UF UNIVERSITY of
2006 Florida Plant Disease Management Guide: Lettuce
Ken Pernezny and Richard Raid2
Specific Common Diseases
Alternaria Leaf Spot (Alternaria sonchi,
Symptoms: Small circular spots are formed, the
centers of which gradually dry and turn black.
Production of spores increases the black appearance
of the spots. Leaf lesions often exhibit a zonate
Chemical Controls: Rarely an economic
problem in Florida, the use of maneb fungicides to
control downy mildew on lettuce or endive will aid in
control of this disease.
Bacterial Blight (Pseudomonas marginalis
Symptoms: This disease begins on older, outer
leaves. First, water soaking begins usually towards
the leaf base. Then brown-to-red or black decay
appears and later infected areas become slimy and
foul smelling. During shipment, a pink rib symptom
Cultural Controls: Avoid mechanical injuries
during harvesting. The use of rapid pre-cooling and
the avoidance of crushing and free water in shipping
containers will aid in the reduction in postharvest
Bacterial Leaf Spot (Xanthomonas
campestris pv. vitians)
Symptoms: This has been one of the most
widespread and serious diseases of lettuce to appear
in Florida in recent years. Lesions begin as small
water-soaked spots on outer leaves. As the lesions
mature, they become brown to black and
greasy-looking. Even mature lesions may remain
water-soaked on the underside of leaves. In some
cases, especially on romaine-type lettuces, a
"peppery" look will develop among lesions. So far the
disease has not been observed on endive.
Cultural Controls: Use only disease-free seed.
Avoid movement in fields where plants are wet.
Some differences in susceptibility exist among
cultivars. Romaine-type lettuces generally are most
1. This document is PDMG-V3-40, one of a series of the Department of Plant Pathology, 2006 Florida Plant Disease Management Guide, Florida
Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Revised December 2005. Please visit the EDIS Web site
2. Ken Pernezny, professor, Plant Pathology Department, Everglades Research and Education Center, Belle Glade, FL; Richard Raid, professor, Plant
Pathology Department, Everglades REC, Belle Glade, FL; Florida Cooperative Extension Service, Insitute of Food and Agricultural Sciences, University
of Florida, Gainesville, FL 32611.
The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products
named, and references to them in this publication does not signify our approval to the exclusion of other products of suitable composition.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and
other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex,
sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service,
University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry
2006 Florida Plant Disease Management Guide: Lettuce and Endive 2
susceptible and butterheads are least susceptible.
Growers should not follow lettuce with lettuce in the
same season, especially if the first crop has had a
confirmed outbreak of bacterial leaf spot.
Chemical Controls: Treat seed with bleach or
other disinfectant before pelletization. Copper sprays
in the field applied for other diseases may provide
some control of bacterial leaf spot, especially since
most strains recovered to date are fairly sensitive to
Botrytis Blight (Botrytis cinerea)
Symptoms: The fungus can invade in the
seedling stage through maturity as well as cause a
post-harvest decay. Young plants or seedlings may
damp-off as a result of Botrytis infection. Invasion
normally begins on lower leaf margins and progresses
throughout the head until it is a rotted slimy mess.
The fungus produces a light-brown to ashen-gray
layer of sporulation on diseased tissue. Spores are
easily blown by wind, air movement, agitation or
water splash. The disease may occur over a wide
range of temperatures but is strongly favored by fog,
light rain or high humidity periods, and moderate
Cultural Controls: Prevent condensation and
late-day waterings in greenhouse production sites.
Sanitize infected tissue carefully and destroy. Prevent
tissue damage in the field caused by other diseases,
pesticide, or soluble salts bum.
Chemical Controls: See PPP-6.
Corky Root Rot (Rhizomonas suberifaciens)
Symptoms: This bacterial disease is
characterized by corkiness on the outer portions of
the taproot, necrosis of the tip of the taproot, and
reduced fibrous root mass. Vascular tissue of the
taproot may be discolored and, in advanced cases, the
taproot may disintegrate internally and become
hollow. In the field, infected plants range from
exhibiting little evidence of any disease to wilting,
stunting, and death, depending on the degree of root
Cultural Controls: Differences in cultivar
susceptibility to corky root rot exists. Avoid planting
in fields with a history of corky root rot.
Damping-off (Pythium spp., Rhizoctonia
Symptoms: Seed fails to germinate due to rapid
colonization of seed by soilbome fungi. Excavated
seed will be rotted and soft, often with evidence of
fungal mycelium. Young, newly emerged seedlings
often collapse at soil line and topple over. The stems
may exhibit an obvious discoloration ranging in color
from a reddish-brown to black and may be dry or
mushy to the touch depending on the soil fungus
Cultural Controls: Avoid planting seed when
soil moisture, soil preparation, temperature or
planting depth do not favor rapid emergence.
Chemical Controls: See PPP-6.
Downy Mildew (Bremia lactucae)
Symptoms: Older leaves are the main ones
infected by this fungus, but it may appear on any
above-ground parts. It produces a yellow area on the
upper surface of leaves, and usually a white or
grayish fluffy growth on the underside of the same
leaf areas. Lesions often coalesce into large areas.
The causal fungus may survive on a number of native
weeds including: chicory, cudweed, sowthistle, and
wild lettuce. It may also arrive from other states on
shipments of lettuce.
Cultural Controls: Infection will necessitate leaf
trimming or stripping at harvest.
Chemical Controls: See PPP-6.
Drop (Sclerotinia sclerotiorum)
Symptoms: The first symptom is a progressive
wilt of older and then younger leaves; outer leaves
collapse around plants in a fan pattern. The inner
leaves become soft, dry rapidly, and turn dark in
color. The fungus usually produces a heavy mycelial
growth around the main stem and in cavities in the
plant. As the plant dies, numerous black, irregular
bodies (sclerotia) develop on and in the diseased
2006 Florida Plant Disease Management Guide: Lettuce and Endive 3
tissue. Sclerotia range in size from 1/4-3/4 inch in
length. These survival structures of the causal fungus
will fall to the soil surface where they persist for a
number of years.
Cultural Controls: Since the disease is not
serious every year, it is difficult to suggest a definite
control program. Each of the following control
measures has reduced drop, but when conditions
become favorable for the development of the disease,
it may be necessary to combine all of them in order to
obtain satisfactory control.
1. Rotate with a crop not susceptible to this disease,
such as sweet corn.
2. Turn soil at least 6 inches deep when plowing.
3. Flood the soil either completely, partially or
intermittently for a period of six weeks during
the summer. Before flooding, find out from local
authorities if drainage into a given body of water
4. Apply a fungicide for severe disease situations.
Chemical Controls: See PPP-6.
Erwinia Soft Rot (Erwinia carotovora pv.
Symptoms: This is a typical soft rot disease
characterized by a wet, slimy decay with extensive
brownish discoloration. Affected plants decompose
rapidly in a loose, wet mass in the field. Although this
soft rot is correlated with mechanical injuries on
many crops, it can affect internal leaves within a head
of lettuce and not be apparent from the external
Cultural Controls: Avoid harvesting from field
areas affected by this disease. Handling infected
heads can result in spread of the soft rot bacterium by
workers, harvesting equipment etc.
Mosaic [Bidens mottle virus (BMV), Lettuce
mosaic virus (LMV)]
Symptoms: Field differentiation of BMV from
LMV cannot be reliably done without laboratory
verification. Typical symptoms of BMV on lettuce
include vein clearing, some mottling, and veinal
necrosis, as well as an increase in leaf serration.
Young infected plants may exhibit severe stunting.
Symptoms on endive are a more subtle leaf mottling
with or without plant stunting. Escarole demonstrates
a more pronounced leaf mottling due to the greater
Symptoms of LMV are similar on these leafy
crops. Infected crisphead lettuce types may exhibit
vein clearing, mosaic and stunting. Wrapper leaves
are duller in appearance, typically fold backward and
exhibit more leaf margin serration. Romaine types
exhibit similar symptoms to crisphead types plus a
characteristic leaf blistering. Affected butterhead
types often are stunted and quite chlorotic. Endive
and escarole plants affected with LMV exhibit
chlorotic dots against the green leaf background.
Both viruses are vectored by aphids-especially
the green peach aphid (Myzus persicae). LMV is
known to be seedbore in most lettuce varieties but
not escarole or endive. BMV is not known to be
seedbore. Survival of BMV in Florida is strongly
dependent on weed hosts, such as hairy beggar ticks
(Bidens pilosa), Virginia pepperweed (Lepidium
virginicum), horse weed (Erigeron canadensis),
butterweed (Senecio glabellus), American burnweed
(Erechtites hieracifolia) and Mexican prickle poppy
Cultural Controls: Strong weed control efforts
directed toward old crop land and ditch banks will
reduce surviving virus inoculum. Weed control is
especially important to minimize BMV. State
regulations for a 0 tolerance of LMV infected lettuce
seed in a 30,000 seed test lot reduces LMV incidence
in Florida. Avoid planting lettuce, endive or escarole
crops near old plantings or crops such as carrots,
Chinese cabbage, or radish that are favored feeding
sites for aphids.
Rhizoctonia Bottom Rot (Rhizoctonia solani)
Symptoms: Disease first occurs as soon as outer
or lower leaves touch the soil surface, trapping
moisture. Leaf petioles and midribs become rust
colored and slightly sunken prior to leaf decay.
Infection may spread upward through the head or leaf
canopy destroying the entire plant. Obvious brown
strands of the causal fungus can be observed on
2006 Florida Plant Disease Management Guide: Lettuce and Endive 4
severely affected heads. Sclerotia (dormant survival
structures) of Rhizoctonia often form on the lower
leaf tissue close to the soil.
Cultural Controls: Severely infested fields
should be plowed deeply and allowed to fallow until
all crop debris has rotted.
Chemical Controls: See PPP-6.
Symptoms: This is a physiological (abiotic)
problem. Disease incidence increases as plants
approach marketable stage. Symptoms first appear as
small, translucent spots near the margin of inner
leaves. These spots discolor and the margins die.
Injury can occur with no external signs unless
soft-rotting organisms invade the tipbum areas
causing a soft rot. Tipbum severity is greatest on
butterhead and Romaine (cos) types. Environmental
conditions that favor disease seem to be those where
the greatest difference exists between soil
temperatures and air temperature. This occurs when a
dry, sunny period follows cool, moist weather.
Cultural Controls: Choose tipburn resistant or
tolerant varieties where this disease has been a