Department of Plant Pathology |
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| LETTUCE
DISEASES |
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Rhizoctonia
Bottom Rot, caused by the fungus Rhizoctonia solani, starts
on leaves next to the ground and later progresses into the head. Affected
areas become dark brown and slimy, but later may dry out-leaving an
erect mummified plant. Rhizoctonia persists in field soil. Disease
is promoted by moist conditions. |
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Downy
Mildew, caused by the fungus Bremia lactucae, affects seedlings
and mature plants. Symptoms appear first on oldest leaves. Yellowish
or light green blotchy areas appear on the upper sides of leaves.
A white downy mold then appears on the underside of the leaf spots;
finally the affected areas die. The fungus overwinters in residue
and in wild lettuce plants. Fungus spores are spread by wind. Spore
production is favored by temperatures cooler than 65 ° F and by
relative humilities approaching 100 percent. |
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Sclerotinia
Drop symptoms, caused by the fungus Sclerotinia sclerotiorum begin on the stem near the soil surface. A severe wet rot develops
rapidly and spreads downward on the roots and upward through the head.
Once the base of a leaf is rotted, the leaf wilts, withers, and dies.
These symptoms successively develop from outer leaves to inner leaves.
The head becomes a wet slimy mass. During wet conditions a white cottony
mold develops on rotted plant parts; hard irregular black sclerotia
(pea-sized persistent fungus structures) may be present in the mold.
Sclerotinia reproduces in soil planted to susceptible crops (such
as tomatoes, cabbage, celery) and then can persist as sclerotia for
many years in soil. Wet conditions favor disease development. |
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Botrytis
Gray Mold caused by the fungus Botrytis cinerea, appears on
plants at all stages of maturity. Affected seedlings look like they
have damping-off. On older plants, rot begins on the stem or on lower
leaves where they touch the soil; a slimy rot spreads upward into
the head. The diagnostic feature of gray mold is development of a
dense fuzzy gray mold on exposed surfaces and affected areas. Dark
hard sclerotia (pea-like structures that function as overwintering
fungus "seeds") may develop on affected heads. Botrytis
is present wherever plants are grown. Disease development is favored
by moist conditions. |
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Aster
Yellows, caused by a mycoplasma, is characterized by a yellowing
and curling of the youngest leaves. At heading time, heart leaves
remain dwarfed and curled and heads remain soft. The mycoplasma overwinters
in many perennial weeds and is spread to lettuce by leafhoppers during
their feeding activities. |
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Mosaic
can be caused by several viruses, including lettuce mosaic virus (LMV)
and cucumber mosaic virus (CMV). Symptoms caused by these viruses
are similar. Infected plants are stunted, yellowish, and do not head
properly. LMV is seed-borne; CMV overwinters in many perennial wild
and cultivated hosts. Both viruses are spread by aphids and by mechanical
means. |
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Big
Vein, a disease characterized by virus-like symptoms, may be
caused by a viroid (a type of infectious nucleic acid). Young plants
that are infected develop a characteristic clearing of the area around
leaf veins; these plants remain small and stunted and never produce
marketable heads. In more mature plants, a distinctive clearing of
yellowing of tissue next to major veins appears. These plants appear
to have "big veins." Infected plants are more upright than
usual and leaves are ruffled. The casual agent is soil-borne; a water
mold, Olpidium brassicae, is the vector. The causal agent persists
in soil for at least 10 years. Symptoms develop best between 42 and
60 ° F; at higher temperatures, leaf symptoms fade. |
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