Title: Downy mildew of crucifers
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00066897/00001
 Material Information
Title: Downy mildew of crucifers
Series Title: Plant Pathology Fact Sheet PP-33
Physical Description: Book
Language: English
Creator: Kucharek, Tom
Affiliation: University of Florida -- Florida Cooperative Extension Service -- Department of Plant Pathology -- Institute of Food and Agricultural Sciences
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: 1985
Spatial Coverage: North America -- United States of America -- Florida
 Record Information
Bibliographic ID: UF00066897
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
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Full Text

Plant Pathology Fact Sheet

Downy Mildew of Crucifers

Tom Kucharek, Professor and Extension Plant Pathologist, Plant Pathology
Department, University of Florida, Gainesville 32611. 1985, Copied January
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean


Downy mildew of cruciferous crops is
caused by the fungus Peronospora parasitica. Cab-
bage, Chinese cabbage (including NAPA and
bok choy), broccoli, cauliflower, radish, turnips,
Brussels sprouts, mustard, collard, rutabagas,
kohlrabi, rape, forage kales, and kale are sus-
ceptible. The susceptibility of many crucifer-
ous weeds in Florida is unknown; they are in-
fected by this fungus but it remains unknown
at this time if the strains infecting such weeds
will also infect cruciferous crops. For practical
purposes it is presumed that at least some cru-
ciferous weed strains of this fungus infect cru-
ciferous crops. Varieties within cruciferous
crops and the crops themselves will vary in
their susceptibility to downy mildew, but in-
formation on this aspect is scant. Therefore, do
not make hasty decisions about changing vari-
eties without first inquiring into the availabil-
ity of current information about varieties or first
experimenting with small plantings of new va-
rieties. If varietal reaction to downy mildew is
available, it would be available for major cru-
ciferous crops such as cabbage, broccoli, cauli-
flower and maybe radish. On the other crucif-
erous crops, personal experience may have to
be relied upon. This issue is further compli-
cated by the presence of different strains (races)
of this pathogen. The presence of different races
is known, but practically speaking this infor-

mation is of minimal value at this time. When
race surveys are conducted and crop varieties
are evaluated extensively each year, such
knowledge could become useful. There is some
evidence that strains that infect radish are not
aggressive on other cruciferous crops. Also, the
weed, shepards purse, has not been success-
fully infected with the downy mildew fungus
using spores from commercial crops even
though this fungus was first described on
shepards purse in 1796.

The fungus causing downy mildew on
crucifers produces two spore types. One spore
is called an oospore and presumably functions
as a survival spore. It is formed within infected
host tissue primarily during crop senescence.
Then, when the next cruciferous crop is planted
it might serve as a new source of inoculum by
germinating, developing microscopic threads
hyphaee) that later produce the second spore
type called conidia. Some reports suggest the
possibility that oospores can be carried on seed
if infected plants are used for seed production.

It is questionable at this time whether the
oospore stage of the life cycle is of any practi-
cal consequence for carry over of this fungus.
First, germination of oospores has not been re-
ported. Second, cruciferous crops or volunteers
infected with downy mildew abound through-
out the year in Florida. During the summer this
disease spreads slowly but it can exist in pre-
viously infected plants either in a systemic or


locally infected manner. Then, when cooler tem-
peratures occur, the fungus will produce
conidia that are disseminated by wind or wind
driven rains. Third, cruciferous weeds may har-
bor the fungus in a similar manner as crucifer-
ous crops but at this time it is unknown if such
weeds act as reservoirs for strains of this fun-
gus that infect cruciferous crops. Some scien-
tists believe that weeds are not a source of in-
oculum while others are in doubt. Whatever the
source of inoculum may be, it is ample because
when environmental conditions are conducive
for downy mildew, this disease can abound on
cruciferous crops.

Cool and wet conditions are conducive
for downy mildew of crucifers. Spore produc-
tion is greatest from 530 to 610F but occurs to
some extent from 390 to 850F and maybe at
slightly higher temperatures. When spores are
in contact with a leaf with moisture they germi-
nate and the germ tube apparatus penetrates
the tissue. Temperatures between 420 to 610F
are ideal for these activities but they can occur
between 390F to 750F. Symptoms can occur
within 3 to 4 days after infection if temperatures
near 750F prevail during days following the
infection period. As temperatures deviate from
previously mentioned optima, disease devel-
opment and spread is progressively slower.

Moisture is required for disease devel-
opment. It is probably correct to say that the
greater the rainfall or overhead irrigation or the
longer the wetting periods from rainfall, dew,
etc., the more severe this disease will be if tem-
peratures are suitable.

All plant parts of crucifers can become
infected with this fungus. In Florida, leaf symp-
toms are commonly observed. Black or dark
specks appear on young leaves, usually on the
underside of the leaf first. Such spots are often
irregular in shape and may appear net-like
(Figs. 1 & 2). The upper side of the leaf will also
develop dark spots similar in shape and may

be accompanied by leaf yellowing (Figs. 3 & 4).
On older leaves, coalescence of these spots can
occur, resulting in larger areas of the leaf blade
having large, sunken, paper tan-colored spots
(Fig. 5). Leaf yellowing, again, may accompany
these symptoms. Early infection on young
plants can cause stunting.

On the underside of the leaf spots, a
white-grey, downy growth can be observed
with or without the aid of a hand lens, espe-
cially when leaves are wet. On mature cabbage,
downy mildew can appear as dark sunken
spots on the head or wrapper leaves (Fig. 6).
Often infections on the cabbage head will re-
sult in a purplish tinge. Such infections predis-
pose the plant to soft rot bacteria or
Sclerotiniose (see PP Fact Sheets 12 and 22),
which can further rot tissue in the field or after

Cauliflower curds and broccoli heads
can become infected with blackened areas on
the outside of the tissue. The infection can be-
come systemic and turn inner curd and stem
tissue dark. Radish and turnip "roots" can be-
come infected by spores that are washed down
to the soil from the leaves. Symptoms might
predominate on the upper part of the root but
the entire root is susceptible. Black spotting or
a netted appearance can be observed on the
outside of the root but an internal, firm rot can
occur as well in some situations. Some root dis-
tortion could occur, especially, if infection oc-
curred early in relation to root swelling. Flow-
ers and seed stalks of cruciferous crops, espe-
cially mustard, are also infected.


Downy mildew is controlled primarily
by fungicides at the present time. Nonsystemic
fungicides should be applied at least weekly
beginning when night time temperatures are in
the range conducive for sporulation and dis-

ease development and when rains or irrigations
are frequent or heavy. Systemic fungicides, as
they are developed, will more than likely re-
quire considerably fewer applications but
should be used as a tank mix with non-systemic,
protective fungicides. Systemic fungicides have
a history of losing their effectiveness because
"new" strains, resistant to systemic fungicides,
increase in population. The tank mix of sys-
temic and non-systemic fungicides may
lengthen the effective "lifespan" of these spe-
cific, systemic fungicides. For details on cur-
rent fungicide usage contact your local county
extension office. Downy mildew favorable
weather may occur at seeding or transplanting

Therefore, be prepared to spray even at an
early stage of crop development.

It is customary with many plant diseases
to integrate fungicide spray programs with cul-
tural controls and resistant varieties. With
downy mildew of crucifers, however, many
sources of spores may exist. The life cycle of
this disease is imperfectly understood and re-
sistant varieties may be difficult to document,
making it difficult to recommend a long list of
definitive non-chemical controls. Certainly, the
grower should purchase or produce downy
mildew-free transplants. Excess plants pro-
duced in ground transplant beds should be
plowed down, preferably with a mold-board
plow, as soon as successful transplanting is
complete. Similarly, excess plants produced in
other types of transplant operations should be
destroyed as soon as possible. Do not dump
such plants in cull piles where they might con-

tinue to grow and serve as sources of inocu-
lum for downy mildew and other diseases.
When growing plants in doors maximize ven-
tilation within the structure. This practice re-
duces moisture retention periods on the leaves.
Transplants grown indoors under partial shade
will retain moisture for a longer period of time
and temperatures might be cool enough for
downy mildew development even though tem-
peratures outside are too high for disease de-
velopment. Crop rotation with non-cruciferous
crops might be of some benefit.

Where ground transplant beds are used,
rotate sites each year. Transplant production
should be at least 1/4 mile from production
fields if possible. Volunteer cruciferous plants
should be eliminated. Cruciferous weeds
should be controlled as best as possible in case
they might serve as a source of spores.

This may be difficult or impractical in
some cases such as in ditch banks. Weed con-
trol in the vicinity of transplant beds should be
maximized. Direct seeded field plantings have
been observed to have less downy mildew, pre-
sumably because the initial canopy is thin and
allows for faster evaporation of leaf moisture
compared to the thick canopy associated with
some transplant beds.

Determine which varieties are more re-
sistant to downy mildew by consulting seed
catalogs and local data sources. When experi-
menting with new sources of seed, isolate them
in transplant production areas and fields to re-
duce chances of introducing black rot (See P.P.
Fact Sheet No. 13) on your farm.

Figure 1. Downy mildew in cabbage leaf.

Figure 2. Downy mildew on underside of
cabbage leaf.

Figure 3. Downy mildew in cabbage leaves.

Figure 4. Downy mildew in radish leaves.

Figure 5. Downy mildew in broccoli leaf. Figure 6. Downy mildew in cabbage head.

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