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Copyright 2005, Board of Trustees, University
SUGGESTIONS FOR THE CONTROL OF SOIL-BORNE
FUNGAL PATHOGENS OF FOLIAGE PLANTS
J. F. Knauss
Agricultural Research Center Apopka (ARC-A)
ARC-A Mimeo 72-4
The following information is an extension of ARC-A Mimeo 71-2 and is
intended to serve as a guide for the control of soil-borne fungal patIhgens-
of foliage plants. The chemical control measures sigge sitfust i '
misinterpreted as a formal recommendation nor is the'use of trade ame
intended to discriminate against products sold by other producers which
might be just as effective or may be the same but sold under a jffe~i* 0orida
trade name. All chemicals and rates mentioned are suggest'eT~ or media
consisting totally or largely of peat and may not be safe in other media
Severe economic disease losses caused by soil-borne fungi are experienced
every year by foliage plant growers. Losses resulting from attacks by soil-
borne fungal pathogens have been estimated to comprise 25 50% of the total
disease loss resulting from attacks by all foliage plant pathogens. Although
these estimates are not verified as fact, the author after 3- years of
observation of Florida foliage plant production, agrees losses resulting
from attacks by soil-borne fungi are high indeed. For the purpose of this
mimeo we will restrict the subsequent discussion to those soil-borne fungal
pathogens which cause the majority of disease, i.e., species of Pythium,
Phytophthora, Rhizoctonia and Sclerotium rolfsii.
SOIL-BORNE PATHOGENS, INFORMATION RELATIVE TO THEIR CONTROL
These pathogens are capable of attacking all manner of plant material,
i.e., seeds, seedlings, cane pieces, rooted and unrooted cuttings, and small
and large potted plants. Although disease is more severe at particular times
of the year, attack may occur almost year-round with most of these pathogens
because of Florida's sub-tropical climate. Because of this, many ornamental
growers who are growing in Florida for the first time are amazed at the
number and severity of disease problems they must face and overcome in
order to realize a profit. An environment ideal for disease development is
truly the main reason Florida is such a haven for plant disease.
In conjunction with an environment conducive to disease development are
several other reasons for the severe soil-borne diseases experienced by
Florida foliage growers. These are listed in the following discussion
according to the growing area involved:
1. Stock Bed Area
In the Florida foliage industry most stock areas have been prepared as
ground beds of native soil in open slat sheds. This soil is usually
amended with peat or some other suitable organic material. Effective
sterilization of these beds, prior to planting, may be accomplished by
chemical fumigation or steam sterilization. The "clean" nature (freedom
from pathogens) of these ground beds, however, usually lasts only a short
time. This occurs because of the relative ease by which pathogens are
reintroduced into the clean area on diseased or infested planting material
or by spread from infested areas by normal foliage plant cultural methods.
There is no sense fooling ourselves, maintaining ground beds in a disease-
free state is an impossible task. The best one can hope for in ground bed
stock production is to maintain a low population of pathogens by utilizing
a year-round soil fungicide program directed at keeping the major pathogens
under control. This can be accomplished for many foliage crops but the over-
all profit realized probably is far less when compared to raised bed production
in covered structures where clean soil mix and plant material are employed.
Even with raised beds, however, a minimum preventative program of soil
fungicide application will have to be employed with the more disease-prone
crops (pothos, dieffenbachia, aglaonema, some philodendrons, maranta, and
others) to insure maximum production and profit.
2. Propagation Area
Foliage plant propagation by seed, cane or cutting is ordinarily done
in raised beds consisting totally or largely of domestic or foreign peat.
Often the media employed in these beds are not sterilized prior to propagation.
This may not be as important as it first seems, however, if the peat is dug
from an old, deep bog. Trouble with propagation peat usually occurs when
a native peat or muck is used which was dug from an area previously cropped
to vegetables or ornamental plants. Peat taken from this type of area
often contains an abundance of pathogens that must be eliminated by fumigation
or steam sterilization in order to produce quality foliage plants.
After propagation of the initial crop in the new bed, the bed should
be worked up, the plant remains (roots, leaves, stems, etc.) pulled out and
new peat mixed in to replace the peat used-up in propagation of the previous
crop. The bed should then be steam sterilized to eliminate all remaining
pathogens. After sterilization, the bed must be leached thoroughly.
Growers who do not sterilize their propagation beds prior to re-use, should
use the other pre-mentioned procedures (other than sterilization) and employ
a preventative soil fungicide program to keep the pathogen population low.
These fungicides are normally applied as a plant dip to the propagative
material prior to sticking or as a drench to the propagative media.
Experience with peat media indicates that drench treatments to the bed
for maximum effectiveness should be applied just prior to sticking leafy
propagative units. The drench may, however, be applied immediately after
sticking non-leafy cuttings, cane sections or seeds. In both cases,
application is at a rate of 1 quart per square foot of propagation area.
Keep in mind that the more times one reuses a propagation bed without a
thorough sterilization, the more likely is the prospect of building up a
high pathogen population which will be more difficult to control.
When reusing propagation areas attempt, whenever possible, to rotate
the foliage species after each propagation. Foliage species that are
especially poor risks to repeatedly propagate in the same media without
media sterilization are syngonium nephthytiss), pothos, aglaonema, dieffenbachia,
neanthe bella palm and peperomia. Always follow these with a foliage
species which is more disease resistant, i.e., one which in your past
experience has given little or no problems.
3. Finishing Area Pots, Combinations, etc.
Often after potting, apparently healthy foliage plants progressively
deteriorate in the holding or finishing area. Plants that respond in this
manner were probably infected at time of potting or were potted in pathogen-
infested media. Even if the plant and media are clean when potted, the
soil may become recontaminated or the plant may become infected in several
different ways. Because the chances of media recontamination are good under
present growing methods, a broad-spectrum soil fungicide drench (at the rate
of 1 pint/6 inch pot) should be applied soon after potting. This drench
will prevent a high pathogen population in the media thus allowing root
growth and proper establishment of the potted foliage plantss. The fungicide
drench will also protect against recontamination of clean soil from contaminated
sources. Although one treatment is usually enough for most potted material,
large potted plants which will be held in the nursery for a long period of
time should probably be retreated once every 4 months. The following combi-
nations appear to give broad spectrum activity: Truban Benlate, Truban -
Terraclor, Dexon Benlate, Dexon Terraclor, with Truban and Terraclor
used at 12 oz. and Benlate and Dexon used at 1 Ib each per 100 gal water.
Other aids in preventing soil-borne fungal disease development in pots
1. Use well-aerated, easily drained soil mixes.
2. Take care not to overwater, water only when needed and then thoroughly.
3. Place pots in a manner which will allow unrestricted drainage from
the holes in the pot. Never put pots with holes in the bottom
directly on a plastic sheet.
THE MAJOR PATHOGENS
This fungus group produces disease year round but is most severe in the
wet, warm months. Pythium attacks seeds, seedlings, cuttings, cane, roots
and occasionally stems. Seed and seedling attack may be inhibited when they
are not planted close or kept excessively wet. Pythium Root Rot may be
inhibited by planting in well-aerated, rapidly drained media. Care not to
overwater also aids in root rot control. This pathogen may be spread on
contaminated tools, hands, feet, media, flats, pots or in or on infected
plant material. Once established in the media, Pythium can persist for long
periods of time, even in the absence of a susceptible plant. Foliage plants
which are particularly susceptible to this fungus group and chemicals which
may be used for control will be listed later.
The information presented for Pythium applies in large part also for
this fungus group. Although closely related to Pythium, Phytophthora is
more difficult to control. It also attacks the leaves and has been observed
to produce severe foliage disease on Philodendron oxycardium and Dieffenbachia
amoena. Another feature of this group which is different from Pythium is
its ability to attack mature plant tissue often causing severe stem and
cane decays of fully matured plant tissue. As with Pythium, susceptible
foliage species and chemicals for control will be listed later.
Attacks by this fungus are generally restricted to the warm to hot
portions of the year, with the most severe disease losses occurring when
wet and humid conditions exist for long periods of time. This fungus is
easily recognized by its heavy, weft-like white growth on the soil surface
and the affected plants. Almost always, numerous light tan sclerotia about
the size and appearance of mustard seeds or small osmocote granules are found
to be present. These sclerotia act as resting structures and allow the fungus
to remain in an inactive but living state from one crop to another and for
much longer periods of time. Once established in a propagation bed, S. rolfsii
may spread rapidly and cause rapid and severe losses if left unchecked. A
wide range of plants are attacked by this fungus with losses occurring
primarily in propagation areas but with scattered instances noted in stock
and finishing areas.
When a crop in propagation is severely diseased by this pathogen, the
bed should be sterilized before reuse. If this is impossible, clean the
bed of all plant residue and propagate a foliage plant known to be resistant
to S. rolfsii. A preplant drench with Terraclor, Fermate or Demosah may help
but safety to the propagative material cannot always be assured with any of
these fungicides. When centers of S. rolfsii infection are noted in a
propagative bed, the diseased plants should be removed carefully and the
infested area plus a 2 foot border drenched with one of the previously-
mentioned fungicides. The infested soil (2 ft border not included) should
then be carefully removed after being allowed to set for 1 day. As stated
for the previous pathogens, a list of susceptible plants and chemicals for
control will be listed later.
Of all the soil-borne fungal pathogens that attack foliage plants, this
group appears to be the most active on a year-round basis. It has caused
untold losses in the propagation and finishing of foliage plants. Rhizoctonia
prefers a warm, not excessively hot, moist environment. This corresponds
in the Florida foliage industry to the periods of middle and late spring and
fall. Although this generalization is normally true, the winter of 1971-72
was an exception and had periodic warm and moist periods. During these
periods Rhizoctonia was particularly active and many growers were unprepared
to combat the problem. Disease resulting from attack by this pathogen often
appears to growers to occur overnight. Although Rhizoctonia grows rapidly,
it does not move quite that fast. This impression among growers is probably
caused by the fungus' wide occurrence throughout the growing areas, its
ability to survive during weather unfavorable to its growth, and its ability
to attack rapidly such a wide variety of foliage plants.
Rhizoctonia is easily recognized by the prominent red-brown threads it
produces. These threads may often be seen when affected plant material is
examined closely. Often these threads are seen on the soil surface in the
diseased area. If Rhizoctonia-infected leaves touch the media surface they
will usually be difficult to lift, if raised slowly, because the threads
are attached to both the soil and leaf surface. In a sense, these threads
act like small ropes holding the leaf to the media surface. Again the
threads will be visible.
When foliar attack of foliage plants occurs, reduction in watering,
wider spacing to allow better air movement among plants, increased air
movement produced by fans or turbulators and foliar applications of Benlate
or Daconil will control the pathogen.
In all types of production, this pathogen as with the previously-
mentioned pathogens, is best controlled when clean plants, soil and pots
are utilized. With problem crops, however, most effective control is
usually achieved when a preventative fungicide drench is employed just
prior to or just after sticking the propagative material and soon after
potting. Of the fungicides tested thus far as a soil drench, Benlate has
generally been the most effective and definitely the safest. Benlate may
be applied several times in the same area or to the same pot several times
a year. This may not be the case with the other fungicides to be listed
later as they should not ordinarily be used more than once a year to the
same media or pot.
FOLIAGE PLANTS PARTICULARLY SUSCEPTIBLE TO THE
MAJOR SOIL-BORNE FUNGAL PATHOGENS
In the following, the major pathogen will be listed. Underneath each
list will be found foliage species and type of tissue which the author has
found to be particularly susceptible to the pathogen. Foliage growers are
encouraged to utilize the lists as a guide to the problems they may face
and to take preventative action to keep the soil-borne fungal pathogens
Monstera deliciosa (P. pertusum)
seeds, seedlings, roots
roots, canes, stems
neanthe bella palm
roots, stems, leaves
roots, canes, stems
Sclerotium rolfsii cont'd
neanthe bella palm
cuttings, stems, stock plants
Monstera deliciosa (P. pertusum)
stems, leaves, roots
cuttings, stems, leaves
cuttings, stems, leaves
tubers, roots, leaves
canes, cuttings, roots, stems
roots, seedlings, leaves
Rhizoctonia spp. cont'd
neanthe bella palm
Purple Passion (Gynura sp.)
roots, cuttings, stems
seedlings, roots, stems
cuttings, roots, leaves, stems
seedlings, stems, roots
Soil Fungicides Suggested for Pythium Control
Truban 30 WP 8-12 oz/100 gal
Very effective; effective often for 8-12 weeks (long lasting); safe
for use on foliage plants at suggested concentration. Application may
be repeated every 3 months in media consisting totally or largely of
peat; should not be applied more than twice a year to sandy, ground
beds; may be mixed with Benlate or Terraclor.
Soil Fungicides Suggested for.Pythium Control Cont'd
Dexon 35 WP 1.0 lb/l00 gal
Effective; effectiveness may last only 1 month where heavy watering
occurs; light sensitive; may injure plants if used more than 3 times
a year in same soil; tendency to injure plants in sandy soils, even
with only one application; compliments Truban in a control program
because of activity against Pythium and fact that recent research
indicates considerable activity against the bacterial genus, Erwinia.
This genus (Erwinia) commonly causes many propagative rots of foliage
plants. Dexon may be mixed with Terraclor or Benlate.
Soil Fungicides Suggested for Rhizoctonia Control
Benlate 50 WP 1.0 lb (0.5 lb as foliar spray)
Most effective chemical tested; no injury as drench on cuttings of Hoya,
Syngonium, Purple Passion or seedlings of schefflera; expensive but
should be part of all Rhizoctonia control programs.
Terraclor 75 WP 1.0 Ib
Very effective; slight injury to cuttings of Syngonium and Purple
Passion and to seedlings of schefflera; moderate to severe injury to
cuttings of Hoya; Use only once a year in same soil, alternate with
Soil Fungicides Suggested for Rhizoctonia Control Cont'd
Daconil 75 WP 1.5 Ib
Effective but when applied as drench moderately to severely injured all
plants tested; as foliar spray effective and safe for control of foliar
Demosan 65 WP 1.0-1.5 lb
Slightly to moderately effective; slight to moderate injury on Syngonium,
Purple Passion and Hoya. Moderate injury on seedlings of schefflera.
Fermate 76 WP 2.0-3.0 lb
Effective; severe injury to s.hefflera seedlings; initially slight to
moderate injury to cuttings of Purple Passion but disappearing after
rooting and extended watering.
Soil Fungicides Suggested for S. rolfsii Control
Terraclor 75 WP 1.0 lb
Most effective and long lasting chemical tested. Slight phytotoxicity
to seedlings of schefflera and cuttings of Syngonium and P. oxycardium;
slight to moderate phytotoxicity to rooted and unrooted cuttings of
Peperomia. Use only once a year in the same soil, alternate with another
Soil Fungicides Suggested for S. rolfsii Control Cont'd
Demosan 65 WP 1.0 lb
Effective for approximately one month but effect decreases with
prolonged watering; slight phytotoxicity to cuttings of Syngonium;
slight to moderate phytotoxicity to cuttings of Peperomia and seed-
lings of schefflera.
Fermate 76 WP 2.0-3.0 lb
Effective and long lasting; severe phytotoxicity to seedlings of
schefflera and cuttings of Syngonium; slight to moderate phytotoxicity
to cuttings of Peperomia.
Soil Fungicides Suggested for Phytophthora Control
To date little information is available on the relative effectiveness
of soil fungicides for control of this pathogen on foliage plants.
Fungicides which have given an indication of some degree of control
when used as a drench are Truban 30 WP (12 oz), Dexon 35 WP (1 lb)
and Dithane M-45 (2 lb). Truban and Dexon appear to be safe on most
plants, while M-45 will cause phytotoxicity to dieffenbachia. For
foliar infections Daconil WP (1.5 lb) and Dithane M-45 (1.5 lb)
applied as foliar sprays appear to provide the best control.