Considerations in the Use of Products for Control of Citrus Greening

As the state-wide production of citrus drops to historically low levels, crops are currently producing relatively positive economic returns.  At the same time, citrus production managers are making significant efforts to combat citrus greening (Huanglongbing or HLB), and its vector the Asian citrus psyllid (Diaphorina citri).  Intensified efforts to control greening and the vector have been widely discussed, and some elements of these control measures have been introduced into grove management programs.  These programs involve different ways of applying crop protection chemicals while others include using materials not previously considered.

One approach for more efficacious treatment for citrus psyllid is that more concentrated sprays (i.e., lower spray volume per acre) can cover more area in less time, thereby lowering application costs.  However, pesticide labels vary in their specifications for application.  For example, the dimethoate label states “for concentrated application, apply 1 to 2 quarts per acre in sufficient water to provide full coverage of foliage.”  In contrast, the fenpropathrin label states “no less than 50 gallons per acre should be used to apply the material by ground spraying.”  When a product label states a range of spray volume per acre, it is the responsibility of the applicator to follow the prescribed spray volumes and/or application methods.  Improper spray volume rates of pesticides may result in smaller spray droplets that may drift and risk movement off-site to non-targets areas or cause potential crop damage or residue issues.    Remember the label is the law and must be followed.

Production managers know that numerous “remedies” for citrus canker were touted as the disease spread statewide after the hurricanes in 2004 and 2005.  The only products recommended in the Florida Citrus Pest Management Guide for canker control are well known copper formulations already in use for control of foliar and fruit fungal diseases. Nevertheless several alternative products were utilized and verified as ineffective by grower experience.  Although thousands of compounds are allowed on food commodities without requirement for a tolerance, the federal Environmental Protection Agency (EPA) maintains lists of these compounds in the form of a “list of inerts” (also known as the 4A and 4B inerts), and the list for 25(b) pesticides.  The 25(b) pesticides are minimal risk materials that are a subset of the inert ingredients.  Materials that can be used in organic production are also drawn from the inerts list and organic producer organizations (such as the Organic Materials Review Institute) determine which can be used in that production scheme.

The term “pesticide” is defined as (1) any substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any pest, or (2) any substance or mixture of substances intended for use as plant regulator, defoliant, or desiccant.  One of the most important words in the FIFRA definition of “pesticide” is “intended.”  A key analytical step to determining whether a product is a pesticide is to consider whether the product is “intended” to be used as a pesticide.  Products are generally considered to be a pesticides if they are  intended for preventing, destroying, repelling or mitigating any pest, or intended for use as a plant regulator, defoliant or desiccant.  EPA determines intent by examining claims on the label, advertising, composition/use and /or mode of action of the products as distributed or sold.  If a person who distributes or sell the product claims, states or implies by labeling that the product can be used to control pest then the product is a pesticide.  Some substances and product are excluded for pesticide registration requirements if they meet certain conditions or criteria.  Example of products that are not pesticides include liquid chemicals sterilants, nitrogen stabilizers, product labeled only for use in living man or animals, vitamins hormone products, products intended only to aid in growth of desirable plants, antimicrobial products used solely in processed foods or feeds, in beverages or in pharmaceuticals, and products with no pesticidal claims.  Products that are not intended to prevent, destroy, repel or mitigate a pest, or to defoliate, desiccate or regulate the growth of plants are not considered to be pesticides.  Some of these products may appear to be pesticides but are not considered as such unless pesticidal claims are made on their labeling or in connection with their sale and distribution.

Conversely, there are products available in the citrus management trade that claim enhancement of plant health as opposed to providing pesticidal activity that reduces the pest or pathogen population. The use labels on these products may contain statements that they affect certain pathogens by eliciting a plant defense response.  Recently a grove manager reported to their colleagues that they are applying one or more alternative products to reduce the progression of greening symptoms in their groves.  The rationale is that these products may initiate or activate a chemical defense pathway in the tree to reduce pathogen invasion.  Indeed, the active ingredient of the well known pesticide Actigard, acibenzolar-s-methyl (ASM), acts as an inducer of systemic acquired resistance (SAR).  Actigard is registered for use against tomato bacterial spot diseases but is currently not registered for use in citrus. 

Citrus juice and by-products are tradable commodities, and as such are subject to significant testing to meet national and international standards.  With this in mind, grove managers must consider all chemical inputs into their crops.  Inadvertent residues of compounds which are not supposed to be present may be grounds for rejection of the entire crop or ‘lot’ of juice.  If a certain compound has suspected utility, grove managers should interact with registrant representatives or Extension personnel to examine the utility and legality for use of the product.  If utility is proven in some manner, registrations are available under the federal pesticide law to make them available during extreme events.