The organisms
 Diagnosis and symptoms
 Control and management

Title: Diseases of agronomic and vegetable crops caused by pythium
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00066823/00001
 Material Information
Title: Diseases of agronomic and vegetable crops caused by pythium
Series Title: Plant Pathology Fact Sheet PP-53
Physical Description: Book
Language: English
Creator: Kucharek, Tom
Mitchell, Dave
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: 2000
Spatial Coverage: North America -- United States of America -- Florida
 Record Information
Bibliographic ID: UF00066823
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
    The organisms
        Page 1
        Page 2
    Diagnosis and symptoms
        Page 3
        Page 4
    Control and management
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
Full Text

Plant Pathology Fact Sheet


Tom Kucharek, Professor and Extension Plant Pathologist; Dave Mitchell, Pro-
fessor, Plant Pathology Department, University of Florida, Gainesville FL 32611.
February, 2000

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


Pythium is a generic term used herein to represent
numerous species of fungal-like organisms within the
Genus Pythium. Pythium spp. occur throughout the
world and are common in association with roots of plants
and other plant parts in contact with or in the proximity
of soil. When environmental conditions are favorable
for these organisms and adverse for the plant, infection
of plants may occur. Infections typically occur in roots,
lower stems, fruit near or in contact with the soil, and
soft plant tissues which exist naturally in seedlings of plants
prior to or shortly after emergence from the soil.Pythium
spp. that are not parasitic in plants have caused diseases
in fish, crustacea, and even other Pythium spp.

Over time, taxonomists (scientists who study sys-
tematics, the system of classification) have changed the
classification position ofPythium spp. from being within
the kingdom Planta to being within the kingdom
Chromista. The newest classification schemes place
Pythium spp. as being more closely related to diatoms
and brown or golden algae than fungi. In the newest
classification systems, based upon single evolutionary
lines and unique morphological and biochemical char-
acteristics, most of the other fungi, such as Fusarium
spp. or Rhizoctonia spp. that were formerly in the plant
kingdom, are now placed in the kingdom Eumycota
(named Mycota or Fungi by some authorities). A few
of the fungi, such as Plasmodiophora spp., are in the
kingdom Protozoa. All of these organisms, however,
may still by referred to by the common or colloquial
term fungi because they are all spore-bearing eukary-

otes, without chlorophyll, that typically have a walled,
filamentous or plasmodial thallus and the ability to ab-
sorb their food. Thus, although different fungi are clas-
sified in three biological kingdoms, they all retain the com-
mon name (fungus; fungi, pl.) based upon common life
forms and functions. The reader should realize that these
academic developments have not changed the life cycles
of Pythium spp. nor have they effectually influenced
our approaches used for control and management of the
diseases caused byPythium spp. This information pro-
vides the reader with some of the dynamic turns and
twists that occur in science.

The most common species of Pythium that cause
important plant diseases in Florida are Pythium
myriotylum and P. aphanidermatum. Other species
of Pythium that are sometimes associated with dysfunc-
tional plants in Florida are P. splendens, P. irregulare,
and P. vexans. It is generally considered that P.
myriotylum and P. aphanidermatum abound in Florida
because they are adapted to high soil temperature.
Growth of these later two species can occur from 40 or
500 F to 1050 F, but the optimum temperatures for their
growth and infection of plants range between 86 and
980 F. In other locations in the world, other species of
Pythium cause plant diseases. For example, in the north-
ern United States, P. ultimum prevails in some situa-
tions. Pythium ultimum prevails in cooler to cold soils;
it can grow across as wide a range of temperatures as
the other two, but its optimum for growth is between 77
and 86 F. Interestingly, the optimum for infection of plant
tissue by P. ultimum is slightly lower (740 F or slightly
below). Typically, Pythium spp. are composed of mi-
croscopic, slender, round tubes (strands) called hyphae


(Fig.l). A mass of these hyphae is called a mycelium
(mycelia, pl.) and is white. Sometimes large mycelial colo-
nies can be seen growing on infected tissue (Figs. 18,
19, & 20). A colony of mycelium can grow more than
an inch per day when moist and warm conditions pre-

Multiplication of Pythium spp. is done by growth of
the hyphae and production of microscopic spores. Four
types of spores are produced, sporangia, zoospores,
oospores and chlamydospores. Chlamydospores are
formed as thickened segments of hyphae, but they are
not considered important for Pythium spp. that cause
plant diseases in Florida. Sporangia can germinate to
form new hyphae, but typically the role of sporangia
within the disease cycle is to produce vesicles in which
zoospores are formed.

Zoospores are fragile, non-sexually produced spores
without cell walls and they have two flagella (tails) that
propel the organism in water. Zoospores are produced
in microscopic vesicles (bulb-like, unwalled structures)
that are attached to sporangia (also a bulb-like or lobe-
like structures, with cell walls ) which are in turn formed
on stalks or formed as enlarged hyphae (Fig 1). Water
is necessary for production of sporangia, vesicles, and
zoospores, and thus, Pythium spp. have been referred
to commonly as belonging to the Awater molds.@ Con-
sequently, standing water in fields, ditches, ponds, lakes,
and hydroponic systems are places that Pythium spp.
can exist, multiply to great numbers and spread. To il-
lustrate the small size of zoospores, it would take about
2500 zoospores in a line to equal one inch. Zoospores
can swim for 20 to 30 hours and move three or more
inches through soil. Without free water they die rapidly
or sometimes they can encyst by forming a thick wall
and may survive up to 7 days in soil. Zoospores swim
against gravity and after reaching the surface of flooded
soil, may be carried to new roots if the water is moving.
The zoospores are then attracted by chemicals to root
tips where infection typically occurs. Natural or artifi-
cial movement of water infested withPythium spp. pro-
vides a method by which rapid spread of these plant
pathogens can occur over greater distances.

An oospore (Fig. 2) forms after mating by male and
female portions of the thallus (the entire mass of the or-

ganism). Most species of Pythium are hermaphroditic
(homothallic) and thus are self fertile. Because oospores
are the result of sexual mating, the organism has a mecha-
nism for recombining genes to enhance variation within
the species. Also, oospores provide an effective method
for survival of Pythium spp. because oospores typi-
cally have thickened walls. Oospores survive in old crop
debris such as undecomposed roots and stems that were
infected earlier and they survive in soil or old containers
or on implements that contain infected plant tissue or
infested soil. Oospores are extremely durable and have
survived for more than 10 years. Pythium spp. have
survived passage through the intestinal tracts or earth-
worms, snails, and birds.

The primary habitat for Pythium spp. is in the soil or
previous crop debris, such as roots and stems. Pythium
spp. can be found typically in the upper 12 inches of
soil, but they have been found as deep as 12 feet.
Pythium spp. can also exist in old plant debris in cull
piles or areas where infected plants are discarded. These
fungi commonly exist in ponds, lakes, or other sources
of standing water. Soil and these bodies of water are
the most common sources of inocula (spores or hyphal
fragments) for pathogenic species. Pythium spp. are
not seedbome, but they can reside in clonally propa-
gated stock such as seed pieces of potatoes, sweet po-
tatoes, sugarcane or transplants of strawberry. Trans-
plants of any crop can carry Pythium spp. in colonized
tissue or by infestation of associated soil and transplant

Most infections from Pythium spp. occur by the
penetration of the root (particularly root tips) or other
susceptible tissue from germ tubes that arise from the
zoospores. Because zoospores can be produced in
extremely high numbers, visible disease appears to oc-
cur Aalmost overight.@ In fact, one generation (i.e.
zoospore to zoospore) can occur in 18 hours or less.
Hyphal infections can occur also, but they are less effi-
cient and they would require significantly more time to
attain an equivalent amount of disease to that resulting
from zoospores. When these organisms colonize and
then penetrate plant tissue, hyphae absorb nourishment
from their living or non-living substrate. Colonization of
plant tissue by P. aphanidermatum is assisted by the
ability of this organism to produce pectolytic or cellulytic

enzymes that breakdown tissue. Pythium myriotylum
also may produce toxins that assist in breakdown of the
host tisuue. Often, infections from Pythium spp. have a
greasy or water-soaked appearance in the plant tissue
because of leakage of the moist cellular contents from
plant cells that occurs from the enzymatic activity men-
tioned earlier. Because the enzymatic activity from
Pythium spp. Abreaks down@ the soft (parenchymous)
tissue, it is common to see the outer portion of dysfunc-
tional roots rotted and the harder vascular tissue in the
center of the root remains intact (Fig. 3 ). This can be
detected in the field by trying to slip the outer portion of
the root off of the central vascular tissue (stele). Con-
trast this effect with a healthy appearing root seen mi-
croscopically in Fig. 4.

Infection is promoted as the population of aPythium
sp. increases. Factors that influence the population of
Pythium spp. in the soil include soil water content, soil
atmosphere (02, CO2, and volatiles), soil temperature,
soil pH, nutrient status of soil, usage of certain pesti-
cides in the soil, and other microorganisms. For ex-
ample, microflora in the soil, such as the fungi Tricho-
derma spp. and actinomycotic organisms such asActi-
nomyces spp., can interfere with the life cycles of
Pythium spp. Roots or plant parts that are infected
with Pythium spp.may be infected with other parasitic
organisms. For example, infection with other organisms,
such as Rhizoctonia solani orFusarium oxyspsorum
and nematodes, such as root knot nematode or sting
nematode, can occur in the same tissue as that infected
with Pythium spp. Conversely, some bacteria or fungi,
such as the fungus Trichoderma, may colonize roots
along with Pythium spp. and reduce damage.

The host range for Pythium spp. is extremely large.
Broadleaf and grass plants are susceptible to multiple
species of Pythium. For example, P. myriotylum and
P. aphanidermatum are both capable of infecting broa-
dleaf plants such as tomatoes, cucurbits, peanuts, or
ornamentals, as well as grasses such as wheat, corn, or
turfgrasses. The crop species that have been commonly
infected with Pythium spp. in Florida include foliage
ornamentals, turfgrasses, Bermudagrass used in pastures,
peppers, tomatoes, beans, soybeans, southern peas, and
small grains (e.g. wheat, oats, rye). Solanaceous weeds,
Crotalaria spp., many weedy grasses, and other weeds

may be hosts of Pythium spp. and serve as important
sources of inocula.

With the anticipated loss of methyl bromide as a soil
fumigant in 2005, it is likely that crops that are now com-
monly grown on methyl bromide/chloropicrin-fumigated
field soil, such as tomatoes, peppers, strawberries, and
many others, will incur greater incidences of visible dis-
ease from Pythium spp. Infection of roots by Pythium
spp. has occurred to some degree in crops that have
been grown in methyl bromide/chloropicrin-fumigated
soil or treated with some other chemical, but the level of
infection has generally been at a lower level and thus,
symptoms have been suppressed. Chloropicrin, which
is commonly formulated with methyl bromide, tends to
be more fungicidal than methyl bromide but the combi-
nation of methyl bromide plus chloropicrin has had a
synergistic effect on soilbome organisms such asPythium
spp., Rhizoctonia spp., Fusarium spp. and many oth-


For accurate diagnoses of plants suspected of hav-
ing Pythium-induced dysfunctions, it would be best to
submit quality samples to a reputable diagnostic labora-
tory. With continued experience, you should be able to
learn which situations will require submission of samples.
You should realize that discolored roots or tissue are
not always associated with parasitic organisms. Excess
fertilizer, flooded soils, insect feeding, and nematode
feeding are just a few of the other causes of dysfunc-
tional roots. Generally, all dysfunctions where root rot
is a portion of the problem should have a laboratory
assessment to determine if Pythium spp. are involved
by themselves, or in conjunction with infections from
other organisms such as Rhizoctonia spp., Fusarium
spp., Macrophomina spp. or a myriad of other possi-
bilities. Diagnosticians use microscopic techniques and
aseptic culturing to assess for the presence of Pythium
spp. Some serological kits are available for use in the
field but the user should realize that because of the di-
versity of Pythium spp. in the soil and in plant tissue,
and because of the likelihood of mixed infections with
other organisms, serological techniques for Pythium
alone are not likely to provide a complete diagnosis un-
less a complete array of test kits are used for the one

situation at hand.

Pythium spp. are often associated with seedling
blights for many crops. Both grass crops and broadleaf
crops incur this type of disease. Seedlings can be in-
fected prior to (pre emergent damping off) or after (post
emergent damping off) emergence from the soil. For
the spinach and soybeans pictured herein (Figs. 5 & 6),
the seedling blights were both pre emergent and post
emergent for both crops and the damage was the result
of infection by both Pythium spp. and Rhizoctonia
solani. It is risky to decide what pathogens are involved
with seedling blights, root rots, and lower stem rots with-
out assessments in the laboratory, but one of the char-
acteristics of tissue infected with Pythium spp. is the
presence of water-soaked or greasy appearing tissue.
In Figure 6, the water soaked tissue of soybean seed-
lings is caused by the infection with a Pythium sp. and
the distinct, orange to red to dark, sunken lesions were
caused by Rhizoctonia solani.

Cultures of a Pythium sp. were isolated from the
water-soaked, lower stem tissue (Fig. 7) of these hy-
droponically grown tomatoes. The symptoms in the
leaves include dried and necrotic (brown) areas that begin
near the tips or margins of the leaflets. When roots or
lower stems are infected with Pythium spp. or other
pathogens, symptoms in the upper parts of plants are
expressed indirectly because the effect from root and
stem rots is to partially or totally inhibit the movement of
water and nutrients from the soil to the upper portions of
the plants. In this situation with tomatoes, the leaves
turned brown, but as shall be seen later, chlorosis (yel-
lowing) of leaves is another common indirect symptom.

In small plants planted thickly, such as in transplant
trays in the greenhouse, Pythium spp. can infect and
colonize the plants with the result that the entire plant is
destroyed (Fig. 8). Note the water-soaked tissue in this
situation with lettuce (Fig. 8). It is common to see white,
mycelial growth associated with dead or dysfunctional
plants that are infected with these pathogens. Mycelial
growth was present in this situation, but a hand lens was
necessary to see the hyphal strands.

Another major symptom in plants that possess root
and lower stem rot caused by Pythium spp. is yellow-

ing of leaves. Typically, the yellowing begins on the
lower leaves first, but cases where yellowing begins on
upper portions of the plant are not rare. In some situa-
tions, the entire plant will become yellow. This leafyel-
lowing occurs in both broadleaf plants and grasses (Figs.
9, 10, 12, 13, and 14). Lower leaf yellowing can also
be caused by a deficiency of nitrogen.

The level of infection can be so severe that plants die
(Figs. 9 and 13). In some situations, infections from
Pythium spp. along with another pathogen may pro-
duce similar symptoms, as is the case with wheat in-
fected with Pythium and soilbore wheat mosaic virus
at the same time (Fig. 14). Field examination of roots
infected with Pythium spp. may reveal an abbreviated
root system, as was the case with the millet pictured
herein (Figs. 10 and 11). Reductions in grain and fruit
yield can occur and the quantity and quality of animal
feed can be reduced. If Pythium root rot occurs early in
the life of the crop, it may cause an infected root to
produce multiple roots that are smaller in diameter, as
occurs in carrot (Fig. 15). Such carrot roots are obvi-
ously unmarketable.

Infection with Pythium spp. can cause wilting of nu-
merous crop species. Peppers commonly wilt when in-
fected with Pythium spp. (Fig. 16). In peanut, Pythium
root rot can cause wilt of the entire plant or a dark brown
to black discoloration of the pods (Fig. 17). Sometimes
infections by Pythium spp. are accompanied by infec-
tion with root knot nematode (Fig 17).

If wet and warm conditions prevail because of fre-
quent rains or irrigation, moist and overcast days, or the
site is typically wet, the white mycelia of Pythium spp.
may become evident in the field. A common name for
this disease is Acottony leak.@ The cottony portion of
the name is from the white mycelial growth and the leak
portion of the name is because the infected tissue is of-
ten greasy and water soaked as a result of tissue break-
down from the enzymes produced by Pythium spp. An
enclosed crop canopy is another situation where mois-
ture is retained, but if ambient conditions are wet, an
enclosed canopy is not necessary for cottony leak to
occur. Some of the crops where cottony leak is seen
include bean (Fig 18), squash (Fig. 19), cucumber (Fig.
20), and salad crops such as lettuce.


Suppression of plant diseases caused by Pythium
spp. requires a different sequence of tactics for differ-
ent situations. For example, suppression of seedling
blights, caused by Pythium spp., requires different tac-
tics than suppression of cottony leak, caused by Pythium
spp., in cucumber fruit. It is best to use all the tactics
available for any situation because no one tactic is a pana-
cea. A planned, systematic approach is best for con-
trolling any plant disease. Resistant cultivars toPythium
do not exist and thus, the control program for Pythium
spp. must be done without this important control mea-
sure. Hardened and woody tissue of all plants tend to
be more resistant that soft and young tissue.

For all field situations, crop rotation with unrelated
crops should be used. Many plant pests, such as nema-
todes, fungi that cause wilts, and some leaf spot patho-
gens, are significantly suppressed by crop rotation. Be-
cause Pythium spp. infect and colonize many broadleaf
and grass crops, crop rotation may not provide the level
of control attained with those pests that are more crop
specific, such as a fungus that causes Fusarium wilt.
However crop rotation may prevent a given Astrain@ of a
Pythium sp. from increasing to unmanageable levels on
a crop that is grown on the same soil over and over.
Keep in mind that crop rotation refers to years and not
seasons. For example, alternate cropping wheat and
soybeans on the same land within the same year is con-
sidered double cropping and not crop rotation. The plant-
ing of wheat or soybean, in this case, one year after
another is generally not considered an adequate period
of time to suppress soil populations of Pythium spp. or
other soilbome pests, such as other fungi or nematodes.

Crops should be planted in well drained soils be-
cause Pythium spp. thrive in moist soils. They multiply
and move in free water with the production and move-
ment of the swimming zoospores. One is most likely to
encounter Pythium-induced diseases at sites that have
standing water or saturated soils for long periods of time.
However, zoospores are produced rapidly and may be
detected within half an hour after a site is flooded and
remains inundated. The sandy soils in Florida are gen-
erally considered quite well drained because of their

porosity, but certain conditions can counteract this natu-
ral porosity. If enough rain or irrigation occurs, water
can stand in portions of fields with sandy soils either
because those portions of the fields are low and water
accumulates or hard pans occur below the soil surface.
Hard pans in soils are formed either from compaction
by repeated use of tractor and implement usage or be-
cause of an underlying layer of a finer sand or clay that
congeals. Hard pans may exist on new land. One of the
characteristics of the Aflatwoods@ soils in peninsular
Florida is the natural existence of a hard pan. Subsoiling
equipment is available to break hard pans; this practice
will enhance drainage and deeper penetration of roots.
Where seepage irrigation is used on a natural hardpan,
destruction of this hardpan would be counterproduc-
tive. Unnatural hardpans above this natural hardpan can
form and may need to be disrupted.

Seed or transplants should not be placed too deep.
The longer it takes for seedlings to emerge, the more
likely seedling blights will occur. Prior to planting every
effort should be made to suppress nematodes and in-
sects in the soil. Damage from these pests to the roots
or lower stems provides entry points for Pythium spp.
and other pathogens.

High density plant populations should be avoided.
While high density plantings have some advantages for
some crops in some situations, they do create more fa-
vorable environments for growth of Pythium spp. and
certain other pathogens because aeration and drying
within the canopy is reduced.

Mechanical cultivation of an established planting for
weed control can influence diseases caused by Pythium
spp. and other pathogens. If the cultivating implement
damages the roots of the crop, it would not be unex-
pected to have increased root or lower stem rots later.
Also, when cultivating, avoid moving soil onto the es-
tablished plant. The soil deposited onto the stem can
contain disease-causing organisms which can infect the
tender stem. An older and hardened stem is more resis-
tant to such infections. Depositing hot soil on stems has
caused heat scald in the stems, which was followed by
infections by several parasitic organisms.

The time of year when a crop is planted may influ-

ence the qualitative and quantitative aspects ofPythium-
induced diseases. For example, the planting of small
grains, such as wheat, oats, or rye, early in the fall (Sep-
tember to mid October) in northern Florida, when the
soil is still quite warm, is desirable for using the crop for
livestock forage. However, such early plantings are com-
monly damaged by Pythium aphanidermatum. which
causes seedling blights and root rots. Planting those small
grains in November to early December progressively
reduces the risk from Pythium spp. Vegetables, such
as collards, turnips, and mustard, that are planted in early
fall can incur the same problem as they grow best at
cooler temperatures. For vegetables that grow best in
warmer soils, such as okra, planting in warm soils pro-
motes earlier emergence and reduces the possibility of
seedling blight caused by Pythium spp.

To reduce problems with Pythium spp. in crops
planted early in the fall, some control techniques are avail-
able in addition to the previously mentioned tactics. For
the small grains planted in early fall, certain seed treat-
ment chemicals are available. Some of the broad spec-
trum, non-systemic chemicals, such as captain and thiram,
provide low levels of suppression. However, certain
systemic chemicals (e.g mefenoxam) provide longer pro-
tection as the seedling enlarges. For the early fall-planted
vegetables or vegetables planted at any time, one can
bypass the highly susceptible seedling crop growth stage
by using healthy transplants. Transplants can be infected
with Pythium spp. or other pathogens during their pro-
duction; thus, it is imperative to use pest-free transplants.
Ask your supplier if the transplants were certified pest
free by an authorized government agency. Although such
an inspection and certification is not a guarantee for per-
fect plants, it is better than no inspection at all. If certi-
fied plants are not available from your supplier, start
dealing with a supplier who provides quality plants.

Production of healthy transplants is the result of the
deliberate use of sanitary practices. If the plants are
produced outdoors in ground level beds, such as those
used for tobacco in northern Florida for over 100 years,
several tactics are available. Do not use the same site
each year for producing transplants. Isolate the trans-
plant production area from crop production fields.
Choose a site that has adequate drainage for water. When
preparing the site with tillage equipment, plow under old

weed and crop debris at least 30 days prior to seeding
the bed, particularly if a soil fumigant is not to be used.
This interval of time allows beneficial organisms to build
up in the soil and reduce parasitic fungi, such asPythium
spp. and Rhizoctonia spp. that can abound on recently
plowed down plant debris. Wash off soil and crop de-
bris from tractor tires and implements when moving from
a field known to be infested with Pythium spp. or other
pests. The finished transplant bed should be raised above
the perimeter area by at least six inches to allow for run
off of excess water and reduce the chance of the pre-
pared bed from being further contaminated with disease-
causing organisms. If a soil fumigant is used to reduce
soilbome pests, such asPythium spp. andRhizoctonia
spp., the use of a raised bed is even more important.
Fumigated, sterilized or pasteurized soil have minimal or
no beneficial, soilbome organisms to compete against
pathogenic organisms. Without competition, Pythium
spp. and Rhizoctonia spp. tend to grow and multiply at
faster rates when compared to their growth in the pres-
ence of competitors. Do not use pond or ditch water
for irrigation; they are commonly infested with patho-
genic organisms.

For transplants that are grown in a greenhouse-type
situation in containers or trays, several sanitary tactics
are available to minimize infestations and subsequent in-
fection from Pythium spp. and other parasitic organ-
isms. A soil mix that has been sterilized or sanitized,
such as MetroMix, should be used. Use a loose mix
that allows excess water to drain. Many commercial
brands of such soil mixes are available or you can blend
your own soil mix and then sanitize it with either dry
heat, steam or an available soil fumigant, such as methyl
bromide. However, note that at the time of this writing,
methyl bromide is scheduled to be totally unavailable for
any use except for quarantine purposes by the year 2005.

The site of the transplant production area should be
away from production fields and the perimeter around
the site should be a well maintained grass. The trans-
plant production trays or containers should be on raised
benches to minimize the chance of contamination with
infested soil. They should never be placed on the floor
anywhere within the premises. Hands should be thor-
oughly washed with soap and water before handling the
planting equipment, containers or plants. Specific per-

sonnel should be assigned to the transplant production
area, and they should not be allowed to return to the
transplant production area if they leave the site and make
any contact with soil until they have washed their hands
and forearms. All implements for planting that are likely
to come into contact with the containers or plants should
be sanitized. If a tray or plant falls to the ground level
soil, do not place it back onto the raised bench until it
has been sanitized. The discharge end of hoses or wa-
tering devices, and other implements should not be al-
lowed to be on the ground where they can become con-

Walkways and access ways within the production
area should be concrete or a material that can be easily
washed. The soil under the raised benches or frames
should be void of weeds and volunteer plants. Some
greenhouse production sites have eliminated exposed
soil by pouring the entire site with concrete for easier
cleaning and maintenance. All soil mixes and containers
should not be stored in contact with the flooring of the
structure even if it is concrete. They should also be stored
in such a way that when the concrete floor is washed,
water splash from the floor will not contact the soil or
containers. Do not use pond or ditch water for washing
or irrigation. Cull piles of plants and used containers
should not be near areas used for plant production or
planting purposes and they should be buried or destroyed
as soon as possible.

Containers used for transplant production can be
reused repeatedly provided they are sanitized between
uses. All cleaning operations should be away from trans-
plant and production areas. Prior to sanitizing the con-
tainers or trays, wash off the clinging soil and plant parts.
The washed containers can then be sanitized by soaking
them for at least 30 minutes in a commercial bleach
(5.25%) solution that is diluted to 10% (one part 5.25%
bleach to nine parts of tap or well water). Other sanitiz-
ing agents and formulations are available, but the above
suggestion is made because it is known to be effective.
One logistical problem that exists when trying to sub-
merge polystyrene trays in a sanitizing solution is their
buoyancy. It will take significant overhead weight or
mechanical force to keep them submerged. Solid, plas-
tic trays are becoming increasingly popular because they
are easily washed and they submerge easily. However,

they will not float if one is using a float-type transplant
production system.

At first sighting of some transplants having disease of
any kind, the container or tray with those plants should
be removed from the production area until the cause of
the problem is determined. If they are not removed, these
first groups of plants with disease may serve as a source
of inoculum for the remainder of your operation. It is
better to play it safe early rather than having a major
problem later.

For crops such as tomatoes or cucumbers that are
produced in hydroponic systems in the greenhouse, the
same level of sanitation used for production of trans-
plants must be utilized. Contamination of the rooting
zone of plants must be eliminated whether the system is
an ebb and flow water system or bags of moistened soil
are used for growing the plants. The same sanitary tech-
niques used for transplant production can be utilized for
crop production in the greenhouse.

Several types of chemicals can be use for suppres-
sion of Pythium spp. As mentioned above, sanitizing
compounds such as bleach are available and effective.
Many chemical seed treatments have been used for de-
cades. At the time of this writing, formulations of seed
treatment fungicides that contain mefenoxam (e.g Apron)
are best because they strongly suppress Pythium spp.
and they are systemic. One of the negative characteris-
tics of mefenoxam and certain other non-broad spec-
trum fungicides is that resistance within the population of
the intended pest develops and renders the product in-
effective against those non-sensitive strains. For this rea-
son it is advised that a seed treatment be composed of a
selective fungicide, like mefenoxam, and a broad spec-
trum fungicide, such as captain and thiram.

Pre-plant, soil fumigation with chemicals such as
methyl bromide (available as limited supplies until 2005),
chloropicrin and metam-sodium are available. They are
effective if applied correctly. Soil fumigants are most
effective if applied into well tilled soil that is slightly moist.
If the soil is too wet, the vapors of the fumigant do not
move adequately through the air pores in the soil. If the
soil is too dry, the fumigant moves too quickly and the
exposure time of the plant pest at any one site in the soil

is not long enough. The linear air pockets from fumigant
chisels, disks, or shanks that remain to the soil surface
after the fumigation process must be solidly sealed. Fail-
ure to do so will allow for an immediate and premature
escape of the fumigant from the soil which in turn, mini-
mizes the needed effects from the fumigant. Each fumi-
gant has a standard waiting time from time of application
to time of planting or transplanting. With chloropicirin
and metam-sodium, the waiting period can be as long as
three weeks in heavier soils with clay. With sandier soils,
slightly shorter waiting periods can be used, but if the
soil is moist, it would be best to wait the full three weeks,
even in sandy soils.

Some fungicides for suppression of Pythium-induced
diseases are available as sprays. These products can
be somewhat effective against diseases caused by
Pythium spp. that occur above the soil surface, such as
cottony leak Most fungicides that are applied as sprays
will not adequately suppress disease below the soil sur-
face. If the fungicide is water soluble and directed to the
soil surface, the compound will leach into the rooting
zone and suppress the fungal activity below the soil sur-
face. Such is the case with mefenoxam-containing fun-
gicides (e.g. Ridomil Gold) for suppression of seedling
blights and root rots in young plants caused by Pythium
spp. However, if the spray is applied after infection
occurs, you will be less likely to attain a positive re-

Heat can be used for suppression of diseases caused
Pythium spp. For suppression of Pythium spp., the
soil must be heated to at least 1200 F for at least 30
consecutive minutes to reduce the population. This can
be done with steam for use in greenhouse operations or
solarization for use in the field or greenhouse. For green-
house soil, steam can be released into an enclosed body
of soil until the temperature of the soil at all sites reaches
1200 F for at least 30 min. In reality, to control other
fungal pathogens, insects and weed seeds, a tempera-
ture of 1800 F throughout the soil mass for 30 consecu-
tive minutes is recommended. Because the steam may
not be able to permeate deeply into soil from its ejection
site, smaller batchs of soil should be used to insure com-

plete pasteurization. Start the 30 minute count when all
the soil mass has achieved 1800 F as indicated by a
reliable thermometer. Dry heat can be used but it is
typically not as effective as moist heat (steam).

In the field, solarization of soil before planting can
increase the temperature of the soil and reduce popula-
tions of soil pathogens, such asPythium spp. Solariza-
tion is done by securing clear plastic over the soil sur-
face for up to six weeks. The soil temperature will in-
crease provided that direct sunlight is available. The
upper few inches of soil will incur the greatest increase
in temperature, so it is likely that solarization will be most
beneficial while the roots of the plants remain in the up-
per few inches of soil. This may provide some suppres-
sion of disease through the seedling or young plant stages.
If cloudy conditions prevail during the solarization pe-
riod, minimal benefit, if any, will result. Solarization has
been most effective in drier climates, such as Israel and
Oklahoma, where the number of cloudless days is greater
than that in Florida.

Fire can be used to reduce levels of pathogens in the
soil. The burning of brush piles on soil increases its tem-
perature and will reduce pests in the soil where brush
piles are burned. This is most likely to be useful in gar-
den or small sites. Another advantage of such bums is
the recycling of nutrients back to the soil.

Trellising crops to grow vertically rather than on bare
soil will significantly reduce infection fromPythium spp.,
particularly in fruit. This technique has been extremely
effective in reducing cottony leak in fruit of tomatoes,
cucumbers, strawberries, and in beans that produce long
vines. Similarly, the use of plastic mulch over the soil to
grow a crop will significantly reduce fruit rots by simply
having a barrier between the soil and the fruit.

Manipulating organic matter, either by the direct ap-
plication of amendments or by planting crops for living
or debris mulches, may by beneficial in developing soils
with high levels of microorganisms that are antagonistic
to or competitive with Pythium spp. Organic amend-
ments, such as sewage sludge or cotton gin trash and
mature composts from yard wastes and animal wastes,
may be used to reduce damage from Pythium spp. in
the greenhouse or field, but generally, very high, and of-

ten uneconomical, amounts of the amendments are
needed. Solarization of soil that has been amended with
aged chicken manure plus aged yard waste has suc-
cessfully suppressedPythium spp. Certain types of peat
or composted hardwood bark have effectively sup-
pressed Pythium spp. in potting media in greenhouse
production systems.

Planting living mulches may support competing or-
ganisms in the field, but unless tested, may result in in-
creased populations of the pathogen. Some soils are
naturally suppressive to diseases caused by Pythium
spp. or may become suppressive with increased organic
matter, manipulation of soil pH, or some crop rotations.
Combinations of minimum tillage and multicropping re-
sulted in lower incidences of damage by Pythium spp.
on some crops, such as soybean, rotated with corn with
minimum tillage when compared to conventional tillage.
Living mulches or plant debris used on the soil surface
for mulch may restrict the movement of inoculum in
splashing water and, thus, reduce spread of the disease.
At the time of this writing, many claims have been made
by others in relation to the use of suppressive mulches,
soil mixes, and field soils. The positive side of this is that
research is being conducted, but the negative side is that
this desirable tactic is still a long way from being em-
ployed reliably.

Several biological control agents, including actino-
mycetes and other bacteria, fungi, and even selected

Fig. 1. Hyphae, zoospores in a vesicle, and
sporangia of Pythium aphanidermatum.

species ofPythium, are available commercially individu-
ally or as combinations for suppression of soilbore plant
pathogens. Although a great deal of effort has been ex-
pended for the development of these products, their suc-
cess rate has been variable, at best. The most likely
sites for their success are in highly controlled locations,
such as hydroponic systems or containerized nursery
systems, that allow the addition of the biological control
agents to a pasteurized or sterilized growth medium prior
to the buildup or recontamination by the pathogen, such
as Pythium spp., or prior to a similar buildup of poten-
tial microbial antagonists to the biological agents.

Constant plant growth and proper nutrition of the plant
are capable of reducing infection from Pythium spp.
This is exemplified by increased plant growth when ni-
trogen fertilizer (ammonium nitrate) was applied to oats
and millet (Fig. 10) that were yellow and slightly stunted
because of infections from Pythium spp. in the roots.
The nitrogen is not fungicidal; it allows the plant to grow
quicker if even a slight nitrogen deficiency exists in the
soil. If the plants are severely stunted and near death,
this tactic will be useless. The suppressive effects of
various nutritive amendments, such as calcium and po-
tassium, have been studied for control of Pythium spp.
No definitive statements can be made with reliability on
this subject at this time except that some individuals claim
that the addition of calcium or potassium ions have had
suppressive effects against Pythium spp.

L i

vig. z. uospores
root tissue.

Fig. 3. Microscopic view showing sloughing Fig. 4. Microscopic view of healthy root with
off of root cortex from infection by Pythium. intact cortex.






Fig. .5. Seedling blight of spinach caused by
Pythium sp. and Rhizoctonia sp.

Fig. 6. Seedling blight of soybeans caused by
Rhizoctonia sp. and Pythium sp.

Fig. 7. Stem rot of tomato caused by Pythium sp. Fig. 8. Pythium blight in lettuce transplants.

Fig. 9. Yellowing of pepper seedlings from
Pythium root rot.

Fig. 10. Yellowing of pearl millet from
Pythium root rot.




~ I vi;


Fig. 11. Root rot of pearl millet caused by
Pythium sp. (on left) and healthier root system
on right.

Fig. 12 Yellowing of wheat leaves caused by
Pythium root rot.

,;. '".. *.
"' i i L' '. ,' ,'" 'pIl

I '.A j a ,'I .
'' \ .* w",

I I , ., ,,.., .. .

Fig. 13. Death of wheat plants from Pythium
root rot.

Fig. 14. Yellowing and stunting of wheat
caused by infection by Pythium sp. And
soilborne wheat mosaic virus.


Fig. 15. Pythium root rot (Brown rot) of

Fig. 16. Wilting of peppers caused by infection
with Pythium sp. in roots.






Fig. 17. Pod rot of peanut (black) caused by
Pythium sp. and galls caused by root knot nema-

Fig. 18. Aerial blight (cottony leak) of young
bean plants caused by Pythium sp.

Fig. 19. Aerial blight (Cottony Leak) of fruit of Fig. 20. Fruit rot (Cottony Leak) of cucumber
yellow summer squash caused by Pythium sp. fruit caused by Pythium sp.

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