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Plant Pathology Fact SheetPP-124Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean 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.

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

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

bulb.

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 (conidia) are produced asexually. In

addition, Botrytis spp. are capable of produc-ing sexually derived spores (ascospores) 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

fungus.

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 59° to 68°F. As temperatures deviate

from this optimum range, longer periods of leaf

wetness are required for infection. For example,

temperatures at 48°F and 79°F 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 48°F and

79°F if leaf wetness periods last for nearly 20

hours.

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

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 (conidia) 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

less.

Temperatures below 55°F are not con-

ducive for infection by the fungus causing PB,

but temperatures above 55°F contribute to the

development of this disease. Because the PB

fungus is active across a wide range of tempera-

tures above 55°F, 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.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 1. Lesions caused by Botrytis in onion

leaves.Figure 2. Purple blotch in onion.

Figure 3. Purple blotch in leek.