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Spur
Blight and Cane Blight of Raspberries
May 2000
Spur and
cane blights are common, serious diseases of raspberries in Illinois,
especially during wet seasons. These blights often occur together on red
raspberries, weakening the canes and reducing yield.
Spur Blight
Spur blight is caused by the fungus Didymella applanata (the imperfect
stage is an unnamed Phoma sp.). This disease is more common and
serious on red raspberries and to a lesser extent on black and purple
raspberries and loganberries. Blackberries and dewberries are highly resistant
to this disease. Spur blight can cause yield losses in several ways. It
can blight the fruit bearing spurs, cause premature leaf drop, and kill
buds on the canes that later develop into fruit bearing side branches.
In addition, berries produced on diseased canes may be dry, small, and
seedy. Affected canes may be more vulnerable to winter injury than uninfected
ones.
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Spur
Blight Symptoms
Chocolate brown, dark blue, or purplish spots with encircling bands
form on the new canes and leaf petioles in the late spring or early
summer, usually at a bud or leaf attachment (Figure 1). The infected
areas, which vary from half an inch to several inches long, gradually
enlarge until the cane is girdled. By late summer the bark in the
cankered area dries up, and the canes may crack and split lengthwise.
Speck-sized, black reproductive bodies (pycnidia) of the fungus
develop in the dead bark. Later in the fall, the bark in these areas
turns silvery gray, and numerous black pustules containing perithecia
(another reproductive structure of the fungus) form on the lesion.
Many buds on fruiting canes that were infected the previous year
shrivel and die. Surviving infected buds develop small, weak, lateral
shoots with stunted yellow leaves that wither and die early in the
season.
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Click
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Figure 1. Red Raspberry Canes Showing Spur Blight Infections
(dark areas). Note dead buds and black specks -- fungus fruiting
bodies.
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Leaves sometimes
become infected and show brown, angular or wedge-shaped areas. The widest
part of the wedge is toward the margin or tip of the leaflet. Spur blight
spreads into the leaf petiole and into the cane at the point of attachment.
Leaflets on diseased canes turn yellow and drop prematurely, sometimes
leaving the petiole (leaf stem) attached to the cane.
Spur Blight
Disease Cycle
The fungus survives on infected canes during winter. The following spring
and summer, during wet and rainy periods, spores are released and carried
by splashing rain and wind to nearby canes and leaves, where they germinate
and penetrate plant tissue. Infections commonly occur where the leaf petiole
is attached to the stem.
Large numbers
of spores (conidia) ooze from pycnidia during warm, wet weather. These
spores are washed and splashed to other canes, where new infection occurs
through wounded or unwounded tissue.
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Figure 2. Cane Blight on Black Raspberry -- Note Gray Spore
Masses.
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Cane
Blight
Cane blight is caused by the fungus Leptosphaeri coniothyrium
(the imperfect stage, Coniothyrium fuckelii). This disease,
more common on black raspberries, also occurs on red and purple
varieties but rarely attacks blackberries and dewberries. Cane blight
can result in wilt and death of lateral shoots, a general weakening
of the cane, and reduced yield.
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Cane Blight
Symptoms
Toward the end of the season, dark brown to purplish black cankers form
where pruning, insect, and other wounds occurred on young canes. The cankers
enlarge and encircle the cane causing the lateral shoots to wilt and die.
On second-year canes, the side branches may suddenly wilt and die, usually
between blossom time and fruit ripening. Close examination reveals the
presence of cankers on the branches or the main cane where insect injury,
pruning wound, or other type of wound has occurred. Infected canes commonly
become cracked and brittle, and break easily. Numerous black specks (pycnidia),
the reproductive bodies of the fungus, develop in the cankers. In wet
weather, large numbers of microscopic, olive-colored spores (conidia)
ooze from the pycnidia, giving the bark a characteristic dark gray, smudgy
appearance.
Cane Blight
Disease Cycle
The pathogen survives over winter on infected or dead canes. The following
spring, conidia, formed in pycnidia, ooze out during wet periods. These
spores are rain splashed, blown, and carried by insects to nearby canes.
Under moist conditions these conidia will quickly germinate and penetrate
any type of wound, rapidly killing cane tissue. Infection occurs at almost
any time of the growing season. Pycnidia are formed in the dead portions
of the older cankers. Dead canes continue to produce conidia and remain
a source of infection for several years.
Control
of Spur Blight and Cane Blight
These blights are not difficult to control if the following suggestions
are followed.
- Select a sunny planting location with good soil drainage and air
circulation. Avoid shady areas. The longer the canes and foliage
remain wet from dew, rain, etc. the greater the chance for spur blight
and cane blight development.
- Plant certified, No. 1 grade, one-year-old, substantially virus-free
stock. When setting out new plants in the early spring, cut off
the "handles" (old cane stubs) at ground level and destroy
them. Before new shoots appear, remove and destroy all dead and
winter-injured canes from established plantings. Also remove all wild
or neglected raspberries and blackberries in the area.
- Fertilize raspberries to maintain plant vigor, but avoid using
an excessive amount of fertilizer.
- Keep the fruit-planting and surrounding region free of weeds and
cultivate carefully to reduce root injury. Keep the rows about 12
to 18 inches (30 to 46 cm) wide for air circulation, to allow penetration
of sunlight, and to improve spray coverage.
- Remove and destroy all old fruit canes immediately after harvest.
Thin the new canes to about 6 for staked-hill plantings, or 4 to 6 inches
(10 to 15 cm) apart for hedgerow plantings. Remove all weak, short,
spindly, and injured canes.
- Topping canes and other pruning to reduce excess growth should
be done in dry, clear weather so the wounds will have a chance to heal
before rain is expected. Weak, broken, or infected lateral branches
on fruiting canes should be cut back to healthy wood.
- Follow the spray schedule outlined in "Illinois Commercial Small
Fruit and Grape Spray Guide", Circular ICSG-2, and "Illinois Homeowners'
Guide to Pest Management", Circular 1354. Thorough coverage of all
canes and foliage with each application is essential for blight control
and for successful fruit production. If possible, apply the sprays a
day or two before rain is predicted.
In the spring, spray when the buds show no more than 3\8 inch (1
cm) of green at the tips. For this spray only, use liquid lime-sulfur
or copper hydroxide 50 WP. Copper hydroxide is available for use on
raspberries as Blueshield 50 WP or Kocide 50 WP. Both products have
a 48 hour reentry interval. This spray also helps control anthracnose,
certain insects, and mites. Delaying this spray after more than 3/4
inch of growth of new shoots will burn exposed foliage.
Where disease has been severe, spray with Captan or Benlate according
to label recommendations. Spray when the new canes are 6 to 8 inches
(15 to 20 cm) tall; when shoots are 12 to 15 inches high (30 to 38
cm) or just before the blossoms open on the fruiting canes; just after
bloom, as soon as the petals fall; and just after the fruit has been
harvested and old canes removed. The first two sprays of Benlate or
Captan may not be needed where these blights and anthracnose are not
severe and the liquid lime-sulfur has been properly applied.
- Keep raspberry plantings free of insects such as crown borers,
stem girdlers, aphids, fruitworms, rose scale, sawflies, plant bugs,
tree crickets, picnic and sap beetles, and mites. Most suggested
insecticides and miticides, except for liquid lime-sulfur, may be safely
mixed and applied at the same time. Follow all the manufacturer's directions.
Further
information concerning fruit diseases can be obtained by contacting Mohammad
Babadoost, Extension Specialist in Plant Pathology, Fruit and Vegetable
Diseases, Department of Crop Sciences, University of Illinois, Urbana-Champaign.
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