Different diseases of the apple fruit are discussed below:
Brooks Spot, Mycosphaerella pomi
Brooks spot is a minor disease that occurs throughout the northeastern and mid-Atlantic apple growing regions of the U.S. and occasionally as far westward as Iowa.
Symptoms: Brooks spot first appears as irregular, slightly sunken dark green lesions typically on the calyx end of immature apple fruit (photo 2-37). As the fruit matures, the lesion turns dark red or purple on red areas of the fruit and remains dark green on green or yellow areas (photo 2-38). The disease is sometimes confused with the physiological disorders Jonathan spot, cork spot, and bitter pit. However, Brooks spot usually appears earlier in the season and shows little browning of the flesh underneath the lesion. Jonathan spot lesions are usually more round and the lesion edge is more abruptly sunken, with a shallow browning of the flesh underneath.
Look for gray or brown corky spots, uneven or deformed fruit, and cracked skin and flesh. Although apple fruit infections usually occur early in the season, pin point
scab with rough, black, circular spots may develop in storage after harvest following
late season infections.
Fruit Rot (Phytophthora)
Cause : Phytophthora cactorum, a soil borne fungus like microorganism frequently carried in irrigation water. Fruit rot has caused significant loss of fruit where irrigation water wetted fruit, usually on lower branches or from overhead irrigation for summer fruit cooling. As little as 1 hour of fruit wetting is needed for infection. Apple susceptibility increases 10 days before harvest.
Symptoms : Firm, tan-colored, rotted spots on areas up to more than 1 inch in diameter on the fruit. Older infections can cause the whole fruit to rot on the tree. Can be confused with fire blight on immature pears but is lighter in color and soft to the touch. Some years, disease also may spread to fruit pedicels and a little way into first-year wood, causing a dieback that resembles fire blight.
Cause : There are many reasons for fruit to russet including cool, wet weather, frost, pesticides, viruses, fungi and bacteria. Golden apples are more susceptible to russeting than red apples. Each cultivator is affected differently by the above factors.
Russet results from the damage to epidermal cells that occur within the first 30 to 40 days after petal fall. Once damaged, a brown layer of suberized cells form in the lower epidermal region. As cork cells develop in this area, they push outward and become exposed to the surface as the fruit matures.
Fuji apples have a newly described but unknown cause of russeting, which has been called flecking. Severity increases during alternate years.
Symptoms : Russeting caused by cool weather and wet fruit often is associated with corky lenticels and tan markings shaped like rain-splashed water droplets. These markings are more abundant at the stem end.
A band that forms either partially or completely around the fruit usually characterizes Frost russeting. Russeting from spray materials is likely to be found where spray droplets accumulate, such as the lowest portions of the fruit. Russeting from powdery mildew is tan to gray and has a netted appearance.
Black pox, Helminthosporium papulosum
Black pox is a minor fungal disease, which can affect apple bark, fruit, and foliage. The disease is more common from southern Virginia southward than in the northern mid-Atlantic region.
Symptoms. Infection first appears on current season twigs as well-defined, conical, shiny black lesions, which enlarge by the end of the first season. Extensive secondary lesion enlargement may girdle small, heavily infected twigs. Smooth barked twigs remain susceptible for several years and twig infection is cumulative throughout this period.
Twig symptoms may be confused with the nutritional disorders measles and internal bark necrosis. Fruit lesions are small, black, circular and slightly sunken (photo 2-44). Leaf lesions begin as red halos with light green centers, and become tan to brown with purple borders. Severe leaf infection may cause defoliation. Differences in cultivar susceptibility have been noted, with ‘Rome Beauty’ and ‘Grimes Golden’ being more susceptible than ‘Yellow Transparent’ and ‘York Imperial’.
Rot (white rot)
The disease first appears as sunken, circular, brown spots, often with a reddish or dark halo. Scattered clumps of fungal reproductive structures may appear on the surface. As the decay expands, the rotted area extends towards the core as a cylinder of affected tissue. This can be seen by splitting the fruit open. The decay is soft and watery, having a clear to light tan color under warm weather conditions.
Although now uncommon, incidence of X-spot as high as 86 percent was reported on unprotected ‘Golden Delicious’ and ‘Rome Beauty’ fruit in 1950. Other susceptible cultivars include ‘Jonathan’, ‘Stayman Winesap’, and ‘York Imperial’.
The names X-spot, X-rot, and Nigrospora spot have been applied to a small, circular, depressed, necrotic spot typically on the calyx end of apple fruit in the mid-Atlantic region (photo 2-43). A fungus has been associated with X-spot lesions, but the typical lesion symptom has not been duplicated by controlled inoculation.