University of Maryland Extension

Apple - Pear Diseases

1) Apple scab and pear scab 

dark spots on apple large apple scab lesion
Apple scab - Credit - R.W. Samson, Purdue University, Bugwood.org (left); Apple scabe - credit - University of GA Plant Pathology, University of GA, Bugwood.org (right)

apple scab lesions Pear scab on fruit and leaves
Apple scab on left (left); Pear scab - credit - Bruce Watt, University of maine, Bugwood.org (right)

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.

Tree Fruits - Pest Control and Spray Schedules

2) Fire blight 

fruit tree with fireblight classic shepherds crook
Fire blight infected branches (left); Fire blight on apple - Credit - Ward upham, Kansas St. Un, Bugwood.org (right)

Branch killed by fire blight
William Jacobi, Colorado St. Un., 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

3) Powdery mildew 

Powdery mildew on branch Powdery mildew on leaves
Powdery mildew infected leaves Photo: William Brown, bugwood.org (left); Powdery mildew on leaves Photo: Un. of GA Plant Pathology, University of Georgia, Bugwood.org (right)

Apple with powdery mildew damage
Photo: Clemson University, USDA Coop 
Ext Slide Series, Bugwood.org 
Powdery mildew damage to fruit

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.

Tree Fruits - Pest Control and Spray Schedules

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

cedar apple rust on apple tree rust lesions on upper surface of leaf
Cedar apple rust on apple tree (left); Cedar apple rust lesions on upper leaf surface (right)

cedar apple rust lesions lower surface Cedar apple rust spore close-up
Cedar apple rust lesions on lower leaf surface (left); Close-up of cedar apple rust spores (right)

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. 

Tree Fruits - Pest Control and Spray Schedules

5) Sooty blotch and flyspeck 

flyspeck and sooty blotch close-up of flyspeck and sooty mold
Photo: Bruce Watt, University of Maine, Bugwood.org Both sooty blotch and flyspeck (small dark dots on fruit) (left); Close-up of flyspeck and sooty blotch (right)

sooty blotch and San Jose scale
Sooty blotch and San Jose scale

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.

Tree Fruits - Pest Control and Spray Schedules

6) Summer rots and spots 

apple with rotten spot Black spots on leaves
Photo: University of GA Plant Pathology, Un. of GA, Bugwood.org Black rot on apple (left); Photo: Paul Bachi, Un. of Kentucky Research and Ed. Center, Bugwood.org Black rot spots on apple leaves (right)

large rotten area on apple Apples on tree with bitter rot
Photo: Elizabeth Bush,Va Polytechnic Ins. and St. Un., Bugwood.org Bitter rot and anthracnose on apple (left); Photo: Gerald Holmes, CA Polytechnic St. Un. at San Luis Obispo, Bugwood.org Bitter rot and anthracnose on apples (right)

White rot spots on apple

Photo: Un. of GA Plant Pathology,
Un. of GA, Bugwood.org 
Botryosphaeria canker, white rot on apple

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.

Tree Fruits - Pest Control and Spray Schedules

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