Mary Helen Ferguson, Louisiana State University Agricultural Center, and Guido Schnabel, Clemson University

Orondis Gold, a fungicide containing the active ingredients oxathiapiprolin and mefenoxam, is now labeled for strawberries for managing Phytophthora crown rot (vascular collapse) and red stele root rot.

This gives you, the grower, an option for Phytophthora root and crown rot management, in addition to mefenoxam or metalaxyl alone (Fungicide Resistance Action Committee [FRAC] code 4; Ridomil Gold SL, Ultra Flourish, MetaStar 2E, and others) and the phosphonate chemicals (FRAC code P07; aluminum tris, phosphorus acids and salts).

Oxathiapiprolin (FRAC code 49) is a welcome addition to existing chemical options in the toolbox for Phytophthora root and crown rot management, especially considering that resistance to FRAC 4s such as mefenoxam has been found in the strawberry crown rot pathogen (Phytophthora cactorum) on some farms.

It is critical to use good resistance management practices to reduce the likelihood that Phytophthora populations will develop resistance to this new mode of action, as well as to preserve existing effectiveness of mefenoxam and metalaxyl. Management of Phytophthora crown and root rot should begin with good cultural practices. These include practicing crop rotation, providing good drainage, and obtaining clean plants from reputable sources. Overhead irrigation is essential for establishing bare-root plants especially, and insufficient water during establishment stresses plants, which may result in increased disease susceptibility. At the same time, excessive water favors Phytophthora diseases, so water should be managed carefully. Water source should be considered. Surface water from ponds that collect runoff from fields may be contaminated with Phytophthora. Irrigation with well-water is preferable. Cultivars commonly used in the southeastern United States are susceptible to Phytophthora, but keep an eye out for new ones with increased resistance. Application of a mefenoxam/metalaxyl or phosphonate product, or now Orondis Gold, is a measure to take where problems with Phytophthora crown or root rot are anticipated.

Table 1 compares Orondis Gold and a commonly available mefenoxam product, Ridomil Gold SL.

Table 1. Comparison of Orondis Gold and Ridomil Gold SL

Orondis GoldRidomil Gold SL
Active ingredient(s) (concentration; FRAC code)Mefenoxam (9.89%, 0.88 lb/gal; FRAC 4) and oxathiapiprolin (3.29%, 0.29 lb/gal; FRAC 49)Mefenoxam (45.3%, 4 lb/gal; FRAC 4)
Application rate20 to 62 fluid ounces per acre1 pint per acre (adjust for root zone width/width of band)
Amount of mefenoxam per acre per application0.14 to 0.43 lb0.5 lb (adjust for root zone width/width of band)
Maximum product usage per acre per year (amount of mefenoxam in this amount)124 fluid ounces (0.85 lb mefenoxam)3 applications / 3 pints (1.5 lb mefenoxam)
Maximum allowed mefenoxam per acre per year1.5 lb (soil-applied) or 0.19 lb (foliar/soil-directed)1.5 lb
Minimum time between applications30 days30 days
Timing instructions for annual strawberry plantings“Make the first application soon after planting when overhead watering for plant establishment has been completed, and a second application 30 days before the beginning of harvest or at fruit set.”“Make the first application after transplanting. Make the second application 30 days before the beginning of harvest or at fruit set. Apply the third application during harvest, depending on disease pressure and environmental conditions.”
Application method(s) allowedThrough drip irrigation system onlyThrough drip irrigation system or banded/ground-applied*
Pre-harvest interval28 days0 days
Re-entry interval48 hours48 hours if banded/ground-applied; 0 hours if applied through drip irrigation
*Application by overhead chemigation is also allowed, but this is not a common practice in the southeastern U.S.

So now what? Do I always have to chemically treat for Phytophthora? Should I use Ridomil Gold SL or Orondis Gold? If I use Orondis Gold, should I use the low rate or the high rate, and why is there such a big rate range anyway? Those are all very good questions, and we are trying to answer them to the best of our abilities.

First of all, if there is no problem then there is no need for action. If your soil is well drained, you have had no history of Phytophthora problems, and there is no word from the nursery that the plants are contaminated, then don’t waste your money and don’t stress your plants and the environment with unnecessary chemical applications. But if you do have good reasons to suspect there will be a problem, then protect your crop. 

From an effectiveness perspective, either Orondis Gold or a mefenoxam/metalaxyl-only product such as Ridomil Gold SL is expected to have very good efficacy against Phytophthora crown and root rots, as long as there is no resistance to mefenoxam in the pathogen population. In general, fungicide mixtures are better able to prevent the emergence of resistance than products with a single mode of action, if no resistance to either active ingredient exists in the field. That is because pathogens develop resistance to active ingredients consecutively over time, often altering one target gene first and then taking a bit of a break (to generate more genetic diversity) before altering a second target gene to become resistant to two active ingredients. Pathogens will have a much harder time developing two different resistance mechanisms at the same time. With that in mind, Orondis Gold is to be favored over Ridomil Gold SL for resistance management reasons unless you mix Ridomil Gold SL with something else, such as a phosphonate product (e.g., ProPhyt). Alternatively, Ridomil Gold SL could be used in rotation with ProPhyt or similar products.

The wide range of allowed application rates for Orondis Gold made us scratch our heads. The company would not recommend a rate that is not effective. From a resistance management standpoint, the lowest effective rate is typically preferable over higher rates. (A ratetoo low to be effective can promote the development of resistance.) There are also good reasons for using the higher rates in some cases. The label states that the higher rate should be used if there is a history of Phytophthora in the field. A representative of Syngenta representative clarified, though, that 28 fluid ounces per acre is the standard recommended rate for Orondis Gold in strawberries.

If used at the highest labeled rate, Orondis Gold can be used twice per year. Using the 28 fluid ounces per acre rate (or another rate between 20 and 41 fluid ounces per acre) will allow for a third application. However, Orondis Gold has a 28-day pre-harvest interval, limiting its utility later in the season. If more than two applications of Orondis Gold are made, a product with a different mode of action (e.g., a phosphorous acid salt) must be used before a third application.

It is possible to legally make one Ridomil Gold SL application even after Orondis Gold use is exhausted. That is because the maximum yearly amount of Orondis Gold contains 0.85 pound mefenoxam, and 1.5 pounds of mefenoxam are allowed per acre annually. Keep in mind that repeated use of mefenoxam poses great selection pressure on the pathogen population and increases the risk of resistance development. Thus, a product with a different mode of action (phosphorus acid salts) should still be used before a third application of a mefenoxam-containing product.

To our knowledge, no field resistance to oxathiapiprolin in the Phytophthora crown rot or red stele root rot pathogens (P. cactorum and P. fragariae, respectively) has been reported at this time. However, we need to use this product with caution because the risk of resistance development to oxathiapiprolin is considered medium to high, according to FRAC.

Information in this article is based on label language at the time of writing. Read the label of the product you have carefully for updates and additional information. Reference to commercial or trade names is made for the reader’s convenience and with the understanding that no discrimination nor endorsement of a particular product is intended.

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