Fungal diseases of the leaf may occur as soon as the first leaves
unfold in early spring and continue until dormancy in the late fall.
Generally, these diseases do not cause significant economic damage.
The primary damage from leaf diseases is a loss of vigor through
reduced leaf area. If outbreaks of these leaf diseases become
significant, the plants will become weakened resulting in increased
susceptibility to root diseases and winter injury. The three major
leaf diseases that are caused by fungi have a similar disease cycle
and are controlled in a similar manner. Leaf spot, leaf scorch, and
leaf blight are the most common leaf diseases and they all overwinter
in infected dead or living leaves. They all produce spores that
spread the disease by causing new infections during moist, warm
conditions.
Leaf Spot
Leaf spot is caused by the fungus, Mycosphaerella fragariae. Symptoms
of leaf spot first appear as circular, deep purple spots on the upper
leaf surface. These spots enlarge and the centers turn grayish to
white on older leaves and light brown on young leaves. A definite
reddish purple to rusty brown border surrounds the spots. On fruit,
superficial black spots may form under moist weather conditions. The
spots form on ripe berries around groups of seeds. The spots are
about 1/4 inch in diameter, and there are usually only one or two
spots per fruit. However, some fruits may be more severely infected.
The fungus overwinters as spores in lesions on leaves. The fungus
produces more spores in spots on the upper and lower leaf surface
that spread the disease during early summer. These spores are spread
by splashing rain. Middle-aged leaves are most susceptible. Lesions
also develop on stems, petioles and runners.
Leaf Scorch
Leaf scorch is caused by the fungus Diplocarpon earliana. Symptoms of
leaf scorch consist of numerous small, irregular, purplish spots or
"blotches" that develop on the upper surface of leaves. The
centers of the blotches become brownish. Blotches may coalesce until
they nearly cover the leaflet, which then appears purplish to reddish
to brown. The fungus overwinters on infected leaves. The fungus
produces spore forming structures in the spring on both surfaces of
dead leaves. These structures produce spores abundantly in midsummer.
In the presence of free water these spores can germinate and infect
the plant within 24 hrs. Older and middle-aged leaves are infected
more easily than young ones.
Leaf Blight
Leaf blight is caused by the fungus, Phomopsis obscurans. Symptoms of
leaf blight infections begin as one to several circular
reddish-purple spots on a leaflet. Spots enlarge to V-shaped lesions
with a light brown inner zone and dark brown outer zone. Lesions
follow major veins progressing inward. The whole leaflet may turn
brown. In severe cases, stolons, fruit trusses and petioles may
become infected which may girdle and kill the stem.
The fungus overwinters as mycelium or fruiting structures on the old
leaves that remain attached to the plant. Spores are spread by rain
splash early in the spring. Leaf blight is most destructive to older
leaves in the late summer. Petioles, calyxes and fruit may also be
infected earlier in the season.
Control
Leaf spot and leaf scorch are controlled most effectively by the use
of resistant varieties. The following junebearing varieties are
reported to be resistant to both leaf spot and leaf scorch: Allstar;
Canoga; Cardinal; Delite; Earliglow; Honeoye; Jewell; Lester; Midway
and Redchief. The ever bearing varieties, Tribute and Tristar, are
reported to be tolerant to leaf spot and leaf scorch. There are no
varieties with reported resistance to leaf blight. These cultural
practices should help reduce infection:
- Remove the older and infected leaves from runner plants before
setting.
- Take care in spacing runner plants in matted-row culture.
- Plant in light, well-drained soil in a location exposed to all-day
sun and good air circulation.
- Control weeds in the planting. Weeds reduce air circulation and
increase drying time for leaves. (Leaves stay wet longer in weedy
plantings.)
- Removing infected leaves after harvest (during renovation) is
helpful in reducing inoculum and controlling all the leaf diseases.
If leaf diseases are a problem in the planting, fungicides will aid in
control. For the most current fungicide recommendations and spray
schedules commercial growers are referred to Bulletin 506-B2
"Ohio Commercial Small Fruit Spray Guide," and backyard
growers are referred to Bulletin 780 "Controlling Disease and
Insects in Home Fruit Plantings." Backyard growers are
encouraged to use resistant varieties. This should eliminate the need
for using fungicides to control these diseases. These publications
can be obtained from your county extension agent or the Extension
Publications Office, The Ohio State University, 385 Kottman Hall,
2021 Coffey Road, Columbus, Ohio 43210-1044.
-----------------
All educational programs conducted by Ohio State University Extension
are available to clientele on a nondiscriminatory basis without regard
to race, color, creed, religion, sexual orientation, national origin,
gender, age, disability or Vietnam-era veteran status.
Keith L. Smith, Associate Vice President for Ag. Adm. and
Director, OSU Extension.
TDD No. 800-589-8292 (Ohio only) or 614-292-1868
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