Purple blotch

Title: Botrytis leaf blight and purple blotch of onions in Florida
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00066815/00001
 Material Information
Title: Botrytis leaf blight and purple blotch of onions in Florida
Physical Description: Book
Creator: Kucharek, Tom
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: 1996
Funding: Florida Historical Agriculture and Rural Life
 Record Information
Bibliographic ID: UF00066815
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Table of Contents
        Page 1
    Purple blotch
        Page 2
        Page 3
        Page 4
Full Text

Plant Pathology Fact Sheet

Botrytis Leaf Blight and Purple Blotch of

Onions in Florida
Tom Kucharek, Extension Plant Pathologist, Plant Pathology Department, Uni-
versity of Florida, Gainesville FL 32611.1996, Revised January 2000.

Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean

Onions are grown throughout Florida in
small-to medium-sized commercial plantings
or in home gardens. Numerous plant diseases
occur in the roots, bulbs, and leaves of onion.
Purple blotch and botrytis leaf blight are the
most commonly occurring leaf diseases of on-
ions in Florida. Downy mildew, a disease that
occurs commonly in some states, has been
rarely found in Florida.


Botrytis leaf blight (BLB), sometimes
called blast, is caused by the fungus Botrytis
squamosa. While other species of Botrytis have
been associated with diseases of the leaves and
bulbs of onions, B. Squamosa is generally re-
garded as the usual cause of leaf spots in onion
foliage and sometimes in the outer scales of the

Botrytis neck rot, typically a postharvest
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
that are similar to those caused by B. squamosa
but such infections are more superficial and
generally regarded as rare.

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 produc-

ing 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 deter-
mined. However, nearby onion plantings, vol-
unteers, and old onion debris in fields or cull
piles are probable sources of inocula. Asexu-
ally produced conidia are likely to be the prin-
cipal type of inoculum. Conidia, dispersed
mainly by wind, are produced abundantly dur-
ing cool, wet conditions on blighted leaves,
onion debris, or sclerotia of the fungus.

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 this fungus. Sclerotia are somewhat elon-
gated structures of compacted masses of fun-
gal hyphae with a black outer rind and a white
to grey interior that may be as large as several
millimeters. In addition to the ability to pro-
duce conidia, sclerotia can produce sexually de-
rived spores called ascospores. Ascospores are
produced in microscopic sacs within mush-
room-like structures (apothecia) that grow on
the outside of sclerotia. Ascospores can serve
as inoculum for disease, and they are impor-
tant as sources of new genetic variants for this

Botrytis leaf blight has been seen most
commonly south of Gainesville in Florida. It is


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 590 to 680F. As temperatures deviate
from this optimum range, longer periods of leaf
wetness are required for infection. For example,
temperatures at 480F and 790F 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 wet-
ness periods approach 14 hours, provided that
temperatures are in the optimum range. Severe
levels of infection can also occur at 480F and
790F if leaf wetness periods last for nearly 20

Symptoms of BLB include whitish flecks
in onion leaves (Figure 1). 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 symp-
toms that appear somewhat similar to those of
BLB in onions. However, lesions of BLB often
begin as distinct watersoaked areas that per-
sist around mature lesions. With increasing se-
verity of BLB, onion fields appear progressively
more yellow in color.

Older (outer) leaves tend to be more sus-
ceptible to BLB than younger leaves. Conidial
production is greatest on blighted and dead
leaves. Small lesions in green leaves may not
produce conidia. Because larger, more numer-
ous lesions occur on older leaves, BLB appears
more as a blight than as distinct spots on older
leaves, particularly near the leaf tips.

Control of BLB includes crop rotation
with crops not related to onions (leeks, garlic,
chives, and amaryllis are in the same plant fam-
ily). Destroying old onion debris in fields by
burying it with moldboard plows will reduce
inocula. Cull piles in fields and loading areas
near packing houses should be destroyed. Dis-
ease-free transplants should be used. A se-

quence of fungicide spray treatments may be
necessary to reduce BLB and improve the yield
and quality of the onions. Fungicide spray pro-
grams are most effective when initiated before
the disease becomes too severe. If temperatures
and leaf wetness are suitable for BLB, spraying
should be initiated when no more than one le-
sion of BLB per leaf has appeared. Spray inter-
vals of five to seven days may be required dur-
ing weather favorable for disease. Thoroughly
covering onion leaves with the spray is essen-
tial for a high level of control.


Purple blotch (PB) is caused by the fun-
gus Alternaria porri. Bulbing onions, green on-
ions, and leeks have been infected frequently
in Florida, but other onion types also are sus-
ceptible. As disease severity increases, leaf size,
bulb size, and bulb weight decrease drastically.
Purple blotch occurs throughout the state and
yield losses due to PB have been measured at
more than 50 percent in Florida.

Symptoms induced by the fungus caus-
ing 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 yel-
low to brown streaks that progress from the
main lesion (Figures 2 and 3). Individual lesions
may be two or more inches long. Individual
lesions may be light brown throughout or have
a dark brown to purple central area surrounded
by a light brown area. Lesions may contain al-
ternating, concentric zones of dark and lighter
tissue. The concentration of asexually produced
spores conidiaa) is greatest in the darker por-
tions of the lesions. With increasing severity of
PB, leaves become generally yellow to brown
and lose erectness.

Conidial inoculum for initiating an epi-
demic can originate in nearby plantings, vol-
unteers, old, infected onion debris, and possi-
bly other debris in the field.

These spores are dispersed primarily by
wind. Upon landing on a leaf, a spore germi-
nates, and then penetrates the leaf when the next
leaf wetness period occurs. From such infec-
tions, new lesions may form in seven days or

Temperatures below 550F are not con-
ducive for infection by the fungus causing PB,
but temperatures above 550F contribute to the
development of this disease. Because the PB
fungus is active across a wide range of tempera-
tures above 550F, increasing 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 devel-
opment of PB are improved.

Figure 1. Lesions caused by Botrytis in onion

Onion leaves become more susceptible
to PB as they age. Emerging leaves become in-
creasingly more susceptible to PB as the bulbs
approach maturity.

Control of PB includes rotating crops
with nonsusceptible crops, accelerating decom-
position of old onion debris in the field, de-
stroying volunteers, and using healthy trans-
plants. Spraying with labeled fungicides is of-
ten necessary. A sequence of spray treatments,
delivered at five-to-seven day intervals, should
be initiated when symptoms of PB first appear
if 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 arrange-
ments and spreader-sticker adjuvants, excellent
control of PB and BLB can be achieved.

Figure 2. Purple blotch in onion.



- .k : I

Figure 3. Purple blotch in leek.

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