of Rhode Island GreenShare Factsheets
Fruit Diseases: Scab
Scab occurs wherever peaches, nectarines, plums and apricots
are grown. The disease affects fruit, leaves and young green
twigs. The principle loss from scab is the unsightly spotting
of the fruit skin, but premature defoliation and a dieback of
infected twigs can also occur. Losses are generally greater on
peaches than on nectarines, plums and apricots. Wet conditions
during spring and early summer after petal fall are necessary
for a severe attack by the scab fungus. The disease is usually
more serious in low-lying, moist, shady areas where air movement
Symptoms first appear on fruit when
they are half formed to nearly full-grown--six to seven weeks
after petal fall. Small,
round, olive-green spots about l/l6 - l/8 inch in diameter develop
on the fruit. Spots are most common near the stem end on the
side of the fruit exposed to the sun. Spots are superficial and
slowly enlarge. They may merge to form large, irregular blotches
that turn velvety, dark olive-green or black. Severely infected
fruit may be stunted, become misshapen, or crack open. When fruits
crack open, they are vulnerable to invasion by other fungi that
can rot the fruit. Scabby fruit may drop prematurely and does
not ship or store well. Leaves may also be infected by scab.
Small (l/4 inch), round, and yellowish-green to yellowish-brown
spots develop on the underside of the leaf. Diseased leaf tissue
may dry up and drop, leaving "shot-holes" in the leaves. If the
season is wet, scab-infected leaves usually drop early.
Scab is caused by the fungus Cladosporium carpophilum. The
fungus overwinters in twigs infected the previous year. During
spring and summer, large numbers of microscopic spores (conidia)
are formed on the twig lesions. At 100 percent relative humidity,
20 to 30 hours are required for abundant sporulation in twig
lesions. 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 (when 1/2 of
the developing peach is exposed by the slipping and splitting
of the shuck) during warm, wet weather. Spore germination and
fungus growth is most rapid at 65-75 degrees F. If weather conditions
are favorable, infection begins to occur at about shuckfall.
The fruit remain susceptible until harvested. Forty to 70 days
elapse from the time the spore lands on the fruit until the disease
is visible. The disease is thus not usually observed until the
fruit are well grown. Spores from the fruit re-infect the twigs
and leaves, completing the disease cycle.
1. When planting an orchard, avoid low-lying or shaded
sites with poor air circulation and soil drainage. Any practice
that promotes faster drying of fruit and foliage will help reduce
the risk of infection. Destroy nearby wild or neglected peaches,
nectarines, plums and apricots.
2. Remove and destroy scab-blighted twigs before growth
starts in early spring.
3. Backyard growers should remember that infections are
generally superficial. If infection is not severe, fruit quality
is minimally affected. Peeling infected fruits make them quite
acceptable for processing (canning or freezing) or eating fresh.
Adapted from Michael A. Ellis,
Ohio State University Extension, 2000
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
of Rhode Island Cooperative Extension provides equal program