Introduction
Pythium species are described as ubiquitous in the environment and opportunistic, capable of lying dormant in greenhouse and nursery settings until proper conditions arise to become pathogenic. Pythium is a water mold, meaning it thrives in high moisture environments such as saturated media and standing water. In fact, rain splashing, leaching or runoff, and irrigation sources are among the best way to transport this pathogen from plant to plant (Figure 1). Once Pythium infects a plant’s root system, it will diminish the plant’s ability to take up water and nutrients, leading to a noticeable drought stress and nutritional deficiencies (Figure 2). These unique characteristics, coupled with repeated use of a limited number of effective fungicides create a scenario wherein resistance is favored for Pythium spp. Resistance can be viewed as a pathogen’s ability to increase tolerance to a specific chemistry. The rate at which an insect, mite, or disease pest develops resistance is driven by multiple factors including the reproductive potential of the organism (e.g., generations per season/year), host range of the pest, and the way in which we use pesticides. In this article we will focus on developing programs that avoid resistance development through sound cultural practices, rotating conventional chemistry appropriately, and integrating biological fungicides when possible.
I. Strong Cultural Tools
The adage “an ounce of prevention is worth a pound of cure” fits well here. In many cases, a grower’s best tools for disease management exist outside of the spray tank. Given the opportunistic nature of Pythium, growers can create production environments that avoid stressors which predispose their crop to an outbreak. Here are several cultural management strategies to consider:
- Media Selection – choosing a growing medium based on crop type and season, with Pythium we want to avoid large fluctuations in wet and dry cycles as well as extremes that create stress favorable to infection; also utilizing wetting agents to ensure optimal media performance
- Crop Timing – understanding your crop’s climatic needs so that scheduling aligns with the ideal temperature range of the species, ultimately reducing environmental stress
- Fertility Management – nutritional programs that avoid excessive nitrogen application and prevent high substrate electrical conductivity (EC), which can damage root systems
- pH Monitoring – growing within the ideal pH range for the crop to reduce associated nutritional deficiencies or toxicities
- Sanitation – implementing rigorous sanitation regimes that maintain a clean production environment from start to finish, such as injecting a sanitizer (e.g., ZeroTol 2.0, SaniDate 12.0, KleenGrow) into your irrigation lines on a pulse or continuous basis
II. Conventional Chemistry Rotation
Because Pythium is a soil-borne pathogen affecting plant roots, growers will need to apply any of the pesticides discussed in this article as a substrate drench. It is important to rotate between MOA in each season rather than repeatedly using a single chemical. This is partially why we have seen Subdue Maxx (MOA 4), an historically popular Pythium product, greatly reduce its efficacy over the past few years due to resistance issues. On a positive note, research conducted by Dr. Ann Chase has shown that tank mixing KleenGrow by PACE 49 greatly increased the efficacy of Subdue Maxx against Pythium. Segway O (MOA 21) in considered as one of the most effective and plant safe Pythium products on the market providing preventative and curative activity. Etridiazole based products (MOA 14) such as Banrot (+ MOA 1), Terrazole L, or Truban EC are cost-effective options, however they can be
relatively hard on plant roots. We recommend using this group of products after plants have become established and not at the young plant stage. Although it has a relatively high cost in use, Fenstop by Gowan is the only MOA 11 that provides excellent Pythium control. Applying all these pesticides at the correct label rate also prevents sub-lethal dosages, which are another way of progressing the rate of resistance. Sub-lethal dosage can also be prevented by ensuring chemicals have been stored correctly and are not past their recommended shelf-life. Please reach out to GGSPro for further information if needed.
III. Integrating Biological Fungicides
Given the limited number of effective conventional fungicides on Pythium currently available and the lack of new introductions coming down the pipeline, biological fungicides offer growers an opportunity to rotate unique modes of action into their programs. Bio-fungicides should be used preventatively and not be considered as “come from behind” products or have curative activity. However, they warrant consideration regarding resistance management and their efficacy when used appropriately. Again, the goal is to reduce our reliance on conventional chemistry with a multi-pronged approach, so that we are working against the pathogen from as many angles as possible. Many growers begin to implement biological fungicides by applying a chemical fungicide to reduce pre-existing pathogens initially, and then follow up with a bio-fungicide one week later. Depending on the type of biological product applied, reapplications may be necessary every 2-3 weeks (e.g., Cease, Triathlon BA, Stargus) with some materials lasting up to 12 weeks (e.g., Actinovate SP, RootShield & RootShield Plus G/WP, Obtego) Growers interested in adding bio-fungicides to their existing programs should reach out to the product vendor or GGSPro to determine chemical compatibility.
| Chemical | Item # | MOA | Biological | Chemical | Ornamental | Edible |
| Actinocate SP | 71-1117 | BM02 | X | X | X | |
| Banrot WP/ G | 71-1210 71-1215 | 1 & 14 | X | X | ||
| Fenstop | 71-14801 | 11 | X | X | ||
| Segway O | 71-31102 | 21 | X | X | X | |
| Subdue Maxx | 71-2979 | 4 | X | X | X | |
| Terrazole L Terrazole WP Truban EC Truban WP | 71-3025 3183862 71-3070 71-3065 | 14 | X | X | ||
| RootShield G RootShield WP RootShield Plus G RootShield Plus WP | 71-2780 71-27901 71-27911 71-27961 | BM02 | X | X | X | |
| Cease Companion WP Triathlon BA Stargus | 71-13301 71-372A2 71-3040 71-2950 | BM02 | X | X | X | |
| Obtego | 71-2100 | BM01 | X | X | X | |
| KleenGrow | 74-21101 | NC | X | X | ||
| SaniDate 12.0 | 71-35501 | NC | X | X | X | |
| ZeroTol 2.0 | 71-35351 | NC | X | X | X |