Don’t Expect Pythium Root Rot to Always Act the Same

February 18, 2002 - 14:28

One new development important to our understanding of
Pythium species comes from the findings of molecular geneticists. We have long
thought of Pythium as a “water mold fungus”— now it has been
reclassified according to information gained from comparing gene
similarities…and Pythium is no longer a fungus! DNA analysis has shown us
that Pythium is more closely related to some of the single-celled algae. It is
in a category of organisms called “Oomycetes,” along with downy
mildews and Phytophthora. Small wonder, then, that Pythium is associated with
wet greenhouse environments and that unique chemistries are needed to control
it and the other Oomycetes.

The “big three”

In a close examination of Pythium-infected plants submitted
to plant disease clinics during recent years, we have found that of the over
120  known species of Pythium,
three are consistently causing crop losses: Pythium aphanidermatum, P.
irregulare and P. ultimum. Pythium aphanidermatum, the most aggressive of the
three, is the one most commonly causing root rot of poinsettias. This species
readily spreads in ebb-and-flow systems because it has a swimming spore stage.
Pythium irregulare also forms swimming spores and is isolated from a very wide
variety of greenhouse crops, almost any crop grown. It is less aggressive than
P. aphanidermatum, often causing stunting but seldom killing plants quickly.
Pythium ultimum, very commonly noted in old clinic records, is much less common
but is still isolated from chrysanthemums, verbenas, geraniums and sometimes
poinsettias. Most of the printed information on diseases of ornamentals
describes problems caused by Pythium ultimum. P. ultimum, a widespread soil
inhabitant, may be less of a problem in modern production systems because of
the switch from soil to soilless potting media over the years. style="mso-spacerun: yes">  Several other species, including P.
myriotylum, have been encountered but much less frequently than the “big

The different Pythium species have different environmental
preferences. The historic problem, P. ultimum, was notorious for attacking
poinsettias in the fall, especially when temperatures were dropped in order to
hold plants. This follows from two traits of P. ultimum. First, it does not
ordinarily have a swimming spore (zoospore) stage, hence it favored the days
when soil was a component of container mixes and imperfect soil pasteurization
could lead to Pythium outbreaks. Second, Pythium ultimum favors cool greenhouse
temperatures: the minimum for growth is 41° F, maximum 95° F and
optimum 77-86° F. When other organisms are inhibited by cool temperature,
P. ultimum can prosper.

P. aphanidermatum has a higher minimum temperature (50°
F) than P. ultimum and a very high optimum temperature at 95-104° F. This
Pythium produces zoospores readily in flooded soil, so it is well-adapted to
spreading in recirculating irrigation systems. P. aphanidermatum is also a
typical resident of soils in warm regions, where much of our off-shore plant
propagation takes place nowadays. Pythium problems on poinsettias have shifted
from late-season and cool weather to mid-summer propagation problems, and P.
aphanidermatum is the species most often associated with poinsettia root
problems in recent years.

The third common species, P. irregulare, is somewhat
intermediate between the other two in terms of its temperature preferences, but
it shares with P. ultimum an inability to grow at high temperatures. It can
grow at 34° F but has a maximum of 95° F and an optimum of 86° F.
Pithium irregulare, like P. aphanidermatum, does produce zoospores that can
move easily with irrigation water.

Telling trials

Trials run at Cornell’s LIHREC in 2000 and 2001 have
shown some interesting contrasts in the effects of the “big three”
Pythium species on five different red geranium cultivars under different
environmental conditions. In 2000, an April trial with temperatures ranging
from 60-70° F showed P. ultimum to have the strongest pathogenic effect,
stunting all cultivars and causing “black leg” stem canker symptoms
on ‘Yours Truly’. In 2001, a June trial showed a different pattern:
only plants inoculated with P. aphanidermatum showed stunting or black leg. The
temperatures prevailing during this 2001 study ranged from 55-97° F, which
favored the heat-loving P. aphanidermatum. One lesson from this: growers who
wish to protect against the aggressive P. aphanidermatum on their poinsettia
crop should take care to make treatments early in production, during warm
weather conditions — even though their grandfathers may have found
poinsettias to be more vulnerable to root rot in the fall.

A few growers believe they have experienced Pythium disease
control failures when using the fungicides Subdue (metalaxyl) or Subdue MAXX
(mefenoxam). Several scientists have looked into this serious threat; although
fungicide resistance isknown to exist, the Á more likely possibility is that the fungicide was applied too late in disease development or that the wrong amount of chemical was applied, rather than the Pythium being resistant to the fungicide. If a fungus is resistant to a fungicide, that fungicide no longer effectively controls the fungus and using the chemical is a waste of time and money.

We are testing the sensitivity of isolates to Subdue MAXX
(mefenoxam) and Banol (propamocarb) by growing the Pythium on an agar medium
with a range of fungicide concentrations. To date, several Subdue-resistant
isolates of P. aphanidermatum and P. irregulare have been identified. For
example, of the 35 P. irregulare isolates thus far tested, 12 are resistant to
Subdue and 10 of 27 P. aphanidermatum isolates are resistant. Also of concern
is the fact that some of these same isolates (three of the 12 P. irregulare)
were able to grow on agar containing high concentrations of Banol. Although we
did not find any P. ultimum isolates resistant to Subdue, five of the nine
tested thus far grew on high concentrations of Banol. Work has been done to
show that if a Pythium can grow well in the presence of high Subdue
concentrations in culture, it can overcome the fungicide and cause disease on
whole plants treated at the label rate of fungicide. This has not yet been
proven to be the case with Banol, and those studies are underway.

Testing Truban, Banrot or Terrazole in agar is tricky
because the active ingredient, etridiazole, works by vapor action. As soon as
the agar is prepared, the concentration of Truban begins to decline, and we are
not sure how much chemical is really present during the tests. However, to
date, we have no indications that any of the Pythium isolates have resistance
to etridiazole. One thing we have observed is that although some isolates are
not resistant to Subdue or Banol, they keep growing very slowly and are not
killed at high concentrations of the fungicides. That may indicate that Pythium
could survive at a low level of activity, waiting until the concentration of
fungicide declines as it inevitably does over time. For that reason alone, it
is extremely unwise to put fungicides directly in subirrigation reservoirs.
They must be put in the pots, where they will maintain the proper concentration
for the longest time.

In addition to testing for resistance in culture, we are
exploring other ways of detecting that an isolate is genetically resistant to
fungicides. Using molecular techniques to examine the DNA, not only can we
identify individual species, but we believe we have found a genetic
“fingerprint” of fungicide-resistant individuals of P.
aphanidermatum. We have not yet found such a fungicide resistance fingerprint
for P. ultimum or P. irregulare or for Banol resistance. In parallel research,
we are testing DNA analysis methods that should allow us to determine whether,
for example, the P. irregulare in a particular crop is identical to or
different from the P. irregulare we may find in unused potting mix, the water
supply or soil under the benches in that greenhouse. By pinpointing the source
of the Pythium causing crop losses, the grower can then target control measures
to eliminate that source.


Cornell University and Penn State are collaborating on
several phases of research to better understand what is occurring in Pythium
root rot. This work is funded by the American Floral Endowment, the Fred C.
Gloeckner Foundation, the Pennsylvania Floral Industry Association, Cornell,
Penn State and by a special cooperative agreement with the USDA-ARS.

Editor’s Note: The use of specific trade names in this
publication does not constitute endorsement of these products in preference to
others containing the same active ingredients. The use of trade names is solely
for the purpose of providing specific information and does not signify that
they are approved to the exclusion of others. Mention of a product does not
constitute a guarantee or warranty of the product by the author or magazine.

About The Author

Gary W. Moorman is a professor of plant pathology at The Pennsylvania State University, and Margery L. Daughtrey is a senior extension associate at Cornell University. They may be reached by phone at (631) 727-3595 or E-mail at

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