apple of tree with apple scab

Apple scab

Updated: April 8, 2021

1) Apple scab and pear scab 

apple with dark blotches
Apple scab infected fruit
University of Georgia Plant Pathology , University of Georgia, Bugwood.org
apple scab spots on leaves
Apple scab spots on leaves
Penn State Department of Plant Pathology & Environmental Microbiology Archives , Penn State University, Bugwood.org
scab disease on a pear
Pear scab
Bruce Watt, University of Maine, Bugwood.org

Both are caused by different fungi, but the symptoms and control measures are similar.

  • Foliar lesions often appear first on the undersides of the leaves but can also develop on the upper leaf surfaces as well as on the fruit. At first, the margins of scab lesions appear irregular and the interior area is velvety and olive green to black in color.
  • Within a short time, however, lesion margins become much more defined and the lesion may cause the leaf to curl.
  • Early leaf drop and defoliation can occur.
  • Fruit lesions appear similar to foliar lesions initially, but later appear as slightly raised, russeted, scabby areas.
  • Young leaves are susceptible for about 1 week after emergence or until they are fully expanded, but the fruit remains susceptible throughout most of the growing season. Fruit infections that occur within the first 3 to 4 weeks after petal fall usually cause severe deformities and early fruit drop.
  • Later fruit infections tend to be limited to the surface of the fruit and do not affect the overall quality of the flesh.

2) Marssonina blotch

Marssonina blotch, caused by (Marssonina caronaria), is a foliar disease first identified in Pennsylvania in September 2017. The symptoms are similar to apple scab. 

PennState Extension - Marssonina blotch

3) Fire blight 

wilting brown leaves hanging on a branch
Symptom of a fire blight infection
Photo: Ward Upham, Kansas State University, Bugwood.org
fire blight wound on stem
Dark fire blight canker on twig
Photo: Joseph OBrien, USDA Forest Service, Bugwood.org
  • A destructive bacterial disease of both apples and pears that destroys blossoms, shoots, and limbs, and sometimes kills whole trees. Infected blossom clusters and shoot tips wilt and then rapidly turn brown and die but remain attached.
  • As the infection spreads into the supporting twig or limb, bark cankers develop and are noticeable by their reddish-brown color and slightly sunken appearance.
  • Where susceptible rootstocks are used, the bacteria from a few infection sites on the scion may pass through otherwise healthy limbs and the trunk without causing symptoms but can initiate a canker on the rootstock that will girdle and kill the tree later in the season or the following spring.

Management: Ugly stub pruning method

4) Powdery mildew 

powdery mildew symptoms on apples
"Russeting" of mature apple fruits, caused by infection with powdery mildew.
The fruit on the right displays the normal, healthy appearance for this variety.
Photo: N.S. Luepschen, Bugwood.org

 

white coating on apple leaves
Powdery mildew symptoms
Photo: University of Georgia Plant Pathology, University of Georgia, Bugwood.org
  • Powdery mildew is a fungal disease of apples that appears early in the season.
  • The fungus overwinters in buds and can appear as early as the pink stage before bloom but is often most visible just after petal fall as the new shoots are rapidly developing. Infected leaves are distorted and covered with white, powdery fungal growth.
  • Mildew can be serious on young trees because it stunts new shoot and branch development.
  • The fungus also can produce a net-like russet on the surface of the fruit, but this is largely cosmetic and does not affect the fruit quality.

5) Rust diseases of apple (cedar-apple rust, hawthorn rust, and quince rust) 

closeup of cedar apple rust fungal spore
Closeup of fungal spore
  • Rust diseases are caused by fungi that complete part of their life cycle on the red cedar (juniper) and part on apple, crab apple, hawthorn, or quince. Cedar-apple and hawthorn rusts produce bright yellow-orange spots on the leaves and fruit. Quince-rust spots develop only on the fruit and are sunken and dark green at first, later becoming purplish. Like apple scab, early season lesions can cause the fruit to be deformed and fall early. Unlike apple scab however, the spores produced on foliar and fruit lesions do not cause more infections on apple but, instead, infect nearby junipers. Rust diseases cause leaf drop and defoliation. 

6) Sooty blotch and flyspeck 

dark spots and blotches on an apple
Both sooty blotch and flyspeck on an apple
  • These are two common fungal diseases that develop during the summer. Both affect the fruit finish but neither affects the quality of the fruit flesh to any great degree.
  • Sooty blotch produces dark patches of black fungal growth on the surface of the fruit, sometimes covering most or the entire apple.
  • Flyspeck disease often occurs in conjunction with sooty blotch and looks very much like fly specks on paper i.e., shiny little pinpoints of black fungi appearing in small to large circular groups on the apple skin.
  • Both these diseases begin developing about mid-season and tend to be more common and severe in areas that remain wet for extended periods of time following rain or dew.

7) Summer rots and spots 

large brown blotches on apples
Bitter rot and anthracnose symptoms on fruit
Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
Large rotten areas on apples
Black rot
Photo: University of Georgia Plant Pathology , University of Georgia, Bugwood.org
  • There are many rots and spots on apples which are caused by a variety of different fungi, all of which are harbored in dead twigs and limbs.
  • Fruit rots can develop in two ways. Primary infections can occur directly through the lenticels on the fruit or through the calyx cup at the tip of the fruit. The rots that develop are usually firm, brown to dark brown in color, and appear as circular, slightly sunken spots that may show concentric rings of color variation.
  • As the rot continues to develop, the flesh decays and, depending on the fungus involved, may remain relatively firm and dry or become soft and watery.
  • Secondary rots occur when pathogens become established in wounds caused by fruit cracking, insect feeding, or hail damage.