A fairly common statement indeed! You know as well as I do that it is frustrating when you have to tell someone that little can be done to prevent certain diseases other than giving the tree some TLC and promptly getting rid of the “corpse” once the pathogen wins. Does the future look brighter? I think so. A number of researchers, practitioners, and pesticide registrants are working together to do new things with not-so-new pesticides.

Abamectin. What is it? Abamectin is an antibiotic derived from the soil bacterium, Streptomyces avermitilis. It is well-known (examples include Syngenta’s Avid and Arbor-System’s Greyhound) by entomologists as effective against insect and mite pests of plants. It is also well-known by veterinarians because it is effective against nematode and insect pests of animals. I know, you may be thinking, “Why is he talking about an insecticide in this article?” Well, not all diseases are caused by fungi, bacteria, and viruses. For example, pine wilt is a devastating disease of Scotch pine that is caused by a nematode. So, if you’re trying to find a pesticide that will prevent pine wilt, it makes sense to consider nematicides. That’s exactly what researchers at Kansas State University are doing; they are experimenting with abamectin to prevent pine wilt.

Does it work? The short answer is that it’s not perfect, but it looks quite promising. Now, for the details. First, they verified that a very low concentration of abamectin does stop the nematode in a petri dish. Second, they compared the Wedgle and STIT (systemic tree injection tubes; Helson et al. 2001. Can Ent 133:729-744) injection treatments in a 2002 field study and found that only the STIT treatment was effective. All field studies included treated (abamectin) and control (water) trees, and all trees were artificially infected with pinewood nematode several weeks after the injection treatments. The following data include only STIT injection treatments. In the 2002 experiment, 75% (15/20) of those injected were healthy 9 months after inoculation, versus only 50% (10/20) for the water-controls. In a separate experiment in 2003, 83% (15/18) of those injected were healthy a month after inoculation, compared to only 42% (7/17) for the water-controls. Late-season data for the 2003 experiment and “following-year data” from the 2002 experiment are not available. However, additional publications are expected, and additional field experiments are under way to validate the results and optimize (for example, dosage, timing) the technique.

Registration status? Abamectin is labeled for the control of a variety of insects and mites on Christmas trees and other woody ornamentals. However, current labels do not address pine wilt or control of the causal nematode. Will registrants amend their labels to include pine wilt? The initial results are promising, but there is more to learn.

Source: “Prevention of Pine Wilt of Scots Pine with Systemic Injections of Abamectin.” Poster displayed during the August 2003 American Phytopathological Society meeting. Authors: R. James, L. Giesler. M. Harrell, N. Tisserat, and T. Todd. Kansas State University and University of Nebraska.

Paclobutrazol. What is it? Paclobutrazol is a synthetic triazole fungicide with more value as a plant-growth regulator (PGR). You’re probably more familiar with its sibling fungicides such as propiconazole, myclobutanil, triadimefon, or miconazole (ok, maybe not mi-onazole, which controls athlete’s foot). As a plant-growth regulator, paclobutrazol is sold under many names, including Profile (Dow), Cambistat (Rainbow Tree Care), Turf Enhancer (Andersons), and Trimmit (Syngenta). It can be applied to a range of trees and shrubs as a basal soil drench or soil injection and is also available for the turf market. Paclobutrazol affects plant growth in two major ways: (1) It reduces cell elongation by reducing the production of gibberellic acid, and (2) it helps conserve water and slows growth by reducing destruction of abscisic acid. Simply stated, paclobutrazol makes plants more conservative.

Does it work? What effect does this “I’m really a PGR” fungicide have on plant disease? Paclobutrazol can control a number of pathogens in the petri dish, (K. Jacobs, Morton Arboretum). Moreover, researchers and practitioners report a reduction in apple scab of flowering crabapple, and at least one researcher (B. Fraedrich, Bartlett Tree Research Laboratories) has observed a reduction in bacterial leaf scorch of red oaks. The really odd part of this last statement is the word “bacterial.” A fungicide that reduces the effects of a bacterial disease? William Chaney (professor of tree physiology, Purdue University) helps to explain this mystery: “The effect of paclobutrazol on bacteria is unknown at this time but may be due to increased resistance of trees to infection by bacteria through alteration in leaf surface structure or even the size of stomatal pores.” In fact, Chaney lists a number of reported or suggested physiological effects on trees, including reduction in shoot growth (20 to 90%); increased shoot-to-root ratio; enhanced fine root development; reduced cambial growth (smaller cells); greener, thicker leaves with smaller stomatal pores; and increased tolerance to environmental stress.

Because of its ease of use (for example, soil drench) and the potential for added benefits, many researchers and practitioners are experimenting with paclobutrazol. At the Morton Arboretum, early field trials against verticillium wilt, armillaria, sphaeropsis tip blight, apple scab, botryosphaeria canker, and cytospora are under way. In terms of obtaining adequate disease control, research is needed regarding dosage and timing. Furthermore, a number of individuals have expressed concerns regarding the long-term effects (for example, “poodle-shape”) of repeat applications to trees.

Registration status? As mentioned, paclobutrazol is labeled as a PGR on a wide range of plants. However, like abamectin, current labels do not address disease control. Will registrants amend their labels to include disease control? Again, most reports look promising, but there is much more to learn.

Source: Chaney, William. “Tree Growth Retardants: Arborists Discovering News Uses for an Old Tool.” Tree Care Industry, March 2003. www.rainbowtreecare.com/pdf/Tree_Care_Industry_TGR_Article.pdf.

Final note. This article outlines experimental research results. Experimentation often requires formal agreements with the pesticide registrant (the owner) and the U.S. EPA. When using any pesticide, carefully read and follow the label instructions. Be sure you are applying it to a legal site (that is, area and plant), using an approved application technique, and staying within the listed dose range. Off-label pesticide use is illegal and irresponsible, and may jeopardize the future availability of pesticides as an option in pest management.