UF UNIVERSITY of
2006 Florida Plant Disease Management Guide: Onion1
Richard Raid and Tom Kucharek2
Specific Common Diseases
Botrytis Leaf Blight/Blast (Botrytis
Symptoms: Botrytis leaf blight (BLB),
sometimes called blast, is caused by the fungus B.
squamosa. While other species of Botrytis have been
associated with disease of leaves and bulbs of onions,
B. squamosa is generally regarded as the cause of
leaf spots in onion foliage and sometimes in the outer
scales of the bulb.
Botrytis neck rot, typically a post harvest
disease, is likely to be caused by B. allii or B.
cinerea. B. cinerea, a common pathogen of many
other crops, also may produce leaf symptoms slightly
similar to those caused by B. squamosa but such
infections are more superficial and generally regarded
The generic name Botrytis refers to the portion
of the cycle of Botrytis spp. during which the spores
conidiaa) are produced asexually. In addition,
Botrytis spp. are capable of producing
sexually-derived spores ascosporess) in the portion of
the life cycle called Botryotinia.
The sources of spores that cause BLB in onions
grown in Florida have not been determined. However,
nearby onion plantings, volunteers, and old onion
debris in fields or cull piles are probable sources of
inocula. Asexually produced conidia are likely to be
the principal type of inoculum. Conidia, dispersed
mainly by wind, are produced abundantly, during
cool, wet conditions on blighted leaves, and onion
Sclerotia are embedded in bulb or leaf tissues on
onions or may exist in soil after the onion tissue has
completely decomposed. They provide a mechanism
for long-term survival of his fungus. Sclerotia are
somewhat elongated structures of compacted masses
of fungal hyphae with a black outer rind and a white
to gray interior that may be as large as several
conidia. Sclerotia can produce sexually derived
spores called ascospores. Ascospores are produced in
microscopic sacs within mushroom-like structures
(apothecia) that grow on the outside of sclerotia.
Although ascospores can serve as inoculum for
disease, they are probably more important as sources
of new genetic variants for this fungus.
Botrytis leaf blight has been seen most
commonly south of Gainesville in Florida. It is
1. This document is PDMG-V3-42, 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. Date Reviewed: January 2006. Please visit the EDIS Web site at
2. R. Raid, professor, Plant Pathology Department, Everglades Research and Education Center--Belle Glade, FL; T.A. Kucharek, professor, Plant Pathology
Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.
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: Onion 2
regarded as a cool, wet-weather disease. Some
infection can occur with only seven hours of leaf
wetness if temperatures are near the optimum range
of 59F to 680F. As temperatures deviate from this
optimum range, longer periods of leaf wetness are
required for infection. For example, temperatures at
480F and 79F are conducive to infection, provided
that leaf wetness periods increase to 10 hours. Levels
of infection should be regarded as being most severe
if leaf wetness periods approach 14 hours, provided
that temperatures are in the optimum range.
Symptoms of BLB include whitish flecks on
onion leaves. Botrytis cinerea can cause similar, but
more superficial symptoms. This latter fungus,
common in Florida, causes diseases in many
broadleaf plants but is not known to cause disease in
onions in Florida. Herbicides, thrips and ozone can
induce symptoms that appear somewhat similar to
those of BLB in onions. However, lesions of BLB
often begin as distinct watersoaked areas that persist
around mature lesions. With increasing severity of
BLB, onion fields appear progressively more yellow
Older (outer) leaves tend to be more susceptible
to BLB that younger leaves. Conidial production is
greatest on blighted and dead leaves. Small lesions in
green leaves may not produce conidia. Because
larger, more numerous lesions occur on older leaves,
BLB appears more as a blight than as distinct spots
on older leaves, particularly near the leaf tips. See
Plant Pathology Fact Sheet PP-124.
Chemical Controls: See PPP-6.
Damping-off (Fusarium spp., Pythium spp.)
Symptoms: The seedlings are infected at or
slightly below the soil line. When this occurs the
tissue shrinks rapidly at or near the point of infection
causing the above-ground parts of the plant to topple
over. These areas of infection usually result in
more-or-less circular spots of various sizes in the
Cultural Controls: Consists of preventing
infection by cultural and other methods and by seed
and soil treatment. Cover crops, grasses, and weeds
should be turned under in sufficient time for the plant
material to decay prior to planting the onion seeds.
Good drainage to prevent the seedlings standing in
saturated or extremely wet soil, and the
encouragement of favorable growing conditions all
help in prevention of damping-off. See Plant
Pathology Fact Sheet No. 1.
Chemical Controls: Use seeds with a seed
treatment. See PPP-6.
Downy Mildew (Peronospora destructor)
Symptoms: This disease has not occurred
frequently in Florida. Downy mildew is favored by
humid weather conditions. The first symptoms are
found by examining onion leaves early in the morning
while the dew is still on the plants. The somewhat
violet-colored furry growth on the surface of the leaf
or seed stem is characteristic of the downy mildew
disease. The affected leaves gradually become pale
green and later yellowish, and the diseased parts
The fuzzy growth, which is the causal fungus,
becomes widespread under conditions of high
humidity. The disease usually starts in areas of the
field where the dew remains longest and then spreads
to surrounding areas. If the weather becomes dry
with short dew periods and relatively low humidity,
spread of the disease will slow significantly.
Infected plants often send out new leaves and
partially recover. Downy mildew lesions on seed
stems are circular and elongated and often affect only
one side of the leaf. This weakening of one side of
the leaf frequently causes it to break over.
Chemical Controls: See PPP-6.
Purple Blotch (Alternaria porr)
Symptoms: Bulbing onions, green onions, and
leeks have been infected frequently in Florida, but
other types are also susceptible. As disease severity
increases, leaf size, bulb size, and bulb weight
decrease drastically. Purple blotch (PB) occurs
throughout the state and yield losses due to PB have
been measured at more than 50 percent in Florida.
2006 Florida Plant Disease Management Guide: Onion 3
Symptoms induced by the fungus causing PB are
of two types. Initially, flecks similar to those induced
by the BLB fungus may occur in leaves. Later oval to
football-shaped lesions appear, sometimes
accompanied by linear yellow to brown streaks that
progress from the main lesion. Individual lesions may
be two or more inches long, light brown throughout
or have a dark brown to purple central area
surrounded by a light brown area. Lesions may
contain alternating, concentric zones of dark and
lighter tissue. The concentration of asexually
produced spores conidiaa) is greatest in the darker
portions of the lesions. With increasing severity of
PB, leaves become generally yellow to brown and
Conidial inoculum for initiating an epidemic can
originate in nearby planting, volunteers, old, infected
onion debris, and possibly other debris in the field.
These spores are dispersed primarily by wind. Upon
landing on a leaf, a spore germinates and then
penetrates the leaf when the next leaf wetness period
occurs. From such infections, new lesions may form
in seven days or less.
Temperatures below 55F are not conducive for
infection by the fungus causing PB, but temperatures
above 55F contribute to the development of this
disease. Long periods of leaf wetness and relative
humidities above 90 percent generate higher
probabilities for infection and disease occurrence.
Leaf wetness periods of nine to eleven hours are
adequate to promote spore production and infection.
As periods of leaf wetness become longer, conditions
for development of PB are improved. Emerging
leaves become increasingly more susceptible to PB
as the bulbs approach maturity. See Plant Pathology
Fact Sheet PP-124.
Cultural Controls: Control of PB includes
rotating crops with non-susceptible crops,
accelerating decomposition of old onion debris in the
field, destroying volunteers, and using healthy
Chemical Controls: Spraying with approved
fungicides is often necessary. A sequence of spray
treatments, delivered at five-to-seven day intervals,
should be initiated when symptoms of PB first appear
and leaf wetness periods exceed 11 hours. Spray
intervals can be lengthened during extended dry
periods, when leaf wetness lasts for less than nine
hours. Thoroughly covering onion leaves with the
spray is essential to achieve control. With proper use
of nozzle arrangements and spreader-stick adjuvants,
excellent control of PB and Botrytis Blight can be
achieved. See PPP-6.