Better Yields Through Science
Peach & Nectarine Diseases & Pests
|Disease||Symptoms, Life-cycle and pest management|
|Coryneum Blight or Shot Hole Disease||Pathogen: Stigmina carpophila, syn. Coryneum carpophilum or Wilsonomyces carpophilus|
Coryneum blight is also called shot hole disease, California blight, peach blight or pustular spot. Severe foliar shot holing may weaken a tree, while the most apparent damage is infection of the fruit.
The main symptoms of shot hole on nectarines and peaches occur on twigs and buds, but fruit lesions may develop when spring weather is wet. Twig symptoms first appear as small, purplish black spots. These turn brown as they enlarge, often having a light center with a purplish brown margin. Tiny, dark brown bumps develop at the center of each lesion. These bumps are spore-forming structures called sporodochia and are easily seen with a hand lens. When buds are affected, the scales turn dark brown or black and the buds may be covered with a shiny layer of exuded gum. Buds killed by bacterial blast have a similar appearance but tend to be much blacker, and nearby foliage of the affected shoot is wilted. Shot hole can be distinguished on peach by the presence of tan twig lesions with dark margins, usually accompanied by profuse gumming.
The fungus survives on infected twigs and buds. Spores are produced throughout winter and are spread by splashing rain and wind. The disease is favored by prolonged wetness in fall to mid-winter (twig blight). Summer rain or sprinkler irrigation encourages fruit infection. There is more infection low in the tree where fruit stay wet longer. Fall application before winter rains begin is the most important application for control of this disease.
|European Red Mite||Scientific Name: Panonychus ulmi|
The female European red mite is about 0.02 inch long and has a brick red, globular body with long curved hairs that arise from white spots or tubercles on the back. Nymphs or unfed females may appear greenish. European red mite eggs are red, slightly flattened, and have a stipe protruding from the top. They overwinter in the egg stage on twigs and spurs. Eggs hatch in early spring just after the trees leaf out, and many generations (8–10) are produced before fall. Ordinarily European red mite populations build up slowly during spring and do not become apparent until large populations are present.
European red mites remove the contents of the leaf cells as they feed, causing leaves to take on a finely mottled appearance. During summer, look for stippling or bronzing on leaves. Rarely do European red mites cause leaf drop in nectarine trees. Generally treatments for this mite are applied in the dormant/delayed-dormant season.
|Peach Leaf Curl||Pathogen: Taphrina deformans|
Leaves produced in spring are thickened, curled, and colored red or yellow instead of normal green. Severely affected shoots die. Irregular, reddish lesions are sometimes seen on fruit. Badly diseased leaves fall by early summer, and repeated infections debilitate trees and kill branches.
Taphrina deformans survives on tree surfaces and buds and is favored by wet weather during spring. One application in the dormant/delayed dormant period is sufficient except in areas of high rainfall or where leaf curl has become an increasing problem. In such cases, an added application at the delayed dormant timing in late winter before bud swell is recommended.
|San Jose Scale||Scientific name: Diaspidiotus (= Quadraspidiotus) perniciosus|
Female San Jose scale lay eggs that hatch immediately and the young emerge from under the edge of the adult scale covering. These tiny, yellow crawlers wander in a random fashion until they find a suitable place to settle. Immediately upon settling, the crawlers insert their mouthparts into the host plant and begin feeding and secreting a white, waxy material (white cap stage); eventually the waxy covering turns black (black cap stage).
|Black Peach Aphid||Scientific Name: Brachycaudus persicae|
In California, this aphid is prevalent in the San Joaquin Valley. Winged and wingless adults of the black peach aphid are shiny black and about 0.1 inch long. The nymphs are reddish brown. Wingless forms overwinter on the roots of nectarine and other closely related trees. In early spring, some migrate from roots to new growth and start colonies on the young leaves. Several generations of female aphids are produced. In early summer, winged adults are produced and migration to other trees occurs. Aboveground colonies usually disappear by midsummer as wingless forms migrate to the roots to feed and overwinter.
Injury consists of leaf curling, yellowing, and premature drop; the leaf curling is first evident at shoot terminals. If aphids are abundant, honeydew excretion may cause black sooty mold to appear on leaves and fruit. Ants may also become a problem, because they tend the aphids while they are on the roots.
|Powdery Mildew||Pathogens: Sphaerotheca pannosa and Podosphaera leucotricha|
Symptoms of powdery mildew can be seen on the terminal leaves of shoots, which are covered in powdery, white fungal growth. Leaves become misshapen and puckered, and fruits develop powdery, white spots that can scar over as the fruit mature.
Sphaerotheca pannosa survives as mycelium in bud scales and as cleistothecia. Growth of the pathogen is favored by cool, moist nights and warm days. Generally, fruit is susceptible only up to time of pit hardening, but later infections can occur.
Lime sulfur spray is acceptable for use in an organically certified crop. Early treatments are the most important and most effective.
|Scab||Pathogens: Cladosporium carpophilum|
Scab affects foliage, young shoots, and fruit, but damage is the result of fruit infections. Fruit infections appear as dark lesions on ripening fruit, most commonly on the upper surface, and may grow together to form large blotches. Lesions may have green or yellowish blotches that turn grayish when spores are produced. On peaches, lesions are flat, circular black spots up to 1/8 inch (3 mm) in diameter. When nectarines are affected, the skin loses its pigment and becomes pale green to cream in color. The center of each spot is dark with the development of spores.
The fungus that causes scab overwinters in lesions on first-year twigs. Spores are produce in these lesions when humidity exceeds 70% beginning at bloom and lasting several weeks. Spores are spread by air movement and splashing water and will infect developing fruit, although it may take several weeks for lesions to appear.
Delayed dormant spray (when the buds swell and begin to show green) is vital to control with sprays in Pre-bloom and Pink stages. Subsequent sprays should be applied at 10-14 day intervals until approximately 1 month before harvest. On peaches the following subsequent sprays should begin about 1 week after petal-fall, on nectarines they should begin at about petal-fall. During the month before harvest, sprays applied for brown rot control will help reduce late season scab infections on the fruit, twigs and leaves.
|Brown Rot Blossom and Twig Blight||Pathogens: Monilinia fructicola, occasionally Monilinia laxa|
Young blossom spurs and associated leaves collapse to form shoot blight. Gum exudes at base of infected flowers. Cankers on blighted twigs have tan centers with dark margins. Gray brown spore masses form on diseased flower parts and twig cankers under high humidity.
Monilinia fructicola overwinters on mummified fruit that either fall to the ground in late fall or early winter, or remain in the trees. In spring, apothecia form on mummies that are on the ground; these apothecia release ascospores at the same time as the trees bloom. Ascospores serve as primary inoculum for brown rot in many orchards. Mummies in the trees as well as those on the ground may also produce conidia, which may serve as the primary source of inoculum in some years. Twig lesions apparently do not produce spores. A fall or early dormant ground application is useful to clean up spores.
Begin treatment pre-bloom, make a treatment at 20 to 40% bloom and again at 80 to 100% bloom on susceptible varieties or if heavy rainfall and other conditions are occurring that result in high susceptibility to infection.
|Cercospora Leaf Spot||Pathogens: Cercospora circumscissa, Cercospora rubrotincta|
The fungus causes gray or brown spots with dark edges on leaves. Spots can be small or large. The centers of the spots can drop out. Leaves turn yellow and fall prematurely. If the infection is severe enough to cause defoliation for two consecutive years, the peach tree can be seriously weakened.
|Rust||Pathogen: Transchelia discolor|
Infections of young twigs and leaves are the most common symptoms of rust, fruit infections may be a major component of the disease as well.
Transchelia discolor survives in twig cankers or on other host parts, and airborne spores depend on wetness for infection. This disease typically has been more prevalent on cling varieties of peaches than on other varieties because the areas in which cling peaches are grown tend to have higher rainfall, making conditions more conducive to disease development. Fruit symptoms may resemble damage caused by stink bugs; confirm rust by the presence of rust spores within the fruit lesion or by leaf or twig symptoms.
To be effective, treatments must be applied before rust symptoms appear on leaves. Examine one-year-old fruiting wood for small blisters or longitudinal splits. If twig cankers are found and rain is forecasted, make a treatment. If wet weather persists, additional applications may be necessary in late May or early June. Disease severity in the preceding year is an important factor in determining potential of disease during current year. Fall and Dormant applications are efficacious in helping manage the disease.