Disclaimer: The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products implies no endorsement by the authors. Criticism of products or equipment not listed is not implied or intended. Due to continually changing laws and regulations, we can assume no liability for the suggested use of chemicals contained herein. Pesticides should be applied according to the label directions on the pesticide container.

Chemicals are used to help supplement—not take the place of—all the other management techniques such as water management, clean stock, sanitation, and planting resistant hosts.

Chemical techniques are used to eradicate Phytophthora from production equipment, water, and soil. Other chemicals are used to protect plant tissues from infection or to inhibit further growth of the organism inside plant tissues.

Integrating several techniques should help prevent losses due to Phytophthora diseases.

Eradication Tactics

Eradication means to kill (or inactivate or get rid of) any spores or mycelium of Phytophthora that might be in or on anything used in the nursery operation.

Disinfectants for Tools and Equipment

Starting with clean tools and production areas will help prevent many diseases.

Several kinds of disinfectants are used to treat greenhouse benches, pots, tools, and equipment used for planting and harvesting. Not only will they kill Phytophthora but also many other disease-causing organisms. Although these products are useful as disinfectants, they have a very short residual time and will not be effective as long-term fungicides or bactericides.

Disinfectants include products with peroxide, quaternary ammonium, or sodium hypochlorite (bleach) bases. Some, such as dilute bleach solutions, are highly corrosive to metal tools or surfaces. Since these products are quickly tied up (inactivated) by organic matter, do not use them on soil or gravel.

To get the best value from these products:

  • Clean soil or plant debris from objects and surfaces before applying the disinfectant.
  • Increase contact time with the disinfectant.

Table 1. Disinfectants for Tools and Equipment

Chemical Name Trade Names
peroxides ZeroTol, OxiDate, TerraCyte, Virkon S
quaternary ammonium

Physan 20, Green-Shield CA

sodium hypochlorite (bleach) Clorox, Agclor

Disinfectants for Water

Many Pacific Northwest growers treat irrigation water to reduce Phytophthora inoculum from infested water sources. Container nurseries treat recycled irrigation water with chemicals such as chlorine gas, sodium hypochlorite, and ozone.

See the water management for more information on disinfecting water.

Soil Fumigation

When fumigating soil or media to control weeds and insects, you will also control many fungi and Phytophthora.

Fumigating soil with methyl bromide, with or without chloropicrin, is effective but too expensive for Phytophthora control alone. In any case, methyl bromide was phased out after being classified as an ozone depleter under the federal Clean Air Act.

Other effective soil fumigants include metam sodium and dazomet products, which break down in soil to isothiocyanate. Metam sodium can be applied with irrigation water. All are restricted use materials and require special licensing to use.

Table 2. Soil Fumigants

Chemical Name Trade Names
methyl bromide none
methyl bromide + chloropicrin  
metam sodium Vapam HL, Metam CLR
dazomet Basamid G

Prevention Tactics

The concept of prevention is to keep spores or mycelium of Phytophthora from getting into plant tissues. Two kinds of chemicals prevent Phytophthora infection—contact fungicides and systemic fungicides.

Contact Fungicides

Several contact fungicides are commonly used to protect plants against foliar infection by various Phytophthora species. These fungicides inhibit germination and/or penetration of the sporangia, zoospore, or chlamydospore into plant tissues.

These chemicals are not systemic, so they are ineffective once the pathogen enters plant tissues. They are best used before the pathogen tries to infect the plant. For example, to manage a Phytophthora that is active in winter, apply these chemicals in the fall before winter rains begin.

Copper-based compounds, such as bordeaux mixture, have been used a long time and can still be effective. Other copper-based protectant fungicides include:

  • copper hydroxide
  • copper oxide
  • basic copper sulfate
  • copper oxychloride
  • copper ammonium carbonate

In each case, the active agent against Phytophthora is the Cu++ ion. Some of these may leave residues on plant foliage, and so are generally used during the dormant season. Acidic conditions, such as tank-mixing with phosphorous acids, will make too many copper ions available and injure the plant.

Table 3. Copper Fungicides

Chemical Name Trade Names
bordeaux mixture (copper sulfate +hydrated lime) mix on site
copper hydroxide Kocide, Champ, Nu-Cop
copper oxide Nordox
basic copper sulfate Cuprofix Disperss
copper oxychloride C-O-C-S
copper ammonium carbonate Copper-Count-N

The ethylene bis-dithiocarbamate fungicides such as maneb and mancozeb are also contact fungicides. Chlorothalonil products, such as Daconil, have also been used effectively to control foliar Phytophthora diseases. Etridiazole is also effective, but since it is sensitive to ultraviolet (UV) radiation, it is used primarily as a soil drench.

Table 4. Contact Fungicides

Chemical Name Trade Names
maneb none
mancozeb Fore, Protect DF, Pentathlon DF
chlorothalonil Daconil, Exotherm Termil
etridiazole Terrazole, Truban

There are many products that can have activity on Phytophthora. For example, the biofungicide Zonix is a rhamnolipid surfactant that lyses zoospores. It looks quite promising based on lab tests with P. ramorum.

Systemic Fungicides

Several groups of chemicals are taken up and moved around in plant tissues:

  • Phenylamide (FRAC group 4)
  • Phosphonate (FRAC group 11)
  • Cinnamic acid (FRAC group 40)
  • Quinone outside inhibitor (QoI) (FRAC group 11)
  • Quinone inside inhibitor (QiI) (FRAC group 21)
  • Benzamide (FRAC group 43)

All these chemicals are used extensively to control Phytophthora and related diseases. Products that contain these chemicals are used as seed treatments (for damping-off diseases), soil drenches (for root and crown rots), or foliar sprays.

Each group moves slightly differently from the others, which has a bearing on how and when they are used to managePhytophthora diseases. Some chemicals move up, from roots to shoots (xylem movement only); others move both up and down in the vascular system (both xylem and phloem movement). This uptake and movement in the tissues provides both protectant and suppressive activity.

For movement to occur, however, the chemical must be applied when the plant is actively growing.

Phenylamide Group

The phenylamide group (FRAC group 4) includes metalaxyl, oxadixyl, and mefenoxam. These have xylem movement only (from roots to shoots).

To prevent root rot, these chemicals must be applied or incorporated into the soil or media. The best time to apply is in spring, just before or as early root growth begins.

To control a foliar disease, these fungicides must be applied to the foliage. Why? Although the chemical applied to roots will move up into the leaves, it will not be at a high enough concentration in leaves to control disease.

The phenylamide group is active only against oomycetes, such as PhytophthoraPythium, and the downy mildews. It suppresses sporangial formation, mycelial growth, and establishment of new infections. It does not inhibit zoospore release, zoospore encystment, or initial penetration of the host.

Table 5. Phenylamide Fungicides

Chemical Name Trade Names
metalaxyl Vireo MEC, MetaStar 2E
mefenoxam Subdue MAXX, Mefenoxam

Phosphonate Group

The phosphonate group (FRAC group 33) includes fosetyl-Al and phosphorous acid. These can move both up and down in the vascular system (both xylem and phloem movement).

A plant can take up these chemicals through roots, leaves, and stems and then move the chemical to other parts of the plant. Trunk, soil, or foliar applications can effectively control Phytophthora root diseases; however, soil microorganisms can degrade some of these chemicals quickly, so foliar applications are preferred. Application can be any time during active plant growth.
    
The mode of action in this group is a bit different from other groups’, because it acts directly on Phytophthora itself. However, chemicals in this group also stimulate host plant defense responses. You may also see this described as systemic acquired resistance or SAR.

Table 6. Phosphonate Fungicides

Chemical Name Trade Names
fosetyl-Al Aliette
phosphorous acid Alude, Fosphite, Reliant

The phenylamide and phosphonate fungicides do not kill Phytophthora. They can either prevent the organism from becoming established in the plant, or, if the organism is already inside the plant, the chemicals can  prevent its continued growth. As a result, the chemicals can delay the appearance of symptoms. Once chemical activity has subsided with time, however, Phytophthora can resume growth within infected plants.

For this reason you may choose to—or be directed to—AVOID use of these chemicals when producing susceptible plants. Avoiding use of these chemicals would allow you to identify infected plants that need to be discarded. This practice also would avoid the shipping of infected nursery stock to places where they do not have the disease.

Cinnamic Acid Group

For the nursery industry, this group (FRAC group 40) includes dimethomorph and mandipropamid. Their properties are similar to the phenylamide group’s. They have xylem movement only (from roots to shoots), are applied to foliage for downy mildew control, and are active only against oomycetes. They also move from roots to shoots but are less effective overall than the phenylamide group.

The cinnamic acid group is active on Phytophthora diseases but does not control diseases caused by Pythium.

Table 7. The Cinnamic Acid Fungicides

Chemical Name Trade Names
dimethomorph Stature
mandipropamid Micora

Quinone Outside Inhibitor (QoI) Group

Some of the fungicides in the QoI group (FRAC group 11) have activity on Phytophthora and many fungi. These chemicals are used to control foliar infections.

Although they are systemic, they have much less movement within plant tissues. By comparison with other systemic fungicides, these chemicals move slowly into green leaves and stems. Once in the tissue, movement within the xylem is limited.

Table 8. Quinone Outside Inhibitor (QoI) Fungicides

Chemical Name Trade Names
azoxystrobin Heritage
pyraclostrobin Insignia, Empress
fenamidone Fenstop

Quinone Inside Inhibitor (QiI) Group

The only member of this group (FRAC group 21) is cyazofamid. Like many other fungicides it only has xylem movement within the plant.

Table 9. Quinone Inside Inhibitor (QiI) Fungicides

Chemical Name Trade Names
cyazofamid Segway

Benzamide Group

The only member of this group (FRAC group 43) is fluopicolide. Like many other fungicides it only has xylem movement within the plant.

Table 10. Benzamide Fungicides

Chemical Name Trade Names
fluopicolide Adorn

Chemical Resistance

The mode of action of each of these chemical groups is so specific that many Phytophthora species have developed resistance to them. Resistance means the organism can grow and cause disease at chemical concentrations that formerly prevented them from growing.

Tank-mixing systemic fungicides with contact fungicides will help prevent the development of resistant types. Some products, as listed below, contain a combination of 2 or more fungicides.

Table 11. Combination Fungicides

Trade Name Chemicals Included FRAC Codes
Banrot etridiazole + thiophanate-methyl 14 + 1
Orvego dimethomorph + ametoctradin 40 + 45

Summary

The best way to control a Phytophthora disease is before it starts. Emphasis should be on recognizing the potential for disease and taking preventive steps before losses occur. Fighting these diseases after they become established is difficult if not impossible, especially if using only chemicals.

Integrating several techniques should help prevent losses due to Phytophthora diseases. Chemicals can help supplement other management techniques.