Don’t Expect Pythium Root Rot to Always Act the Same By Gary W. Moorman and Margery L. Daughtrey

One new development important to our understanding ofPythium species comes from the findings of molecular geneticists. We have longthought of Pythium as a "water mold fungus"Ñ now it has beenreclassified according to information gained from comparing genesimilaritiesÉand Pythium is no longer a fungus! DNA analysis has shown usthat Pythium is more closely related to some of the single-celled algae. It isin a category of organisms called "Oomycetes," along with downymildews and Phytophthora. Small wonder, then, that Pythium is associated withwet greenhouse environments and that unique chemistries are needed to controlit and the other Oomycetes.

The "big three"

In a close examination of Pythium-infected plants submittedto plant disease clinics during recent years, we have found that of the over120 known species of Pythium,three are consistently causing crop losses: Pythium aphanidermatum, P.irregulare and P. ultimum. Pythium aphanidermatum, the most aggressive of thethree, is the one most commonly causing root rot of poinsettias. This speciesreadily 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 widevariety of greenhouse crops, almost any crop grown. It is less aggressive thanP. aphanidermatum, often causing stunting but seldom killing plants quickly.Pythium ultimum, very commonly noted in old clinic records, is much less commonbut is still isolated from chrysanthemums, verbenas, geraniums and sometimespoinsettias. Most of the printed information on diseases of ornamentalsdescribes problems caused by Pythium ultimum. P. ultimum, a widespread soilinhabitant, may be less of a problem in modern production systems because ofthe switch from soil to soilless potting media over the years. Several other species, including P.myriotylum, have been encountered but much less frequently than the "bigthree."

The different Pythium species have different environmentalpreferences. The historic problem, P. ultimum, was notorious for attackingpoinsettias in the fall, especially when temperatures were dropped in order tohold plants. This follows from two traits of P. ultimum. First, it does notordinarily have a swimming spore (zoospore) stage, hence it favored the dayswhen soil was a component of container mixes and imperfect soil pasteurizationcould lead to Pythium outbreaks. Second, Pythium ultimum favors cool greenhousetemperatures: the minimum for growth is 41¡ F, maximum 95¡ F andoptimum 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. ThisPythium produces zoospores readily in flooded soil, so it is well-adapted tospreading in recirculating irrigation systems. P. aphanidermatum is also atypical resident of soils in warm regions, where much of our off-shore plantpropagation takes place nowadays. Pythium problems on poinsettias have shiftedfrom late-season and cool weather to mid-summer propagation problems, and P.aphanidermatum is the species most often associated with poinsettia rootproblems in recent years.

The third common species, P. irregulare, is somewhatintermediate between the other two in terms of its temperature preferences, butit shares with P. ultimum an inability to grow at high temperatures. It cangrow 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 canmove easily with irrigation water.

Telling trials

Trials run at Cornell's LIHREC in 2000 and 2001 haveshown some interesting contrasts in the effects of the "big three"Pythium species on five different red geranium cultivars under differentenvironmental conditions. In 2000, an April trial with temperatures rangingfrom 60-70¡ F showed P. ultimum to have the strongest pathogenic effect,stunting all cultivars and causing "black leg" stem canker symptomson 'Yours Truly'. In 2001, a June trial showed a different pattern:only plants inoculated with P. aphanidermatum showed stunting or black leg. Thetemperatures prevailing during this 2001 study ranged from 55-97¡ F, whichfavored the heat-loving P. aphanidermatum. One lesson from this: growers whowish to protect against the aggressive P. aphanidermatum on their poinsettiacrop should take care to make treatments early in production, during warmweather conditions — even though their grandfathers may have foundpoinsettias to be more vulnerable to root rot in the fall.

A few growers believe they have experienced Pythium diseasecontrol failures when using the fungicides Subdue (metalaxyl) or Subdue MAXX(mefenoxam). Several scientists have looked into this serious threat; althoughfungicide resistance isknown to exist, the ç 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 mediumwith a range of fungicide concentrations. To date, several Subdue-resistantisolates of P. aphanidermatum and P. irregulare have been identified. Forexample, of the 35 P. irregulare isolates thus far tested, 12 are resistant toSubdue and 10 of 27 P. aphanidermatum isolates are resistant. Also of concernis 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 wedid not find any P. ultimum isolates resistant to Subdue, five of the ninetested thus far grew on high concentrations of Banol. Work has been done toshow that if a Pythium can grow well in the presence of high Subdueconcentrations in culture, it can overcome the fungicide and cause disease onwhole plants treated at the label rate of fungicide. This has not yet beenproven to be the case with Banol, and those studies are underway.

Testing Truban, Banrot or Terrazole in agar is trickybecause the active ingredient, etridiazole, works by vapor action. As soon asthe agar is prepared, the concentration of Truban begins to decline, and we arenot sure how much chemical is really present during the tests. However, todate, we have no indications that any of the Pythium isolates have resistanceto etridiazole. One thing we have observed is that although some isolates arenot resistant to Subdue or Banol, they keep growing very slowly and are notkilled at high concentrations of the fungicides. That may indicate that Pythiumcould survive at a low level of activity, waiting until the concentration offungicide declines as it inevitably does over time. For that reason alone, itis extremely unwise to put fungicides directly in subirrigation reservoirs.They must be put in the pots, where they will maintain the proper concentrationfor the longest time.

In addition to testing for resistance in culture, we areexploring other ways of detecting that an isolate is genetically resistant tofungicides. Using molecular techniques to examine the DNA, not only can weidentify 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 fingerprintfor 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 ordifferent from the P. irregulare we may find in unused potting mix, the watersupply or soil under the benches in that greenhouse. By pinpointing the sourceof the Pythium causing crop losses, the grower can then target control measuresto eliminate that source.


Cornell University and Penn State are collaborating onseveral phases of research to better understand what is occurring in Pythiumroot 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 thispublication does not constitute endorsement of these products in preference toothers containing the same active ingredients. The use of trade names is solelyfor the purpose of providing specific information and does not signify thatthey are approved to the exclusion of others. Mention of a product does notconstitute a guarantee or warranty of the product by the author or magazine.

Gary W. Moorman and Margery L. Daughtrey

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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