Peach Scab

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Beginner

This pest affects:

Scientific Name
Cladosporium carpophilus

Identification
Fruit:

Initially small (l-2 mm diameter), greenish-grey to olive, circular spots
Gradually enlarge (2-3 mm diameter) and deepen in color to black with a raised corky appearance; may have a yellow halo around them
Lesions may lead to cracking of the skin as the fruit enlarges
May dry out, shrivel and not ripen
Affected fruit do not ship or store well

Leaves:

Lesions first appear on the underside of the leaf as pale green areas, usually in late summer
Dark brown, narrow lesions develop on mid rib
If infection is severe, premature defoliation may occur

Twigs:

Superficial, oval, reddish-grey spots, 1-2 mm in diameter
More evident later in the season
During fall and winter, spots enlarge
Late spring of second year black, rough surface and spores produced in olivaceous tufts

Often Confused With
Bacterial spot on fruit - small (~ 1 mm) spots that break the fruit skin surface and do not develop the olive-black color caused by the fungal spores on scab lesions.

Period of Activity
Shuck split through harvest.

Scouting Notes
Monitor for peach scab where there was a problem the previous year. Look for overwintering twig lesions while pruning, record their presence on sample trees, and mark several locations to monitor for sporulation activity later.

Beginning in mid to late season, monitor 25 fruit on each sample tree for lesions.

Threshold
A total of 10-20 fruit infections and greater than 20 fruit infections represent moderate and high risk, respectively. These damage levels indicate that improvements in disease management are needed.

Advanced

This pest affects:

Scientific Name
Cladosporium carpophilus

Identification
Fruit:

Initially small (l-2 mm diameter), greenish-grey to olive, circular spots
Gradually enlarge (2-3 mm diameter) and deepen in color to black with a raised corky appearance; may have a yellow halo around them
Lesions may be numerous over the upper fruit surface, scattered over the sides on the side of the fruit exposed to the sun and entirely absent on the lower surface
Where numerous, lesions often coalesce and may lead to cracking of the skin as the fruit enlarges, allowing rot organisms to enter.
May dry out, shrivel and not ripen
Skins of scab-infected fruit do not slip off easily when dipped in hot water prior to canning or freezing
Affected fruit do not ship or store well

Leaves:

Lesions first appear on the underside of the leaf as pale green areas, usually in late summer
Dark brown, narrow lesions develop on mid rib
If infection is severe, premature defoliation may occur

Twigs:

Superficial, oval, reddish-grey spots, 1-2 mm in diameter
Start to develop about the same time lesions are seen on fruit but become more evident later in the season
During fall and winter, spots enlarge to form reddish-brown, irregularly circular to oval lesions 3 × 6 mm. Lesions tend to elongate along the stem and, as they mature during the winter, they develop a slightly raised, dark to brown-purple border
Late spring of second year black, rough surface and spores produced in olivaceous tufts

Often Confused With
Bacterial spot on fruit - small (~ 1 mm) spots that break the fruit skin surface and do not develop the olive-black color caused by the fungal spores on scab lesions.

Biology
Peach scab can occur on nectarine and apricot but is most common on peach.

The peach scab fungus overwinters on 1-year-old peach twigs as mycelium in lesions and as chlamydospores on the surface of lesions. In the spring, conidia are produced on the lesion surface and are washed or splashed primarily by wind-blown rain to fruit, leaves, or new growing twigs. Maximum conidial production requires average daily temperatures exceeding 16ºC and high relative humidity for at least 24 hours. The spores remain firmly attached to the twig until they are moistened. When moistened, they are spread by splashing rain or windblown mist to developing fruit, twigs, and leaves. Spores are produced in the greatest numbers about two to six weeks after the shucksplit stage of development during warm, wet weather. Spore germination and fungus growth is most rapid at 18 to 24ºC.

Conidia are carried to new infection sites (fruit and current season’s shoot growth) by wind and rain-splash, with splashing being the more important mode of dispersal. Infection of current-season shoots can occur any time during the growing season.

Infection is favoured by the presence of free water during periods when air temperatures exceed 10ºC; the optimum temperature range for infection is 22º to 30ºC. Considerable infection can occur, even in the absence of rain, during periods of prolonged fruit wetness caused by dew or fog.

Fruit are first susceptible starting 5-7 days after shuck split.

The susceptible period for nectarine and apricot begins earlier (shuck split) due to the lack of protective hairs on the fruit.

Periods of rainfall with temperatures of 18-24 C are optimal conditions for infection. but growth and infection can occur between 2° and 35°C. Although fruit remains susceptible until harvest, the 4-week period following start of shuck split is the most critical period for infection to occur with 40 to 70 days is needed for symptom development. Thus, infections occurring within 4-6 weeks of harvest do not result in visible lesions. In early varieties only infections that occur during the shuck split to pit hardening stage of development show symptoms before harvest. On late-season varieties, both large lesions (from primary infection) and small spots (from secondary infection) may be present on fruit at harvest.

Twig infections take place as soon as new shoot growth is available in early spring, and they can continue to occur throughout the summer. Sporulation does not generally occur on current year’s lesions, but it occurs on mature lesions during the spring and summer of the year following infection. Examination of such lesions in late spring reveals a black, rough surface, which is evidence of sporulation. Spores are produced in large numbers as olive-coloured tufts of conidia. These conidia can lead to additional (secondary) infections of fruit and twigs.
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Period of Activity
Shuck split through harvest.

Beginning in mid to late season, monitor 25 fruit on each sample tree for lesions.

Management Notes
The critical time for effective disease control begins at the shuck split stage of fruit development. By the time the disease appears, it is too late to do anything about it during the current growing season.

All varieties are susceptible to scab, with some being more severely effected than others. Generally, scab is most severe the first year the trees bear fruit (usually 3rd growing season). This results from the large numbers of twig lesions that develop during the first two growing seasons.

Avoid planting sites that are low-lying or have poor drainage.

Prune trees to allow good air circulation, thus promoting rapid drying of fruit, twigs and leaves.

On peaches, sprays should begin at shuck split. Sprays on nectarines and apricots should begin at about petal-fall. Subsequent sprays should be applied at 10-14 day intervals until approximately 1 month before harvest.

Management with fungicides – Fungicides are used to control peach scab in commercial orchards. See OMAFRA Publication 360, Fruit Crop Protection Guide:

Chapter 5 - Peaches : Recommendations for peach scab shuck split to shuck fall, second generation OFM.

Some information included above excerpted from;

Tender Fruit

Peach Scab

Beginner

Advanced