Strobilurn Fungicides Update
In 2000, I wrote a summary of then current information on one of the newest classes of fungicides for ornamentals –strobilurins. The original compound, strobilurin A, was isolated from a mushroom called Strobiluris tenacellus found growing on decaying wood in a European forest. The products that have been developed, however, were synthetically derived. Zeneca (then ICI) started working on this new chemistry in 1982, with BASF following in 1983 and Syngenta (then Ciba Geigy) following thereafter.
The first of the strobilurins to reach the ornamental market was Cygnus 50WDG from BASF Corporation with the active ingredient kresoxim-methyl. Ornamental marketing of Cygnus has recently been transferred to Scotts Professional Horticulture. Cygnus is described as a surface-systemic (translaminar movement) with anti-sporulant activity. This means that the product moves into and through the leaf, distributing chemical on the other side. Practically speaking, the active ingredient reaches areas on the same leaf that were not directly sprayed, which is a big help in controlling many leaf diseases.
Heritage 50WG (Syngenta) was the next strobilurin to reach the ornamental market. The active ingredient of Heritage is azoxystrobin, which has systemic activity with upward movement. This means that when sprayed onto the base of a leaf, Heritage will move out to the edges (tip). If drenched, it will move into the root system and throughout the plant. If it is sprayed onto leaves, however, it will not move downward into the stem and roots.
Compass (trifloxystrobin) was originally developed and marketed by Novartis Crop Protection. With the merger of Novartis and Zeneca last year, Compass was sold (factory and all) to Bayer. Olympic Horticultural Products now markets Compass to our industry. Movement of trifloxystrobin is described as mesostemic to indicate both protectant and curative capacity. In this case, the active ingredient "penetrates the plant tissue, has laminar activity, but there is little or no transport within the vascular system of the plant."
The newest strobilurin is Insignia (formerly BAS 500F) from BASF Corporation. This product has mainly been developed for control of turf grass diseases and its use in ornamentals is not widely known at this time. The product will be recommended primarily as a protectant.
Heritage and Compass have been extensively tested on ornamental crop diseases over the past five years (Table 1). The two products work to a similar degree on Alternaria leaf spots, black spot on rose, downy mildews, Fusarium diseases, Myrothecium leaf spots, powdery mildews and poinsettia scab. Heritage tends to be more effective on downy mildews – probably because of its systemic capacity. Compass tends to be more effective on powdery mildews since movement into the leaf is not as desirable as presence on the leaf surface and immediate vicinity. Addition of a nonionic surfactant (NIS), such as Latron B 1956, has improved efficacy of both fungicides for powdery mildew diseases. This is presumably due to better penetration of the powdery mildew colony when an NIS is added.
Some more obvious differences in efficacy have been found for Botrytis diseases where Compass is slightly more effective than Heritage. In contrast, Cylindrocladium is more effectively controlled by Heritage than Compass, although the level of control provided by either fungicide is rarely better than "good" for this pathogen.
Since these tests were limited in scope and number, I summarized results that were reported for these active ingredients on vegetable, fruit and other nonornamental crops (Table 2). Extensive tests on powdery mildew indicate very good to excellent control with all three strobilurins currently available. As with ornamental trials, trifloxystrobin works better than azoxystrobin on Botrytis. A higher degree of control of Phytophthora and Rhizoctonia is achieved with azoxystrobin compared to trifloxystrobin (similar to ornamental trial results).
There appears to be an overall profile for the strobilurin fungicides with the highest efficacy on powdery mildews, sometimes rusts, downy mildews and leaf spot diseases (Alternaria, Septoria, and Myrothecium). Relatively, a lower efficacy is found for control of Botrytis, Sclerotinia (related to Botrytis), Cylindrocladium, Phomopsis and Pythium. The differences noted between the systemic azoxystrobin and locally systemic trifloxystrobin are due to that factor and the nature of the target disease. Those diseases that are deeper in the host tissue (especially systemic infections with Fusarium) are more effectively controlled with azoxystrobin and the more superficial are better controlled with trifloxystrobin.
The introduction of Cygnus, Heritage and Compass into the ornamentals market has been interesting. The strobilurins are broad-spectrum in their activity, effective at low rates on many diseases, environmentally friendly and fairly safe for our crops. These chemicals should be considered by most producers as an alternative to the products they rely upon presently. The new class of chemistry affords an excellent opportunity for resistance management. Adding one or more of these new fungicides to most control programs should result in superior control of the target disease.