An overview of apple diseases

Alternaria leaf blotch of apple has been a serious issue in North Carolina for several decades. The disease was first observed in West Virginia in 2008. In Pennsylvania, Alternaria leaf blotch of apple is present, but hasn’t been an economical issue. Severity can vary from year to year; however, Alternaria leaf blotch severity is affected by severe mite infestation. Consequently, a good mite management program is important for managing this disease.

Crown rot continues to be a major cause of tree death in Pennsylvania orchards. It is often observed on 3- to 8-year-old trees grown on Malling Merton (MM) 104, MM.106, M.7, and to a lesser degree MM.111 rootstocks. The disease is often observed in low-lying areas of orchards with heavy, poorly drained soils.

Apple scab is Pennsylvania’s most important apple disease, attacking wild and cultivated apple and crabapple. Early season disease management is primarily directed at controlling apple scab.

Bitter rot is an important disease in the southern states but also occurs in Pennsylvania. Bitter rot on apple and pear fruit is caused by the pathogenic fungi Colletrotrichum gloeosporioides and C. acutatum. The same causal pathogens are also responsible for anthracnose disease on peach, anthracnose fruit rot on blueberry and strawberry, ripe rot on grape, anthracnose on pepper, and blossom-end rot of green burrs on chestnuts. The sexual stage of C. gloeosporiodes, Glomerella cigulata, can also cause fruit rot and often is associated with a leaf spot disease. The discussion below is limited to the disease as it affects apple and pear trees.

The black rot and frogeye leaf spot fungus, Botryosphaeria obtusa, covers a wide geographical range, attacking the fruit, leaves, and bark of apple trees and other pomaceous plants. The fungus is a vigorous saprophyte and may colonize the dead tissue of many other hosts. However, its parasitic activities are confined mainly to pome fruits.

Crispin apples are highly susceptible to the blister spot bacterial infections about 2 weeks after petal fall for a period of 2 to 4 weeks. The causal bacteria, Pseudomonas syringae, overwinter in the infected buds and multiply on the leaf surface in spring. Rain washes the bacteria onto the fruit where they infect through the lenticels to cause the reddish spot.

Blossom end rot of apple is not a major problem in Pennsylvania orchards. Because it occurs only infrequently, very little is known about its cycle and control.

Blue mold, a common rot of stored apples and pears, is caused by the fungus Penicillium expansum. Blue mold is the most important postharvest disease of apples worldwide. Other names for the disease are soft rot, bin rot, and Penicillium rot. Aside from losses in fruit caused by rot, sound fruit in the same container as decaying fruit may absorb a moldy odor and flavor.

Caused by the fungus Mycosphaerella pomi, Brooks fruit spot is also known as Phoma fruit spot. The disease attacks apple and crabapple trees and is rarely found in well-sprayed orchards. When cover sprays are stopped too soon, or when trees are not well-pruned and sprayed, severe losses can occur. Varieties such as Rome Beauty, Stayman, Jonathan, and Grimes Golden are quite susceptible.

Crown gall is caused by a bacterium, Agrobacterium tumefaciens, and affects apples, pears, quince, peaches, nectarines, apricots, plums, and cherries. The disease is common in tree fruit nurseries and can occur in orchards.

Fire blight is destructive to apples and quince and is the most serious pear disease in the eastern United States. Caused by the bacterium Erwinia amylovora, the disease can attack some 75 species of plants of the rose family. Fire blight also occurs frequently on pyracantha, spirea, hawthorn, and mountain ash. In fruit trees, the disease can kill blossoms, fruit, shoots, limbs, and tree trunks. Certain varieties of apples are more susceptible than others. Susceptible varieties include Braeburn, Fuji, Gala, Granny Smith, Jonathan, Rome, Yellow Transparent, and Idared.

Gray mold is the most important postharvest disease of pears and is second to blue mold in importance to apple. The disease develops very quickly at cold storage temperatures. Also known as nest rot or cluster rot, gray mold can cause large losses because of its ability to spread from infected to adjacent healthy fruit in storage.

Moldy core is caused by several different fungal pathogens. Many cultivars of apples are affected, including Delicious, which is very susceptible. Moldy core may develop into dry core rot if the pathogen penetrates into the core flesh, but the fungus is generally restricted to the core or carpel region.

Mucor rot is a fungal disease of apples and pears. The disease is a postharvest storage problem. It does not occur as frequently as blue mold, however, losses due to Mucor infection can be serious.

The cause of necrotic leaf blotch of apple is not known, and it is considered a physiological disorder. The disorder is most common on Golden Delicious worldwide.

Nectria twig blight, caused by the fungus Nectria cinnabarina, is a minor disease that breaks out occasionally. Because its symptoms are similar to those of fire blight, growers need to be able to recognize it. The chemical controls used for fire blight would be wasted on nectria twig blight.

Powdery mildew, caused by the fungus Podosphaera leucotricha, attacks buds, blossoms, leaves, new shoots, and fruit of wild and cultivated apples and crabapples. It interferes with the proper functioning of leaves, reduces shoot growth, reduces fruit set, and produces a netlike russet on the fruit of some cultivars. It is often a serious problem in apple nurseries.

There are three rust diseases: cedar-apple rust, hawthorn rust, and quince rust. The most common is cedar-apple rust. All three must spend part of their life cycles on red cedar. These diseases can cause economic losses in several ways. Severe leaf infection and defoliation may make trees susceptible to winter injury. Severe defoliation reduces fruit size and quality, and infected fruit is deformed, sometimes very seriously. The hosts of cedar-apple rust are leaves and fruit of apple and crabapple trees. Of hawthorn rust, hosts are leaves of pear, hawthorn, apple, and crabapple; and of quince rust, hosts are the leaves and fruit of quince and the fruit of pear, apple, and crabapple.

Affecting apple, crabapple, and pear trees, sooty blotch and flyspeck of apple are separate diseases, but both are normally present on the same fruit. They cause surface blemishes that detract from fruit appearance, lowering fruit quality and market value. Sooty blotch also shortens fruit storage life because of increased water loss. Sooty blotch is a disease complex caused by several unrelated fungi. Flyspeck is caused by the fungus Zygophiala jamaicensis.

Apple union necrosis and decline is primarily a problem on trees propagated on MM106 rootstock. It is especially serious on Red Delicious trees, which are on MM106. The disease is caused by the same virus that is vectored by the dagger nematode causing Prunus stem pitting.

The white rot fungus, Botryosphaeria dothidea, often referred to as “Bot rot” or Botryosphaeria rot, is most important on apple trees, but it also attacks crabapple, pear, grape, and chestnut. On apple trees it can be observed as a distinct canker on twigs, limbs, and trunks. The fungus produces two types of fruit rot, but leaf infections do not occur. Losses from fruit rot can be considerable, especially in southeastern regions of the United States. Drought stress and winter injury have been associated with an increase in infection and canker expansion.