Apple trees can suffer from many different diseases. Learn the symptoms early so you can treat them as best you can. Apple trees suffer from three groups of diseases – fungal diseases, viral diseases and bacterial diseases.
Apple Tree Fungal Diseases
Fungal diseases require a food source to survive – in this case, your apple tree. The fungal spores spread by wind and rain during wet cool weather between 55-75 degrees Fahrenheit in the spring and early summer and enter the apple tree in different ways – through the plant tissue, leaves, flowers, fruit, or wounds. New leaves that bud out in the early spring are most susceptible to fungal diseases as rain can splash fungal spores from one tree to the next, or splash fungal spores from fallen debris to newly budding leaves.
The most common fungal diseases in apple trees are apple scab, canker, brown rot, and powdery mildew.
Apple scab is the most common apple tree fungal disease and affects both the leaves and fruit. Apple scab forms pale yellow or olive-green spots on leaves. A severe infection could cause the leaves to twist and drop early in the summer.
On the fruit, the symptoms are similar, forming dark olive-green scabby spots that are sunken and turn a dark brown as the spots mature.
Are Apple Scab Apples Safe to Eat?
While the fruit may look unappealing to eat, they are perfectly safe to eat as apple scab does not produce any toxin. Simply cut out the affected scab, and the apple will have the same taste and texture as the rest.
Can Apple Scab Kill My Apple Tree?
Apple scab will rarely kill an apple tree. The severity of the infection will vary from year to year depending on how wet the leaves have gotten. Young apple trees may suffer more if they are severely defoliated. Make sure to prune your apple tree correctly to allow adequate airflow to dry wet leaves.
Apple canker is a fungal disease that causes a sunken area of dead bark to form on the trunk and branches. Canker fungi enter the tree through areas of damage, such as cracks in the bark, pruning cuts, leaf scars or lesions resulting from scab infections. Canker is said to be more severe in wet and acidic soil. Once cankers are formed they stay year long.
To prevent apple canker from spreading, prune infected branches back down to green wood.
There is strong evidence that canker is spread inside rootstocks, so you could be buying trees that are already infected. Always check out new trees for signs of canker, especially around the graft union, and reject any suspect trees.
Can Apple Canker Kill My Apple Tree?
Mature apple trees can sometimes grow new bark over the infected area, but more likely if the canker is not pruned off and left to spread, will cause death of entire limbs, spread to the trunk and infect – and eventually kill – the entire tree.
Powdery mildew on apple trees forms a white powdery layer and affects the leaves, buds, shoots and fruits and stunts the growth of apple trees. Powdery mildew overwinters in terminal buds that were infected during the previous season. Terminal buds infected with powdery mildew will look pinched and shrivelled. Once weather conditions become favourable and new leaves grow out of these buds, the leaves will grow abnormally – narrow, brittle, or curled and covered in a white powder.
Unlike apple scab and apple canker, powdery mildew is favoured by dry conditions. Powdery mildew develops in the absence of rain, and at temperatures between 66°F and 71°F. Temperatures below 50°F or above 86°F considerably slow down powdery mildew infections.
Maximum germination of powdery mildew occurs when the relative humidity is at least 70%. Apple trees that are pruned well and have open canopies have more air flow passing through and are less susceptible to powdery mildew than trees that have dense foliage, because the airflow dries the leaf surface and reduces its relative humidity. Plant your apple tree in full sun – this will also reduce the relative humidity versus a shaded area if your apple variety is susceptible to powdery mildew.
Can Powdery Mildew Kill My Apple Tree?
Powdery mildew will not kill your apple tree, but can cause it to grow slower with reduced vigour, and it can inhibit the formation of flower buds, so that there will be fewer or no apples produced the following season.
Black rot is a fungal disease that infects the fruit, leaves, and wounded bark of twigs and limbs of the apple tree. The fungal spores spread by splashing rains, wind, and insects.
Infected bark on branches and limbs will show reddish brown sunken areas that will grow larger each year. Infected leaves show symptoms of purple spots 1/8″ to 1/4″ in diameter which later turn yellow or brown in the centre before eventually dropping from the tree.
Black rot infested apples will start to show signs at the opposite end from the stem – the calyx – and originates from a wound that penetrates the skin of the apple, including insect bites. As the apple begins to grow, small red or purple spots will grow with the apple. As the rotting increases, it forms concentric circles of dark brown or black. As the apple rots, the flesh stays firm and eventually the apple will dry out, shrivel and die.
Can Black Rot Kill My Apple Tree?
Black rot can cause cankers on twigs, branches or trunks. Some locations can be pruned away, but black rot cankers that form on the trunk of your apple tree will more than likely kill the tree.
Cedar Apple Rust
Cedar apple rust is a fungus that requires two host plants, a juniper species – usually eastern red cedar and an apple tree. The wind can blow the fungal spores from a juniper up to 1 mile away onto your apple tree where it will germinate and creates yellow to orange coloured spots on the leaves and fruits of apple trees. In late summer, brown clusters of cylindrical tubes – called aecia – appear on the underside of the leaf surface below the yellow leaf spots, and heavily infected leaves may drop prematurely.
Cedar apple rust cannot travel from apple tree to apple tree. It must travel from apple tree to juniper tree and back to apple tree in order to spread from year to year. It will rarely kill your tree but can stunt its growth and cause leaves and fruit to drop prematurely.
Get Rid of Yellow Spots on Apple Tree Leaves
Because cedar apple rust requires two hosts – an apple tree and a juniper species – removing infected galls on nearby juniper species is the most effective organic method of controlling cedar apple rust on your apple tree. In extreme cases, removal of the juniper species may be necessary.
As with all fungal diseases, remove any fallen debris from the tree in the fall to prevent cedar apple rust from over wintering in fallen leaves and if possible, plant apple cultivars that are resistant to cedar apple rust.
Prevent Apple Tree Fungal Diseases
Fungi can survive the winter in dried leaves, limbs and fallen fruit on the floor below you apple tree. To prevent fungal diseases from spreading, clean up infected debris under your apple tree in the fall to prevent the fungal spores from multiplying and spreading the following spring. The longer the fallen leaves remain wet, the more severe the infection will be.
Water in the evening or early morning hours at the base of the apple tree. Avoid overhead irrigation to prevent the leaves from getting wet which would allow the fungus to thrive.
Apple Tree Bacterial Diseases
Bacteria are microscopic organisms that reproduce by simple division during ideal conditions and require the presence of water to spread. Bacterial infections require an entry point to get inside the apple tree. An open wound, pruning cuts, animal and insect damage, even wind damage can act as an entry point for bacterial diseases to infect your apple tree.
Once inside the plant, bacteria can spread throughout the plant by “swimming” to different areas of the apple tree. The most common bacterial disease in Apple trees is fire blight.
Fire blight is a highly contagious bacteria that can spread quickly in warm humid weather to different areas of your apple tree. It enters through an open wound, insect or wind damage and attacks tender new growth at the tip of branches first, and travels down towards the centre of the apple tree. Normally you will notice die back from the outside, in.
The leaves will look scorched, dark-brown or black but will not drop off the tree. Bark fire blight will show signs of reddish cankers that will ooze a dark-orange bacteria on warm days.
Fire blight will over winter in infected bark and spreads through wind or splashing rain. If left un-treated, fire blight will eventually kill your apple tree.
Prevent Fire Blight In Apple Trees
Clean up fallen leaves, twigs and fruit from the understory of your apple tree in the fall. This will prevent the bacteria from overwintering in the debris and spreading the following spring. Dispose of the debris – do not compost it. Composting infected debris will allow the bacteria to survive the winter in your compost heap.
Remove infected limbs and branches by pruning 8-12 inches below the infection to healthy wood, then dispose of the debris. Remove any suckers or water sprouts the growing season. They grow quickly and are a vulnerability for bacterial diseases to enter your apple tree.
When pruning your apple tree, disinfect your pruning shears between cuts by dipping the blades in rubbing alcohol or a bleach solution to prevent fireblight from infecting other trees, or other areas of the same tree.
Apple Tree Viral Diseases
Viral diseases enter the apple tree through grafting or propagation, pollen or seeds, and use the apple trees nutrients to reproduce. Apple tree viral diseases can spread from one tree to the next by carriers like insects, birds and nematodes. Once an apple tree is infected with a viral disease, there is no cure.
Apple Mosaic Virus
Apple mosaic virus is the most common virus found on apple trees and is introduced while grafting an infected scion to a healthy tree or rootstock, or by pollen. It does not spread by insect.
Symptoms of apple mosaic virus start early in the season and will develop pale yellow to cream coloured banded areas on young leaves, that will drop prematurely. Apple mosaic virus only affects leaves and shoots, not the apples themselves.
Once infected, there is no way to eliminate the virus. Mosaic virus will not kill your apple tree, but will slow growth of new shoots and reduce apple yields by up to 40% in some cultivars.
Apple Union Necrosis and Decline
Apple union necrosis and decline is caused by the tomato ringspot virus and affects the graft union of certain apple cultivars on specific rootstocks. The most common root stock affected is the MM.106. Read our guide on the different root stocks and how they affect the growth of your tree.
Symptoms start to appear around year 3 or 4 once the tree reaches its bearing age. Leaves develop to be smaller, more sparse, and paler green. Infected apple trees will flower heavily and set large crops of small fruit. The leaves will discolour and drop prematurely.
Eventually apple union necrosis and decline will cause the graft union to separate and can cause the apple tree to die, though sometimes the infection is not lethal. The combination of scion and rootstock will determine how severe the infection becomes, for example, Delicious cultivar on MM.106 is the most commonly affected.
Apple Chlorotic Leaf Spot
Apple chlorotic leaf spot is a latent virus, meaning it can live in the apple tree without showing any symptoms, and is transmitted when an infected scion is grafted to a susceptible rootstock. Normally apple chlorotic leaf spot is detected with other latent viruses, like apple stem pitting virus and apple stem grooving virus.
Apple Green Crinkle
Apple green crinkle is a viral disease transmitted through grafting that creates distorted apples with deep depressions that appear on only a few limbs. There are no leaf symptoms, and the growth of the tree is not affected. Research shows it has an incubation period of 3-8 years before any symptoms show, and generally only show during a cool spring.
Apple green crinkle is only present in apple trees infected with all three latent viruses, apple chlorotic leaf spot, apple stem pitting virus, and apple stem grooving virus – and does not spread to other apple trees.
Flat Apple Virus
Flat apple virus is a nematode transmissible virus that can travel between apple and cherry trees. Symptoms appear on the apples and do not affect the leaves or the growth of the tree. The apples will grow ‘flatter’ than normal and the stem cavity will be shallower and normally affects apples on lower limbs first.
Prevent Viral Diseases in Apple Trees
Since most viral diseases are transmitted by grafting and propagation, be sure to plant certified virus-free apple trees from your local nursery to avoid inheriting viral diseases.