Pythium solutions

Wilting on part of a coneflower shows typical Pythium root rot symptoms. Photo courtesy of A.R. Chase

Pythium diseases are very common on ornamentals and one of the most reported diseases when it comes to both preventive and curative fungicide applications. Symptoms of Pythium infection are variable and can include:

• stem rot — called black leg at times
• chlorosis — yellowing, especially on lower leaves
• stunting
• wilting — can recover overnight sometimes
• loss of lower leaves
• poor root development
• root loss — outer core can die leaving just the core

Avoiding disease through the use of pathogen-free plugs and liners, new potting media and pots and using the amount of water and fertilizer for optimal crop production, help to minimize Pythium losses. But sometimes use of a fungicide or biological product is needed.

Trial Review
Over the past few years, we have seen quite a few new fungicides registered that are labeled for Phytophthora and downy mildew, but few labeled for Pythium diseases. We are still looking for more products that are highly effective on Pythium diseases. The IR-4 program (provides data for specialty crops product registration) supported trials on Pythium control for several years and came out with a summary of that work in the middle of 2011 (Table 1).

Unfortunately very few products give us better than 50 percent control of Pythium root rot and sometimes efficacy is affected by the exact species of pythium involved. The Pythium species included in the IR-4 trials were aphanidermatum, dissoticum, irregulare, mammilatum, ultimum and vipa.

The best product overall was Terrazole at 92 percent control in eight trials with Subdue MAXX, providing an average of 65 percent control in 21 trials. Both of these products are industry standards. Although both have been used for more than 30 years, resistance has not developed to etridiazole (ai for Terrazole, Truban and Banrot). In contrast, resistance to mefenoxam in Pythium isolates has been reported consistently over the past 18 years. Other growers rely on use of phosphonates (MOA 33) like Aliette and Alude. The IR-4 trials show that Aliette and Alude also provide about 50-58 percent control.

One of the newest fungicides for Oomycetes (includes Pythium, Phytophthora and the downy mildew fungi) is Adorn. Adorn showed 64 percent Pythium control in 25 IR-4 trials. This product must be tank-mixed (label requirements) with another MOA group fungicide for any use, including Pythium control.

Strobilurins (MOA 11) provide varying levels of control but can reach the 50 percent mark. The newest strobilurin to be registered is Empress, which has pyaclostrobin as the active ingredient. At the time of writing this article, the label was not yet available but the most recent research has shown good efficacy for Pythium on a number of crops at different stages.

RootShield Plus
The biological control product most often used for soil-borne diseases like Pythium root rot is RootShield. BioWorks recently introduced RootShield Plus which has two different fungi (Trichoderma harzianum T-22 and T. virens G-41). The IR-4 trials showed inconsistent results with this new product. In most cases, control was less than acceptable with an average of 23 percent. Trials run at Chase Horticultural Research Inc. (2010-2011) showed similar results but those performed by Ramon Georgis, Ph.D., (AgroSci R&D in Palo Alto, CA) in 2010 and 2011 were consistently better. Georgis tested Pythium root rot on mum (P. aphanidermatum) and Pythium root rot on poinsettia, hibiscus, Buxus and mum (P. ultimum). Results ranged from some to very good control with P. ultimum, and very good to excellent control with P. aphanidermatum.

It’s clear that you should trial this material yourself under your specific greenhouse conditions to determine how it can fit into an IPM program on your crops.

Empress
In 2011, we completed a few trials using Empress on newly seeded plug flats. We tested Pythium damping-off on celosia, as well as Rhizoctonia (celosia) and Fusarium (lisianthus). The rates of Empress tested were: 0.8, 1.5, 3.1, 4.6 and 6.1 oz/100 gal. They were compared to Heritage at 1 oz/100 gal, and the inoculated and non-inoculated controls. Fungicides were applied once only before inoculation which was after seedling emergence. Results for Pythium irregulare on celosia showed that 0.8 oz/100 gal was too low, but 1.5 to 3.1 oz/100 gal gave very good control of Pythium damping-off.

In 2012, we worked in larger containers (2.75- and 4-inch pots) with Buzz Uber (Crop Inspection Services, in Valley Center, Calif.). Trials were run with Subdue MAXX (1 oz/100 gal) as the standard and rates of Empress from 0.5 to 3 oz/100 gal. They were treated once before inoculation and then twice more on a 14-day interval. These trials showed variable results based on pot size (Table 2). We found that the smaller pots dried more quickly, and disease was therefore less severe in the smaller pots.

Overall, Empress appears to be very good to excellent for Fusarium, Phytophthora and Rhizoctonia in many container sizes (other trials). It is slightly less effective on Pythium but is still good. Be sure to read and follow the labeled use rates and intervals carefully once the product reaches the market. It will be an excellent addition to our tools for control of root diseases on ornamentals.

Conclusions
At this point the most effective products are those that include:

• etridiazole (Terrazole, Truban and Banrot)
• mefenoxam (Subdue MAXX - resistance is possible)
• fluopicolide (Adorn - remember to follow label directions to tank mix with another effective fungicide)
• phosphonates (Aliette, Alude and many others - I have seen these work only 50 percent of the time against Pythium root rot)
• strobilurins (Disarm O, Empress, Heritage, Insignia) or fenamidone (FenStop - use lower end of label rates to avoid possible root damage that can be confused with root rot.)
• Trichoderma sp. (RootShield and RootShield Plus)

As always read labels before use and rotate between MOA groups. These are the two most important ways to get the most our of your fungicide dollars.

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