Knowledge is power when it comes to pythium

Pythium is probably the most common plant pathogen associated with root diseases of potted plants. The genus is found in nearly all soils and even some potting media and has a worldwide distribution. The ability of an isolate to cause disease varies according to the susceptibility and nutritional status of the host, the amounts of moisture and oxygen in the root zone, temperature, and the species of Pythium. Understanding these issues is the key to avoiding this common and costly disease.

Pythium root and stem rot typically results in yellowing, stunting, or wilt of the aboveground portions of the plant. Leaves turn yellow, droop and some fall off. Often large portions of the root system are decayed before the disease is found. The decayed root cortex is easily stripped off, leaving the vascular strand intact — one of the most typical symptoms of a Pythium root disease. The fungus can move from the root system into the lower stem, causing stem cankers or crown rot. In propagation, bases of unrooted cuttings may be infected and stem cankers occasionally form in the absence of extensive root rot.

The genus Pythium contains about 125 species, most of which are nonselective regarding the hosts they attack and many are not known to attack plants at all. Some species, such as P. ultimum, P. irregulare, and P. aphanidermatum, are common as pathogens of potted plants. Although it may hard to believe, Pythium is not a vigorous soil competitor and survives in soil largely in a dormant state. The pathogen can also survive in dust and soil particles on greenhouse floors and in soiled flats and pots. Commercial peat or peat mixtures have been reported to be occasional sources of Pythium. Germination of dormant propagules is stimulated by exudates from roots or germinating seeds. Pythium oospores can be induced to germinate as quickly as an hour and a half after seed germination. This rapid germination and fast growth rate allow Pythium to be a successful pioneer invader of roots. Pythium is much more aggressive in pasteurized soils or soilless media than in field soil because of the lack of competing microorganisms. The presence of biological control agents like Trichoderma and Bacillus can be effective at this stage.

Many plant pathologists despair of ever creating Pythium root rot in fungicide trials, presumably due to favoring the plant too much to allow the Pythium sp. to flourish. In one trial I performed on pansy, we saw significant root rot in the middle of the trial but absolutely no disease at the end of the trial. These plants had outgrown the Pythium used to inoculate them, leading to the conclusion that it isn’t easy to create Pythium — at least not for plant pathologists.
 

The best defense is a good offense

Knowing what doesn’t and what does promote Pythium is the first line of defense in avoiding the disease. First, it’s important to note that Pythium is not normally moved by air currents.

There are, however many ways Pythium can be introduced and promoted. For starters, adult shore flies can introduce Pythium into the growing medium. Furthermore, while fungus gnat adults do not efficiently disseminate Pythium, their larvae eat spores, which remain viable in their feces until adulthood. The extent to which these two insects contribute to the dissemination of Pythium is not known.

When it comes to growing medium, keeping it Pythium-free is critical. Use only the highest quality media you can afford since sometimes Pythium has been found in new, bagged commercial potting media. Soiled tools and hands should not come in contact with growing media or plants. The growing medium should be well drained, and irrigation should be applied only as needed to avoid prolonged saturation. Plants, especially seedlings, should be grown at temperatures conducive to optimum growth and development.

Excess levels of some nutrients have enhanced Pythium root rot on geranium seedlings. Plant death increased as nitrogen or phosphorous increased. Further research showed that use of high fertilizer levels actually resulted in higher losses of geranium cultivars previously identified as resistant to the disease. Fertilizers should be supplied according to plant requirements for adequate growth.

One of the most interesting aspects of Pythium root rot is that many plants can recover from an initial root loss if conditions are improved or effective fungicides are applied. Sometimes, the plants simply outgrow the damage. It is possible for the crop to experience damage from Pythium without showing any overt symptoms. This may happen when growers see improved roots using a product that is not very effective for Pythium diseases. We rarely make only one change when a problem develops and the improved culture is really what is responsible for the improved crop.

Relying on either chemical or biological control for Pythium root rot is possible, but this tactic has significant drawbacks: few products are highly effective; Pythium resistance to some of them has been demonstrated; and they must be used preventatively for optimal effects. It is actually more effective to avoid Pythium diseases by growing the plants with the best conditions (water, fertilizer, and potting medium) and using only healthy propagation materials.

Finally, it you rely on fungicides alone and do not correct gross problems with your irrigation, potting medium, or methods of producing the crop, you will still have Pythium.

A.R. Chase is the part-owner of Chase Agricultural Consulting LLC and Professor Emeritus of Plant Pathology at the University of Florida. Learn more at www.chaseagriculturalconsulting.com