Apple scab is one of the most common diseases on apple trees and other plants in the apple family. This disease pops up everywhere apple-family plants grow.
If you’re growing any of these trees, it’s essential to understand apple scab. You must know what to look for and how to treat this common, destructive disease to maintain a healthy orchard.
Recognizing Apple Scab
Venturia inaequalis is a fungal pathogen that causes apple scab. It’s much more common in areas with cool, wet springs. It also tends to spread quickly in orchards where last season’s fallen leaves are left to rot at the base of the trees.
Apple scab attacks both the leaves and fruit of the tree. It produces lesions on the leaves, buds, flowers, and fruit. These lesions can eventually weaken the health of the whole plant, and they can also destroy your apple harvest for the year.
Many people think that apple scab can also afflict pear trees. Pear trees are plagued by the closely related fungus V. pirina, known as pear scab.
Pear scab and apple scab, while technically different pathogens, behave similarly and respond to similar treatments. But they can’t cross-contaminate. So your apple trees are safe from pear scab, and your pears are safe from apple scab.
Apple Scab on Leaves and Flowers
Usually, you’ll start to see signs of apple scab in the spring – particularly after a stretch of rainy weather. You’ll notice small, round spots that are dark olive green or tan with fringed, light green or dark brown borders.
If you feel them, you’ll notice a soft, almost velvety texture to the spots. Then, as the disease progresses, the spots darken to nearly black. They grow together and spread across most of the leaves. Finally, the leaves turn yellow, die, and fall to the ground.
The infection looks similar on blossoms and buds.
Apple Scab on Fruit
On fruit, the infection looks much like its name: an ugly scab on your apples. The condition starts similarly to the leaves, with small, olive-colored spots on the fruit.
As the disease progresses, the spots grow into a scabby, brown spot with raised cracks and a corky texture. Eventually, the fruit may even split along the scab.
When fruits are affected early on, they may become deformed as they grow. The scab will cause the fruit to warp as it matures. Fruits with a severe infection often drop to the ground and rot – giving the fungus a chance to overwinter.
The Lifecycle of Apple Scab Fungus
Apple scab overwinters in those rotting leaves and apples. A light, unnoticed infection from the previous year can wait in orchard refuse for the right conditions to permit a full-blown outbreak.
At some point in the growing season, when the temperatures are between 55-70°F and steady moisture is coating the orchard for at least 3 hours, that latent fungus produces spores.
Then, the fungal spores are carried by wind or splashing mud up to the tree – where they land on leaves, fruit, or flowers. A new infection is born.
The longer the leaves and fruit remain wet in the cooler temperatures, the more severe the infection can become. That long-term dampness enables the V. inaequalies spores to take a firm hold on their new surroundings.
Once the infection takes root on one leaf, it can spread quickly to other leaves and fruit. Then, if rainy, cool conditions continue, your trees can be reinfected repeatedly. The infected leaves and fruit then fall from the tree and wait in the soil for the next opportunity to infect the tree.
Preventing Apple Scab
In rainy seasons, treating apple scab can feel like a full-time job. When there’s a continual infection risk, you must keep a close watch on your trees. But you can take a few steps to minimize the chances that apple scab will impact your trees.
Start by choosing apple and crabapple cultivars that are resistant to apple scab. Ensure that some or all of your apple trees will be a dead-end for V. inaequalies spores to reduce its impact on your whole orchard.
Resistant apple varieties like ‘Honeycrisp,’ ‘Liberty,’ ‘Freedom,’ ‘William’s Pride,’ and ‘Redfree’ are a great addition to any orchard. There are also resistant crabapple trees. Try ‘Silver Moon,’ ‘Prairie Maid,’ ‘Bob White,’ and ‘Indian Summer.’
Remember that many of our best love apple varieties, like MacIntosh and Cortland, are susceptible to apple scab. That doesn’t mean you can’t grow them alongside these more resistant varieties, but it does mean you have to take extra care to keep these trees safe.
To keep your trees as safe as possible from apple scab (and a host of other diseases), keep the orchard clean. Rake away dead leaves, dropped fruit, and other debris. Keep the base of each tree well maintained so that apple scab spores have nowhere congenial to overwinter.
Make sure each tree has enough space around it to grow comfortably. If you’re planting a new orchard, space out your trees generously – so they’re in full sun, with plenty of airflow around each tree. Fresh, moving air is one of the best ways to shorten the life of fungal spores.
Of course, if you’re working with an established orchard, you can’t exactly move the trees to a sunnier spot. You may be able to remove one or two low-producing trees, though, to open up space between healthier ones.
The cleaner and more open your orchard is, the less opportunity apple scab has to infect the trees.
If you’re watering your trees, water them in the early morning, and concentrate on the tree’s base. Don’t spray water all over the whole plant. Avoid wetting the leaves whenever possible. The less water on the plant itself, the less opportunity for infection.
Some people like to use compost as an additional safeguard against apple scab. Once you’re raked away the debris from the tree’s base, spread a 3-6 inch layer of clean compost on top of the soil.
The compost gives your tree a nutritional boost and provides an extra barrier between the spores in the soil and the tree.
Treating an Infection
It’s time to take action when you see signs of an apple scab infection. If you see one or two infected leaves, blossoms, or fruits, prune them immediately to prevent continued infection. Burn or throw away these infected parts; never compost them.
Once you have an active infection, fungicidal sprays are your best bet. There are a few successful organic options on the market. Most are copper-based sprays, but sulfur-based sprays are also effective on apple scab.
On apple trees, a copper-based spray can cause discoloration known as russeting on your fruit. You may want to be very careful about how much of a copper spray you use and discontinue use around midsummer to prevent this russeting effect.
Most sprays can be used to treat an active infection and prevent infections. If you’re concerned about a potential resurgence of apple scab in your orchard, don’t wait until you see signs of infection.
Preventative spraying can start early in the year. If repeated every two weeks (depending on the manufacturer’s recommendations), a good spray, like Organocide Plant Doctor or even a simple neem oil solution, can keep infection at bay.
When Not to Spray
If your infection is already very advanced, it’s too late for most commercial sprays. Instead of spraying, do your best to minimize the spread. Then, prepare for next year by cleaning up the environment and nourishing your trees.
Light treatments of neem oil – especially at the base of the tree – are the exception here. Neem oil won’t do much to stop a full-blown infection, but it will damage the spores ability to overwinter successfully.
A few light treatments of neem oil in the soil may contribute to fewer apple scab spores.
All Purpose Sprays
A few all-purpose fruit sprays claim to be effective against apple scab. But these sprays present a few problems. Most importantly, they contain not only fungicides but insecticides.
That means that while they may be killing apple scab spores, they’re also killing pollinators.
If you use all-purpose sprays before or during flowering, you’ll be slaughtering your pollinators before they can pollinate your trees. You’ll also be killing a host of other beneficial insects.
Some all-purpose sprays are not safe for use on edible fruit at all. If you do choose to use an all-purpose spray, confirm it can be used on edible foods, and avoid using it when pollinators are out. Spray late in the evening.