Peach scab
Peach scab is an important disease of peach and nectarines and can be extremely damaging in warm, humid climates. The causal agent is the fungus Cladosporium carpophilum. The disease can also occur on plums and apricots.

Symptoms on young fruit are green to olive circular spots, concentrated near the stem end. As these spots enlarge, they deepen in color and become black and velvety in appearance. Fruit lesions are most common on the shoulders of the fruit, but can occur anywhere on the surface. Where numerous, the lesions often coalesce and lead to fruit cracking, allowing rot organisms to enter, especially Brown Rot. Twigs and leaves can also be infected but symptoms are more noticeable on fruit.

Peach leaf curl
Peach leaf curl is caused by the fungus, Taphrina deformans, is a common disease of peach and nectarine. This fungus destroys young peach leaves. New leaves will develop, but not enough to help the tree. Defoliation by peach leaf curl in successive seasons may kill the tree.

Nectarine pox
Nectarine pox is a widespread disorder. The symptoms initially are observed about 40 days after bloom as a pale white or light-colored circular spot. The area can be a raised protuberance or wart, and are sometimes clustered together into an irregular, rough outgrowth. This has also been confused with scab, spot or other diseases.

Nectarine pox may be a nutritional or hormonal imbalance, such as excessive levels of nitrogen, potassium or magnesium and can result in reduced yields.

Brown rot
Brown rot is caused by the fungus, Monilinia fructicola, and is one of the most important stone fruit diseases. The disease affects peaches, apricots, nectarines, plums, cherries, and most commercially grown Prunus species. The fungus can infect blossoms, twigs and fruit. Infected fruit may rot on the tree or after being harvested.

Brown rot first affects blossoms, which wilt and turn brown. Later, fruit decay occurs as the fruit ripens. The infections begin as small brown spots. Under wet and humid conditions, ash-gray to brown tufts of fungus develop over the surface of the fruit, which can spread into small twigs, and cause cankers to form. The entire fruit can rot within a few hours under favorable conditions. Rotted fruits dry out, become mummified and either remain attached to the tree or fall to the ground.

Peach anthracnose
Peach anthracnose is often called ripe rot is usually rare and considered a minor disease of peach. If left unchecked, peach anthracnose can cause serious fruit rot at harvest time.

Peach anthracnose is caused by the fungus, Colletotrichum sp. ; also known as Glomerella cingulata, the fungus that causes bitter rot of apple. This disease has a very broad host range, which includes apple, pear, nectarine, plum, sour cherry, grape, nuts, vegetables, various legumes, herbaceous annuals and perennials. Because of this wide host range, the disease can become established quite readily.

Symptoms
Anthracnose occurs only on ripe or nearly ripe fruit. The disease begins as lesions characterized by small, brown spots, which become darker, circular and slightly sunken as they age. These large sunken anthracnose lesions are firm to the touch and are often covered with concentric rings of salmon-colored spore masses. This salmon-pink sticky spore mass is a characteristic symptom of anthracnose on peach and other fruits.

Young lesions may be confused with those of brown rot caused by Monilinia species. Early in their development, fruit rots often cannot be differentiated and may be confused with rots caused by various pathogens. At this stage, identification is dependent upon laboratory isolations.

Rhizopus rot
Rhizopus rot is caused by the fungus Rhizopus stolonifer, can be very destructive to harvested fruit. While it can develop in hail-injured or cracked fruit on the tree, it most commonly affects fruit in storage, during transit, and at the marketplace. Ripe fruit of peaches, nectarines, sweet cherries, and plums are most susceptible. Rhizopus fruit rot is usually of minor importance in the field but can cause important postharvest losses.

Symptoms
Rhizopus rot begins much like brown rot, as a small, brown, circular spot but with a detectable difference. The skin of Rhizopus rot-infected fruit slips readily from the underlying flesh, while the skin of brown rotted areas is tough and leathery. At normal temperatures, the small spots of Rhizopus rot enlarge rapidly and can cover the entire fruit in 24 to 48 hours. A white, fuzzy mold appears on the surface of infected fruits, spreading to nearby fruit and the walls of the container. By this time the fruit tends to "leak" and to smell like vinegar. Tiny, black fungal structures give the mold a black appearance.

Bacterial spot
Bacterial spot occurs in most countries where stone fruits are grown. Other names for the disease are bacteriosis, shot hole, and black spot. Common hosts include peach, nectarine, prune, plum, and apricot. Other hosts are sweet and tart cherry, almond and wild peach. The causal bacteria, Xanthomonas campestris pv. pruni, can attack fruit, leaves, and twigs. Fruit loss on some cultivars can be very high.

Symptoms

Leaves
The symptoms of bacterial spot are quite different from other diseases of stone fruits. They may be confused with nitrogen deficiency or spray injury on leaves. The disease first appears as small, water-soaked, grayish areas on the undersides of leaves. Later the spots become angular, purple, black, or brown in color. The mature spots remain angular and are most numerous at the tip ends and along the midribs of leaves. The infected areas may drop out giving the infected leaves a shot hole, tattered appearance. On plum, the shot hole effect is more pronounced than on other stone fruits. Infected leaves eventually turn yellow and drop.

Other leafspot diseases and spots due to spray injury tend to be much more circular in outline. Often, these are not confined by veins in the leaf, as is bacterial spot. (See also nitrogen deficiency).

Fruit
Fruit infected early in the season develop unsightly blemishes and may exhibit gumming. Since the infected areas cannot expand with increased fruit size, the spots crack. Pits or cracks on the fruit surface extend into the flesh and create large brown to black depressed areas on the fruit surface. Lesions that develop during the preharvest period are usually superficial and give the fruit a mottled appearance. On plum, the fruit symptoms are likely to be quite different in that large, black, sunken areas are more common.

Rusty spot of peach is characterized by the presence of rust-colored spots that can cover the entire surface of the fruit. The cause of rusty spot is uncertain, although many plant pathologists believe it to be the apple powdery mildew fungus, Podosphaera leucotricha. Many observations have shown that peach orchards with rusty spot are usually next to apple orchards that are infected with powdery mildew.

Symptoms
Rusty spot is recognized only on the fruit. The earliest symptoms on young green fruit appear as small, orange-tan spots, which can change the color of the fuzz or trichomes on the fruit. These first spots may become noticeable three to four weeks after shuck fall. The discolored area enlarges slowly, and the older discolored trichomes begin disappearing, leaving a fuzzless, smooth, center spot surrounded by a non-uniform band of orange to tan hairs. Finally, the spots become quite spread out leaving brownish or reddish centers of hard, smooth skin.

Powdery mildew
Powdery mildew is caused by the fungus, Sphaerotheca pannosa, and infects leaves, twigs, and fruit. Powdery mildew on the fruit, however, causes the greatest economic loss.

Symptoms

Leaves
Diseased leaves often fail to unfold normally, while those of new shoots become narrow, straplike, and distorted. New shoots are shorter than normal and distorted. The white mycelium and spores may cover infected leaves and shoots or may appear as whitish patches.

Fruit
On fruit the disease first appears as round, whitish spots 2 to 4 weeks after shuck fall. The spots enlarge until they cover much of the fruit. The fungus mycelium and its spores produce the white spots. Later the mycelium sloughs off and leaves a rusty-colored patch. About the time of pit-hardening, the skin of the fruit under the spot turns pinkish. Eventually the skin becomes leathery or hard, turns brown, and may crack.

X Disease
X-Disease is caused by a phytoplasma, a cellular pathogen about half the size of a bacterial cell. the X-Disease phytoplasma can infect peaches, nectarines, cherries and Japanese plums. It also infects wild chokecherry, which serves as a reservoir for the pathogen that can later be transmitted by leafhoppers into orchards.

Symptoms
Symptoms on peach appear after two months of growth in the spring. After infection, the disease may take more than two years to spread throughout the tree. Summer leaves will appear irregularly shaped with yellowing and dead areas, which eventually fall out, giving the leave a shot-hole appearance.

Black Knot
Black Knot is caused by the fungus Apiosporina morbosa (Syn. Dibotryon morbosum) and is commonly seen on susceptible plums and wild cherry and plum trees. It can also appear on apricot, peach and ornamental Prunus species. Once established, this disease is very difficult to control.

Symptoms
The symptoms appear on the woody parts of the plant, twigs and branches, as warty swellings, first visible on new shoots in the late summer. The knots are at first dark green and corky, but later turn black and become hard and brittle. Older knots may become covered with a pinkish-white mold or other fungus.