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The Pathogens Are Knocking

August 18, 2004

For well over 40 years, public and private researchers have been trying to trick plants into protecting themselves by making them “think” they are being attacked by a pathogen. This may sound a little crazy until you think about how the human body reacts to immunization shots. Of course, this analogy only goes so far; plants do NOT have an immune system at all like ours. However, plants do have an amazing capacity to produce some interesting chemical and physical responses to infection by a pathogen. In this article, I’ll explain and examine several commercially available compounds that are being used to induce something called systemic acquired resistance (SAR) in plants.

What exactly is SAR? Systemic acquired resistance is a broad-spectrum, systemic, plant defense response that is triggered by exposing a plant to certain natural or synthetic chemical compounds or to a weak (hypovirulent) strain of a specific pathogen. These inducers or elicitors do not have a direct effect on pathogens. Rather, they are more like an intruder setting off the house alarm. Depending on the plant species and its genetic makeup, the response to a perceived intruder might include one or more of the following: signal the distant plant cells (for example, salicylic acid), chemical warfare (for example, enzymes), build stronger barriers (for example, lignin), or localized suicide (hypersensitive reaction). Depending on the plant, the environment, and the inducer, peak SAR activity may take several days or weeks to develop and may last for several weeks or even months. For a much more thorough explanation of SAR, consider reading the selected references listed at the end of this article.

Many SAR compounds are being researched around the world. However, only a few have been commercialized. Although not are labeled for the turf and ornamental market at this time, I’ll include several that you may hear more about.

Harpin. This protein is marketed by Eden Biosciences Corp. as Messenger. The harpin protein in this formulation is identical to the protein that was initially discovered and naturally produced by Erwinia amylovora (the fire blight pathogen). The label states that this is “A biochemical pesticide used for plant disease management, insect suppression, and plant growth enhancement.” Although the label states generally that Messenger will “aid in the management of diseases,” several viral, bacterial, and fungal pathogens are specifically mentioned. Messenger is labeled for application on a wide range of vegetable, fruit, field, tree, and vine crops. However, it is not labeled for turfgrass (except seed production) or ornamentals. A cursory review of several recent experimental turf-disease trials revealed that Messenger did not protect against infection by Pythium or Rhizoctonia. However, I encourage researchers and practitioners alike to recognize the importance of the following statement found on the Messenger label: “Plants require 5-7 days to fully induce resistance.”

Acibenzolar-S-methyl (ASM). This compound is marketed by Syngenta as Actigard. The proposed common name of acibenzolar-S-methyl has many variants, most notably benzothiadiazole (BTH). The label states that Actigard “is a selective, systemic compound used for the control of downy mildew of cole crops and leafy vegetables, bacterial leaf spots of tomato, and blue mold of tobacco.” This product is not labeled for turfgrass or ornamentals. A cursory review of several recent experimental turf and ornamental disease trials revealed that Actigard offered no protection against Entomosporium leaf spot of photinia or Rhizoctonia brown patch of turfgrass and less than acceptable protection against Phytophthora root rot of snapdragon and dollar spot of turfgrass. Once again, I encourage researchers and practitioners to recognize the importance of the following statement found on the Actigard label: “Maximum disease control is normally obtained 4 days after an Actigard application.”

Phosphonates. Following the Fungicide Resistance Action Committee’s (FRAC) lead, I have combined the fosetyl-aluminum (marketed by Bayer as Aliette and Signature and by Lesco as Prodigy) and phosphorous acid (marketed by Cleary’s as Alude; ArborSystems as Whippet; and JH Biotech as Fosphite) compounds into the phosphonate group. From the literature, it appears that the debate continues as to whether or not phosphonates technically fit the SAR definition (they are plant nutrients, and they are slightly but directly toxic to certain fungi). Nevertheless, and for practical purposes, I feel that their inclusion in this article is warranted. As you read labels, literature, and research reports, it is important to note that fosetyl-aluminum is commonly listed as “aluminum tris(ethyl phosphonate),” which is the chemical name. Similarly, phosphorous acid has several common synonyms, such as potassium phosphite and mono- and di-potassium salts of phosphorous acid. The phosphonate compounds are widely available in many markets, including turf and ornamentals. Their protection against Pythium, Phytophthora, and downy mildew diseases is well documented.

Although the SAR phenomenon has been studied for quite some time, many basic questions remain unanswered. As research explains more about molecular plant--microbe interactions, we will learn how to manipulate these and future SAR compounds like the relative precision instruments that they are.

Selected references.

Gozzo, Franco. “Systemic Acquired Resistance in Crop Protection.” Outlooks on Pest Management, Research Information Ltd. February 2004. <http://www.researchinformation.co.uk/pest/sample/15-1/11-Gozzo.pdf>. Accessed July 28, 2004.

Percival, Glynn. “Induction of Systemic Acquired Disease Resistance in Plants: Potential Implications for Disease Manage-ment in Urban Forestry.” J. Arboriculture 27(4): July 2001. <http://www.treelink.org/joa/2001/july/02_INDUCTION_OF_SYSTEMIC_ACQUIRED_PLANT_ DISEASE_RESISTANCE_percival.pdf>. Accessed July 28, 2004.

“FRAC Fungicide List 1 (arranged by FRAC Code).” Fungicide Resistance Action Committee. <http://www.frac.info/publications.html>. Accessed July 15, 2004.

Author: Bruce Paulsrud


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