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Apple Tree Diseases and Invasives


Apple Scab
Black Rot
Cedar-Apple Rust
Fire Blight
Sooty Blotch and Flyspeck


Apple Maggot
Woolly Apple Aphid


What is it? -  Scab infections on leaves start as olive green to brown spots with an irregular or feathered edge. As leaf infections grow, they may merge together and assume a dark brown velvety appearance. Severely infected leaves may turn yellow and drop prematurely. Scab infections on young fruit start out as olive green to brown spots. As the lesions enlarge, they harden, and eventually become black, corky, inedible areas on the fruit. Severely infected fruit may be deformed and often crack open.

How does it spread? - Apple scab overwinters primarily in fallen leaves and in the soil. Disease development is favored by wet, cool weather that generally occurs in spring and early summer. Fungal spores are carried by wind, rain or splashing water from the ground to flowers, leaves or fruit. During damp or rainy periods, newly opening apple leaves are extremely susceptible to infection. The longer the leaves remain wet, the more severe the infection will be. Apple scab spreads rapidly between 55-75 degrees F.

What can you do about it? - The apple scab pathogen overwinters in infected leaves on the ground from the previous year. In spring, overwintered spores (ascospores) mature and are discharged over a period of 5 to 9 weeks. Wind and splashing rain carries apple scab spores from infected leaves to new growth on nearby trees where new infections begin. In late summer or early fall,  primary apple scab infections produce secondary spores and create new infections, which can continue through the growing season during wet periods. Raking and containing the leaf litter in the fall will help limit the spread of the disease. 

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Black Rot.jpg


What is it? - Black rot is a disease in apples that infects the fruit, leaves, and bark caused by the fungus Botryosphaeria obtusa. Black rot in apples is a common fungal disease that can spread from infected apple trees to other landscape plants. It can spread to healthy tissue on pear or quince trees as well, but is typically a secondary fungus of weak or dead tissues in other plants. 

How does it spread? - Signs of the disease start on the upper surfaces of the leaves. As these spots age, they expand, which results in the more heavily infected leaves dropping. Infected branches and limbs will have red-brown sunken areas that expand more and more each year. Fruit infection is the most destructive form of this pathogen and begins with infected flowers, before the fruits even grow. When fruits are tiny and green, you’ll notice red flecks or purplish spots that grow with them. Mature fruit lesions take on a bulls-eye appearance, with bands of brown and black areas expanding outward from a central point in each lesion. Commonly, black rot disease causes blossom end rot or mummification of the fruits on the tree.

What can you do about it? - Because fungal spores overwinter on fallen leaves, mummified fruits, dead bark and cankers, it’s important to keep all the fallen debris and dead fruit cleaned up and away from the tree. Make sure to check for cankers and cut/prune them away along with any affected limbs (minimum six inches beyond the wound). Destroy any infected tissue as soon as possible and watch for new signs of infection.

Cedar-Apple Rust

What is it? - Cedar apple rust is part of a group of pathogenic fungi that encompass numerous species from the family Pucciniaceae. These are called rust fungi due to their rusty orange appearance when fruiting on a juniperus species. These pathogens often require two host species to exist. In this case, these species include juniperus existing with malus, pyrus or crataegus.

How does it spread? - Golfball-like growths form on juniperus species. These growths produce orange, gelatinous telial horns. The telieal horns emerge after warm spring rains during April and May and produce microscopic spores. These spores can potentially travel up to three miles via the wind and infect any malus, pyrus, or crataegus (apple, crabapple, or hawthorn species), where the pathogen begins its next cycle. Without any of these three species present, the pathogen cannot complete its next cycle, and thus hits a dead end in reproduction.


What are they? -  They affect apple, crabapple, and pear trees and are separate diseases, but both are normally present on the same fruit. Both sooty blotch and flyspeck are surface problems on the skin and the only loss is poor appearance, although they may shorten storage life. 

How do they spread? - Spores of the fungi are windblown into and throughout the orchard; fruit infection can occur any time after petal fall but is most prevalent in mid- to late summer.

What can you do about it? - Prune properly so branches are not too close together and there is good air movement in the tree. You can also use Fungicides such as Captan to help prevent this disease. 

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Apple Maggot.jpg


What is it? - Apple maggot (Rhagoletis pomonella) is a small fly that leaves its larvae in developing fruit.

How does it spread? - Adult flies will leave and feed outside the orchard, and will return to lay eggs under the skin of apples. Each female fly can lay hundreds of eggs and once the eggs hatch, the larvae will feed for three to four weeks. The apples will eventually drop to the ground and the larvae transform into pupae in the soil. The pupae will then spend the winter underground, and will emerge from the soil in the summer.

What can you do about it? - Keep your garden clean from debris, use sticky traps, or pesticides to prevent the spread of apple maggots.


What is it? -  Woolly apple aphid is a sucking insect pest that weakens the tree by feeding on limbs and roots. Colonies form at wound sites on trunks, limbs, and twigs, where they feed on tender bark. Pruning and hail damage can create the wound sites for attack by this pest. As the population grows, the aphids like to attach on water sprouts in the center of the tree. This causes the tree to swell and form galls in affected areas.

How does it spread? - They spread either through a mechanical agent or directly by crawling. Birds and insects can also transport aphids. 
What can you do about it? - The most important natural enemies are general predators such as green lacewing larvae, lady beetle adults and larvae, and syrphid fly larvae. Some pesticides, such as certain carbamates and pyrethroids, encourage outbreaks by killing parasites and predators. These products should be used sparingly when woolly apple aphids are present. An application of a summer aphid treatment should control woolly apple aphids. There are presently no control methods for underground aphids. The best control of woolly apple aphid is genetic such as with plant resistant rootstocks like M.111.

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