Understanding and Managing Sudden Oak Death in California

SIDEBAR 3-2—Selecting and Using Fungicides

To date, only phosphite fungicides have been studied extensively for their ability to protect trees from SOD.  Other materials may be effective, but controlled studies are needed to determine how well a given fungicide will work for a given host/pathogen combination and what dosage is required.  The following factors should be considered when selecting and using fungicides.

Type of activity:  Most fungicides work best when applied as protectants (before infection has occurred), rather than as eradicants or curatives (after the plant has been infected).  Using fungicides as eradicants increases the chance of selecting for fungicide resistance in the pathogen population. 

Contact or surface fungicides are not absorbed by plants and act mainly to inhibit germination and growth of spores on plant surfaces.  Contact fungicides need to be applied as a complete coating over the susceptible plant and must be reapplied as the fungicide residues are washed away or spread out as the plant parts expand.  Systemic fungicides are absorbed by plant tissues and are translocated within the plant.  Fungicides transported in the water-conducting xylem tissues can move upward only, whereas those transported in the phloem (stem tissue that transports carbohydrates) can move downward as well.  Because systemic fungicides move within the plant, it is less critical to obtain complete plant coverage.  Systemic fungicides typically have longer residual activity than contact fungicides and need to be reapplied less frequently.  Materials that need to be reapplied more than one or two times per year may not be practical for protecting large numbers of trees from SOD.

Spectrum of activity:  Most systemic fungicides are selective.  Many fungicides that are effective against true fungi have little or no activity against Phytophthora species, and vice versa.  Many contact fungicides (e.g., copper salts) are relatively nonselective and are active against a wide variety of fungi as well as Phytophthora species

Application methods:  Fungicides are applied by a variety of methods (e.g., spray, injection, soil application) which are specific to the fungicide/disease/plant combination.  The application method can affect cost, practicality, absorption, distribution of the chemical in and on the plant, and the risk of nontarget exposure.  Application methods that may be practical for treating small numbers of trees may be too expensive to use for entire forest stands.

Phytotoxicity:  Some fungicides can cause damage to plant tissues (phytotoxicity) under certain conditions.  Phytotoxicity can be influenced by factors such as plant species and variety, plant maturity, temperature, or interactions with other materials applied to plants.  Fungicides need to be effective at a dose that is well below levels that cause serious phytotoxicity on that plant. 

Effects on nontarget organisms:  Most fungicides and other pesticides work by interfering with vital metabolic processes in the target pathogen.  Fungicides also have the potential to interfere with metabolic processes of other nontarget microorganisms, invertebrates and vertebrates.  Fungicides used in settings such as forests should have little or no negative effects on the wide variety of nontarget species found in these environments.

Toxicology:  Humans are one of the most important nontarget organisms to consider when selecting a fungicide.  Potential effects of fungicides on human health are related to the toxicity of the material (potential to cause acute and/or chronic health effects) as well as the amount and routes of exposure to the material.  Persons that mix and apply fungicides to trees have the greatest likelihood of exposure to these chemicals.  Dietary exposure to fungicide residues used for SOD prevention may occur if acorns are used for food or if animals that consume acorns (e.g., livestock, deer, wild turkey) are eaten.

Applicator safety: Proper safety procedures and personal protective equipment (PPE) should be followed to minimize exposure to applied chemicals.  Appropriate PPE for applicators, such as gloves, faceshields, respirators, and protective clothing, is related to both the type of chemical used and the method of application.  The fungicide label indicates what types of safety equipment should be used and which precautions should be taken. 

Persons applying pesticides for hire in California are required to have a Qualified Applicator's License or Certificate issued by the California Department of Pesticide Regulation (CDPR).  Qualified Applicators must pass tests related to pesticide application and safety and must take continuing education classes to maintain their certification.

Regulatory issues:  Chemicals or natural products that are sold expressly for the purpose of controlling a plant disease, such as SOD, need to be registered as pesticides by the US Environmental Protection Agency (EPA).  In addition, pesticides used in California must also be registered by CDPR.  Registered pesticides that are classified as general use pesticides may be applied by most users, including homeowners.  Pesticides that are more hazardous to human health or nontarget organisms may be classified as restricted use pesticides and may only be applied by CDPR-certified applicators.  Fungicides or other products that are used to manage SOD should be registered for that use and should be applied in a manner consistent with the label, observing all precautions listed.