Pythium Root Rots in Greenhouse Crops

Carla D. Garzón, Postdoctoral Scholar and Gary W. Moorman, Professor, The Pennsylvania State University, Dept. of Plant Pathology, University Park, PA

‘What is causing the root rot and where did it come from?’, are two important questions to be answered when trying to decide how to control a root disease. Of the over 200 known species of Pythium, we have found that Pythium irregulare and P. aphanidermatum have been responsible for the majority of root rot cases in Pennsylvania from 1996 to the present. Pythium irregulare attacks a very wide variety of crops, generally under cool conditions. Pythium aphanidermatum, which thrives under relatively warm conditions, appears to be the species responsible for most of the root rot problems in poinsettias. Historically, Pythium ultimum was considered the most important species of Pythium involved with damping-off and root rot. The significance of this information is that P. irregulare and P. aphanidermatum readily form a swimming spore stage, making it possible for them to spread in ebb-and-flow irrigation systems or be brought into overhead systems from surface water supplies. P. ultimum does not form the zoospore stage but is usually associated with contaminated soil. We speculate that P. ultimum is much less of a problem now because so few growers use field soil as a component of their potting mix.

From our DNA studies, we believe that P. irregulare can be of local origin in many different crop residues and in soil in the greenhouses where root rot outbreaks are occurring. Knowing that, growers must step up sanitation measures to prevent stray, contaminated soil from getting into the potting mix, into pots of plants, and into ebb-and-flow reservoirs. On the other hand, the P. aphanidermatum causing root rot in poinsettias does not appear to be of local origin in each operation where disease occurs. More likely, this pathogen is in infected poinsettias being shipped among greenhouses as cuttings and pre-finished plants. In this case, growers must carefully examine new, incoming plants for any root rot symptoms and be prepared to reject the shipment or discard plants with symptoms and treat the remaining plants with a fungicide immediately.

Complicating the whole issue of the identity of Pythium causing root rot, we discovered that what scientists have been calling Pythium irregulare for many years may actually be a complex of true P. irregulare plus at least one additional organism that looks very much like P. irregulare but is actually, an as yet unnamed species. We are currently doing extensive measurements on these organisms to determine whether small structural differences, if any, exist among them; The new species is more genetically diverse and shows resistance to mefenoxam/metalaxyl fungicides more often than the true P. irregulare.

Our research has revealed that resistance to fungicides in P. irregulare and P. aphanidermatum should be a concern to every grower. Over 40% of the root rot cases involving these species are caused by isolates with resistance to Subdue (metalaxyl) and Subdue MAXX (mefenoxam). Some isolates are also resistant to Banol (propamocarb). Even if you do not use these chemicals on a regular basis, Pythium with resistance to them may be in your greenhouse because they were on infected plants you purchased and placed in your facility.

As noted, several of the clinic isolates studied were resistant to mefenoxam (Subdue Maxx) or propamocarb (Banol). We have found that a low concentration of propamocarb actually stimulates the growth of some Pythium isolates and their ability to cause disease. This effect of a low concentration of a toxic material being stimulating to the target organism is termed ‘hormesis’ (similarly, many human medicines are beneficial to humans at low concentrations but are lethal at high concentrations). When geranium seedlings are treated with a very low concentration of propamocarb, more disease results as compared to when fungicide-free plants are inoculated ;up to 38% more disease develops. This could be a problem if Banol was in an ebb-and-flow tank at very low concentration; or when potted plants on the bench or flood floor are drenched with Banol and some leaches through and enters the reservoir. The other practical significance of hormesis is that if a stimulatory level of chemical is reached during the natural break down of the product on the plant or in the soil, more disease will develop on a treated plant than on a plant that was not treated. Understanding the phenomenon of hormesis may be crucial to the recommended timing of fungicide re-applications.

Research on Pythium has been an ongoing effort at Penn State since 1999. Some of the work is coordinated with researchers at Cornell and other universities in a combined Pythium and Phytophthora project funded through the USDA Floriculture and Nursery Crops Research Initiative. Through these efforts, a better understanding of root pathogens is being developed upon which we hope to improve root disease control in floricultural crops.