By Matt Gibson and Erin Marissa Russell
Are you having issues with apple scab or want to make sure your apple trees stay free of this disease? Maybe instead, you’re looking for a list of apple tree varieties you can plant that are resistant to apple scab or tolerant of it. Look no further—we have compiled all the information you need to know in an easy to access format, including how to identify apple scab, the symptoms of the disease, prevention methods, control advice, and a list of disease resistant varieties to avoid future issues with the fungal infection.
It’s pretty clear why the fungal disease scab got its name. Ornamental plants, vegetables, and fruits affected by the fungal pathogens that cause the disease scab develop disfiguring wounds on their skins that look like scabs. Scab pathogens can affect all exposed parts of a plant, but the fruit it bears takes the most severe damages from infection.
Warm, wet environmental conditions are a bad recipe if you’re looking to avoid fungal growth, and this type of environment is where the scab fungus grows and thrives. The pathogens continue to live in the soil long after they are born, waiting to infect any plant that tries to grow anywhere in its vicinity.
Identification and Symptoms of Apple Scab
Apple scab is a very serious disease for apple and ornamental crabapple crops caused by a soil-borne fungal pathogen (Venturia inaequalis) that attacks both the leaves and the fruit of apple and crabapple trees. The symptoms of the disease’s infection have a similar effect on both the leaves and the fruit of an infected tree.
Pale yellow or olive-green spots form on the surface of leaves, near the stem. Darker, velvety spots sometimes appear on the leaves’ surface, near the tip. Serious infections can cause the leaves to become mangled and puckered, and may even fall off the branch early in the summer. Similarly, on the fruit, tan, scabby, sunken areas appear on infected plants. These sometimes have velvety spores near the center of the lesions. As they mature, the spots grow larger and become brown and cork-like. Infected areas can also crack open, which allows secondary organisms to enter the flesh of the fruit, causing further damage. Heavily damaged fruit may drop off the vine, especially when a serious infection develops on young fruit.
Overwintering in fallen leaves and in the topmost layers of the soil, apple scab pathogens thrive in wet, cool to warm weather, spreading rapidly in temperatures ranging between 55 and 75 degrees F. Fungal spores typically develop in the spring and early summer. The spores are then carried by rain, wind, and splashing water, moving from the ground, to your plants, attaching to the leaves, fruit, or flowers when the plant is wet. The more time the plant is allowed to remain wet, the greater the consequences of the infection will be. Newly opening apple leaves are extra susceptible to infection.
Other types of scab include citrus scab, which targets lemons, limes, oranges, tangerines, and grapefruits, stone fruit scab, which targets peaches, apricots, plums, and nectarines, potato scab, which targets potatoes, beets, carrots, parsnips, turnips, rutabagas, and radishes, and cucurbit scab, which targets cucumbers, melons, pumpkins, squash, gourds, and luffa.
The symptoms of scab disease differ from plant to plant and depending on which scab-causing fungal pathogen is involved. Cucurbit scab, one pathogen which affects cucumber and summer squash specifically, caused infected foliage to form pale, greenish-gray, water-soaked blemishes. On the fruits of the affected cucumber and summer squash plants, small sunken spots formed, which were oozing like open wounds. No scab-like texture appeared, which caused some confusion, mostly in the form of misdiagnosis, as it was often attributed to insect damage.
Winter squash and pumpkins are affected by a fungal pathogen, on the other hand, that cause scorky, scabby areas, and it is often diagnosed correctly. Peach fruits that develop scab disease form multiple green-gray lesions that grow larger and eventually become raised, dark, scab-like patches. This also causes confusion with early diagnoses, until the final form reveals the culprit.
Unfortunately, early identification is of little use, as there is no treatment for plants that have been infected with scab. The only action that a gardener can take at this point, is the swift removal and destruction of all plants that show any signs of infection. Isolating, separating and destroying all plants infected with scab may help avoid the spread of the disease to other plants in your garden.
How To Control Apple Scab
An ounce of prevention is worth a pound of cure when it comes to helping the plants in your garden stand strong against pests and disease. A lot of what goes into determining whether your plants will contract a disease depends on the plant varieties you choose and how you maintain your garden. These factors are called cultural controls or environmental controls in a lot of the literature you’ll read. Although they do require you to plan ahead, and in many cases prevention methods take some elbow grease, time, and a significant investment of energy or supplies, they’re very effective.
However, we understand that sometimes you’re in the middle of a growing season and find yourself battling an infestation or disease, and in those cases, you need help now. Unfortunately, there is no effective treatment for an apple scab outbreak. Even if your trees are already infected, however, we urge you to go ahead and learn about the prevention methods as well, although it may seem useless at the moment. The value is that once you know how to avoid ever dealing with a disease in the first place, you can put those prevention methods into practice in your garden after you’ve cleared up the outbreak you’re handling right now. In many cases, facing a disease or pest one season makes it more likely the problem will return in later years, so it’s important to be prepared for repeat offenders.
Preventing Apple Scab
Working to prevent plant diseases and pest infestation is the reason you’ll see a lot of gardening advice that may seem fussy when you don’t yet know the reason behind it—like when experts recommend cleaning and sanitizing your tools and equipment between every plant you work with, watering plants at the base so moisture doesn’t splash their foliage, clearing out plant debris and leftover roots before winter, or quarantining newly purchased plants to ensure they’re healthy before you add them to your garden, for example.
Although these practices can be a little time-consuming or require you to do a bit of planning before you plant (sometimes whole seasons before you plant), these environmental and cultural controls are the best way to ensure your plants stay healthy, especially in varieties susceptible to diseases like apple scab that have no effective treatment. Keep reading to learn how to prevent apple scab in your garden.
#1: Select Varieties Resistant to Apple Scab When You’re Choosing What to Plant.
If you’ve already planted your garden and are dealing with a. apple scab problem that’s already in process, it’s too late to use this tip—but it’s a good idea to use this as a best practice in the future. If you’re planning a new area of the garden and have had issues with apple scab in the past, or you’ll be planting a species that’s particularly susceptible to apple scab, it’s a good idea to make a point of opting for resistant varieties of plants. These varieties have either been purposefully bred to stand strong against apple scab or they’ve evolved to have a resistance after generations of fighting off apple scab. Either way, picking resistant varieties when you shop for new plants for your garden significantly lessens the chance you’ll be faced with an outbreak of apple scab.
However, there are strains of apple scab that are immune to the Vf gene that is responsible for scab resistance in most apple varieties. Those immune types of apple scab are located in Europe, Illinois, Indiana, Ohio, and New Zealand. If you are gardening in one of these regions, you should opt for apple types that use a gene other than Vf to make them resistant. Alternative scab resistant genes include Vr, VM, and VA. Gardeners in regions with apple scab that’s immune to the Vf resistance gene can also choose to plant apple varieties such as Akane or Honeycrisp, which are not technically immune to scab but still have shown a strong tendency toward resistance. Suncrisp is another such tolerant variety that isn’t immune to apple scab but doesn’t tend to have problems with it, but Suncrisp has the drawback of being susceptible to fire blight.
Here are lists of some notable plant varieties that are resistant to apple scab. Whie the lists are not all-encompassing, they include the most common and most popular varieties resistant to apple scab out there. The list after the resistant varieties includes apple types that aren’t resistant but have shown strong performance against apple scab. These types are sometimes referred to as “tolerant” varieties.
Varieties Resistant to Apple Scab
Crimson Crisp: Prone to fire blight; about 144 days to ripeness.
Crimson Topaz: Prone to fire blight; about 152 days to ripeness.
Dayton: About 148 days to ripeness.
Eden:About 155 days to ripeness.
Enterprise: About 188 days to ripeness.
Freedom: About 165 days to ripeness.
Galarina: About 129 days to ripeness.
Goldrush (also called Co-Op 38): About 204 days to ripeness.
Initial: About 129 days to ripeness.
Jonafree (also called Co-Op 22): About 148 days to ripeness.
Juliet (also called Co-Op 43): About 171 days to ripeness.
Liberty: About 152 days to ripeness.
Pixie Crunch: About 148 days to ripeness.
Prima: About 127 days to ripeness.
Priscilla: About 131 days to ripeness.
Pristine: Prone to fire blight; about 85 days to ripeness.
Redfree: Prone to fire blight; about 106 days to ripeness.
Scarlett O’Hara: Prone to fire blight; about 148 days to ripeness.
Sir Prize: About 155 days to ripeness.
Sundance: About 172 days to ripeness.
William’s Pride: About 100 days to ripeness.
Winecrisp: About 190 days to ripeness.
Varieties Tolerant of Apple Scab
Akane (also called Tokyo Rose): About 136 days to ripeness.
Crimson Crisp (also called Co-Op 39): About 154 days to ripeness.
Initial (also called X 6163): Prone to fruit drop; about 129 days to ripeness.
Nova EasyGro: Moderately susceptible to fire blight; about 148 days to ripeness.
#2: To help your trees stand strong against apple scab, pair a limited fungicide routine with resistant varieties.
Even when you plant varieties resistant to apple scab, please be aware that, according to the Michigan State University Extension, you should also protect your tree with a limited fungicide routine as well. Using fungicide in conjunction with planting resistant varieties lessens the chance that the apple scab pathogen will eventually win out against the resistant genes of your plants. There are plenty of organic fungicide options out there on the market that you can use regularly without worrying about whether they contain harsh chemicals or will end up having a negative effect on your garden. The best time to apply a fungicide is when the temperature has climbed above 60 degrees Fahrenheit and the leaves or blooms on the trees are damp from rain or morning dew.
The fungicides also offer the benefit of protecting your plants against other diseases they might face, including common apple tree problems such as apple rust, sooty blotch, fly speck disease, and powdery mildew.
If your garden will include some resistant varieties and some that are not resistant, the Michigan Extension also advises keeping the resistant and nonresistant trees in separate areas, making it easier for you to use the appropriate fungicide programs for each group. Dwarf and semi-dwarf tree types are the easiest to thoroughly cover when you treat with fungicide, so if you don’t opt for resistant or tolerant apple varieties, consider choosing smaller trees to make the most of your fungicide applications.
Fungicides that experts recommend you use to prevent apple scab include those listed below. Be advised, however, that these treatments may not protect your trees against other common apple tree diseases.
- Bacillus subtilis, also called Serenade: Effective as a preventive when applied previous to the day of harvest or on the day of harvest except for in the Northeast; also controls powdery mildew and fire blight (partially, when used alone and applied to blooms before infection or within 24 hours of infection)
- Elemental sulfur: Do not apply within 7-10 days of horticultural oil.
- Liquid lime sulfur: Do not apply within 7-10 days of horticultural oil.
- Copper fungicide: Controls apple scab for seven days after treatment; not to be used on sweet cherry trees. If applied between half-inch green and blossoming, expect fruit russeting. If applied between bloom and the period around Fourth of July, you may see blackened lenticels. Late July application controls sooty blotch and flyspeck quite well, but will severely discolor yellow apples. If using near peach trees, be aware that built-up residue can cause serious leaf injury.)
#3: Cleaning isn’t just for spring—tidy up your garden before winter sets in to keep apple scab and other diseases or pests at bay.
Like many pathogens or garden pests, the fungi behind apple scab generally spend their winters in garden debris like fallen fruits or dead apple tree leaves. Then, once the temperature begins to creep up and rainfall increases to signal spring, the scab spores launch from their hiding places and hitch a ride on air currents, gliding and drifting until they reach the newly sprouted leaves of the apple trees they’ll infect. Before fruit buds open, they’re much more likely to contract scab than mature fruits are, likely because of this springtime exodus from hibernation.
Don’t let apple scab fungi and other pathogens or garden pests be wintertime squatters in your garden. Take away their secret hideouts, and they’re much more likely to find somewhere else to overwinter. After the harvest season is over and before the bare chill of winter sets in, set aside some time to do a thorough cleaning of the ground around your apple trees as part of getting your garden ready for winter. Make sure the ground is free of any fallen fruit, and rake up all the leaves from the ground. Your goal should be bare dirt under the trees, without so much as a leaf or twig where a bit of apple scab fungus can find refuge.
You can add the leaves and fallen fruit you clean up to your compost heap as well, but only include fruit, leaves, and other debris in compost if your plants have been healthy. If your trees have had any problems with illness or infestation, even if you’ve beaten it, they should be discarded instead of composted to avoid inadvertently re-introducing that issue you worked so hard to overcome.
In fact, it’s best to do a cleanup like this periodically throughout the growing and harvest season as well as every once in a while during the winter. As we mentioned, it isn’t just apple scab that takes sanctuary in garden debris, particularly during the coldest part of the year. Creating a routine to keep your garden shipshape won’t just ensure your property looks handsome; maintaining a level of cleanliness that would make Mary Poppins proud also substantially decreases your chances of facing apple scab and other diseases or infestations.
#4: When leaves cover the ground in fall, take steps to ensure they decompose so apple scab can’t hide in them.
Even before the winter, the leaves that fall from your apple trees are a prime sanctuary for apple scab and other pathogens or insects that you’d rather not stick around. You can do a few things to help the leaves decompose more quickly, simultaneously evicting garden baddies and nourishing the soil with the nutrients in the leaves. Nitrogen applied to the fallen leaves will both aid in the process of decay and entice earthworms to eat the leaves, doing their part to help the leaf layer decompose. As an alternative, you can apply a fish solution by hand, or use 16-16-16 fertilizer to encourage efficient decay of fallen leaves in your garden.
Treatment of Apple Scab
Sadly, there is no way to turn back the clock and effectively treat apple scab once it has taken hold and symptoms can be seen. The good news is that apple scab is not fatal; it will not kill your trees. However, left completely untreated, apple scab can lead to your trees dying as a result of other problems.
This happens because apple scab can cause complete defoliation, which is so stressful to the tree that when it occurs year after year, the tree is weakened to other diseases or infestations that may, unlike apple scab, be fatal. If your trees are symptomatic and an outbreak has already taken hold, this season you can expect a reduced harvest as well as fruits that show the symptoms of scab, making them almost impossible to sell. The best thing a gardener can do when they’re facing an apple scab outbreak is to start a routine that will prevent scab from returning in subsequent years.
Even if your trees are already showing symptoms and you’ll be feeing the loss of part of your crop and seeing a loss of profits from scabby fruit this year, take heart. Apple scab is a manageable disease, and unlike some of the potential pathogens out there, it isn’t fatal to your trees. As long as you learn what you can do to keep scab from coming back and put those techniques into practice, you can make sure this is the last season you have to deal with the effects of apple scab in your garden.
Learn More About Apple Scab
https://www.canr.msu.edu/news/a_review_of_apple_scab_resistant_varieties_for_commercial_growers
https://www.gardentech.com/disease/scab
https://netreefruit.org/organic/fungicidebactericide-options-organic-apple-production
https://today.oregonstate.edu/news/7-tips-controlling-frustrating-apple-scab
https://polk.extension.wisc.edu/files/2014/02/Apple-Culitvars-for-WI-A2105.pdf
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