both apples and crabapples, which are members of the same genus, Malus. And, cue the scary music, some of these strains have become resistant to fungicides.
Pear scab is caused by a different species of Venturia,V. pirina syn. V. pyrina, which doesn’t infect apple trees.
This disease is incredibly costly to the commercial apple industry, and about half of all crop protection products used in orchards are applied to treat scab, requiring an average of 15 applications each year. This is no minor issue.
Symptoms of Scab Fungus
As the name suggests, the disease results in corky scabs on the skin of your apples, but the damage starts well before the fruit develops.
Initially, just over a week after inoculation, the leaves develop yellow, olive green, or brown necrotic lesions.
As the disease advances, these lesions develop a clear margin and a fuzzy coating of spores that extends across the entire leaf surface. Young leaves are typically more symptomatic than older ones.
The lesions can cause distorted growth and puckering, and might even cause the leaves to crack. Heavily symptomatic leaves will drop from the tree.
The flowers and shoots might also be affected, with green lesions appearing at the base of the flowers and dark brown lesions on the shoots.
As the fruits develop, they will display yellow or brown lesions. As the fruit matures and the disease progresses, the lesions become darker and develop a corky texture.
These corky lesions prevent any further expansion in the immediate area, so as the fruit forms, it puckers and twists around the corks. The fruits might also crack and/or drop from the tree.
When they crack, the pommes are vulnerable to other pathogens and common apple pests.
Sometimes a tree will only develop symptoms on either the foliage or the fruit, other times they appear on both.
In the case of a bad infection, a tree can become completely defoliated by the fall and the harvest of usable fruits could be reduced by up to 70 percent.
If a tree is infected a few years in a row, it can become weakened, leaving it susceptible to winter damage and infection from other diseases.
Biology and Life Cycle
The fungus overwinters as fruiting bodies known as pseudothecia in leaf debris and fallen apples that are shed by the tree.
Within these develop the sexual spores called ascospores that reproduce in the debris. When the rain arrives and the tree starts to bud out in spring, the spores are ejected and carried on the breeze or water to the host.
The fungus can also reproduce asexually during wet periods.
When water is present, the spores germinate on the leaves, stems, twigs, and any developing fruit.
In addition to moisture, the fungi also need the right temperature. During cool periods with temperatures around 45°F, it takes the fungi 17 hours on a moist leaf to germinate.
But once it warms up by about 20 degrees, to 65°F, the spores only require moisture for about six hours. So during warm, damp periods the infection really takes hold.
Symptoms begin in the spring shortly after infection but the trees won’t become infected later in the year which means if we can get it under control during the spring, it’s possible to save the harvest.
Control Methods
For such a nasty disease, you might imagine that strong and potentially dangerous chemicals are required. The good news is that it can be controlled using organic methods alone.
To start with, let’s go over cultural control options.
Cultural
Currently, plant breeders are working hard to create scab-resistant apple cultivars.
Some breeders are using M. floribunda, the Japanese flowering crabapple, as a parent because this species is resistant to scab.
The cultivars ’Bonita,’ ‘Dayon,’ ‘Discovery,’ ‘Florina,’ ‘Freedom,’ ‘Fujion,’ ‘Gold Rush,’ ‘Jonafree,’ ‘Liberty,’ ‘Prima,’ ‘Topaz,’ and ‘William’s Pride’ are all resistant.
If you live in a cool, wet region, avoid ‘Cortland,’ ‘Empire,’ ‘Fuji,’ ‘Gala,’ ‘Golden Delicious,’ ‘Granny Smith,’ ‘Jonathan,’ ‘McIntosh,’ ‘Pink Lady,’ ‘Red Delicious,’ and ‘Winesap.’ These cultivars are all extremely susceptible.
You should plant your trees with the appropriate spacing for the expected mature size to ensure good air circulation, which reduces the amount of moisture on and around the trees.
Always rake up all of the fallen leaves in autumn, to prevent the fungus from overwintering on dead and dying foliage. This is very important as it’s almost impossible to fully control the disease if you don’t remove the fallen leaves.
In the late winter, add a thick layer of mulch around the base of the trees, ensuring that it doesn’t touch the trunk.
You should also keep up with your pruning. Apples produce better when they’re pruned appropriately, but appropriate pruning also improves air circulation in the canopy and allows sunlight to penetrate, denying the fungus the wet, warm conditions it prefers.
Biological
Bacillus subtilis strain QST 713 can be used as a preventative but not as a treatment. This beneficial bacteria forms a protective barrier on the tree that guards against fungus.
Serenade ASO contains this bacteria and is available at Arbico Organics in two-and-a-half gallon containers.
Serenade ASO
Apply it as a foliar spray in the early spring before buds and shoots start to develop, following the manufacturer’s directions.
Bacillus amyloliquefaciens strain D747 also works as a preventative to help suppress scab, though it won’t cure the disease. Use it in rotation with one of the fungicides mentioned below.
Bonide Revitalize
You can find Bonide Revitalize available at Arbico Organics in 32-ounce ready-to-use or 16-ounce concentrate forms.
Organic Fungicides
You can start applying fungicides just as the buds break and before the last of the petals fall.
This will kill the fungal spores before they can take hold and start breeding. It’s important to rotate between products.
Start with one product, then use a different one a few weeks later, and then you can go back to the original product.
In the past, lime sulfur was the go-to treatment for scab, but we have lots of other options now.
Leave lime sulfur to the commercial growers and choose one of the many options more suitable for home growers instead.
Copper, Bordeaux mixture, sulfur, mineral oil, and neem oil all work well and are even better if you rotate between two of them.
Bonide Copper Fungicide Powder
Arbico Organics carries one- and four-pound containers of copper powder, which you mix with water and spray on your trees.
Apply your copper and then a week later, apply one of the other options. The following week, go back to copper. Continue until the flower petals have fallen.
Scab Be Gone
Apple scab sucks, there’s no two ways about it. But the good news is that it can be controlled, unlike some apple diseases, so look on the bright side.
What symptoms are your trees showing? Do you have any questions about how to identify the problem or fix it? Let us know in the comments section below.
And for more information about growing apples and dealing with other common diseases, add these guides to your reading list next:
