November 2000

Bringing in the New and Bringing Back the Old in the Next Millennium By A.R. Chase

The bridge spanning 1999 and 2000 marks an interesting time for the ornamental plant producer. We have never seen so many new products for disease control as we have during the past few years. The prospects for the coming year are excellent — many new products are entering the marketplace.

These products run the spectrum from totally new chemistry (i.e., the strobilurin group) to new formulations of some of the oldest chemistry (i.e., the coppers). At first glance, an intriguing new chemistry like strobilurin seems to share little in common with an old standby like copper. However, one very important shared characteristic of these two active ingredients is a truly broad spectrum of activity. We have come to assume that most of our industry standards have a limited range of action. For instance, Subdue Maxx is targeted toward Pythium and Phytophthora control. Systhane is mainly for powdery mildew and rust control. This can be said even of the biological products such as RootShield (Pythium and Rhizoctonia control).

Of course, we also recognize the broader spectrum of leaf spot control afforded by mancozebs (Dithane and Protect, to name two), triazoles (Banner Maxx and Terraguard), and chlorothalonil (Daconil Ultrex). Even so, we seem to have forgotten that the copper-based products have the broadest base of activity, ranging from fungicidal to bactericidal and even algaecidal in some arenas.

With this in mind, we have been very impressed with the newest group — the strobilurins. Some of these fungicides have very broad ranges of activity, ranging from control of leaf diseases such as Alternaria, downy mildew and powdery mildew to root and stem diseases caused by Phytophthora, Pythium and Rhizoctonia. Over the next two months, we will examine these seeming extremes in the arsenal of disease-control chemicals. This month we focus on the strobilurins; next month (GPN, January 2000) we will rediscover the coppers.

History of Strobilurins

Where did the strobilurin chemistry originate? Manufacturers do not routinely relate the particular origin of their new products, but in the case of the strobilurins all parties concerned decided it was worth publicizing. The original compound, strobilurin A, was isolated from a fungus, specifically a mushroom called Strobiluris tenacellus, which was found growing on decaying wood in a European forest. However, the products that have been developed are not naturally occurring; they were synthetically derived. Zeneca (then ICI) started working on this new chemistry in 1982, followed by BASF in 1983, and thereafter by Novartis (then Ciba Geigy).

The Products

The first of the strobilurins to reach the ornamental market was Cygnus 50WDG from BASF Corporation with the active ingredient kresoxim-methyl. Cygnus is described as a surface-systemic (translaminar movement) with anti-sporulant activity. This means that the chemical moves into and through the leaf, enabling distribution on the other side.

Practically speaking, this means that active ingredient reaches areas on leaves not directly sprayed — a big help in controlling many leaf diseases.

Cygnus is mainly applied as a protectant against powdery mildew, downy mildew, rust, black spot, Botrytis blight, anthracnose and several other leaf spots. Although the reported research on Cygnus is somewhat limited, the available information supports very good to excellent control of powdery mildew on ornamentals (Table 1).

Heritage 50WG (Zeneca Professional Products) was the first strobilurin to reach the ornamental market. The active ingredient of Heritage is azoxystrobin, which has systemic activity with upward movement. This means that the active ingredient will move out to the edges (tip) when sprayed onto the base of a leaf. If applied as a drench the active ingredient will move into the root system and throughout the plant. However, if sprayed onto leaves it will not move downward into the stem and roots.

Heritage has been extensively researched and many reports have been made regarding efficacy on ornamental diseases. We have worked on the product at Chase Research Gardens for three years (Table 2). Additional research has been reported across the US (Table 3).

Heritage provides excellent control of downy mildew, powdery mildew, rust, aerial Phytophthora, Myrothecium leaf spot, Alternaria leaf spot and Rhizoctonia stem rot. It also provides good control of Botrytis blight, Cylindrocladium diseases, Fusarium wilt (cyclamen), Entomosporium leaf spot, Phomopsis blight, black spot, anthracnose and Choanephora blight on poinsettia. Results with Pythium root rot have been mixed; reports have ranged from complete failure to control, and even good control.

The final product to enter our ornamental market is Compass 50WG (trifloxystrobin from Novartis Crop Protection). Novartis expects labeling for Compass in most states by the end of April 2000. Novartis describes the movement of trifloxystrobin as mesostemic, which indicates both protectant and curative capacity. In this case, the active ingredient “penetrates the plant tissue, [and] has laminar activity, but there is little or no transport within the vascular system of the plant,” according to Novartis.

Compass has also been researched at Chase Research Gardens during the past two to three years (Table 4). Our results have demonstrated good to excellent control of powdery mildew, downy mildew, Myrothecium leaf spot, Fusarium wilt (cyclamen), Botrytis blight and Pythium root rot. This testing was primarily performed in 1999 and should be confirmed by other researchers across the U.S.

How to use the strobilurins

The capability of fungi to develop resistance to fungicides is a serious and continuing problem worldwide. With this in mind, the manufacturers of strobilurin products have taken a proactive stance by writing specific labeling guidelines to minimize the potential for resistance development. You must read each label carefully and completely to determine the legal rates of use, application intervals and maximum applications per crop cycle or season. It is very important that you recognize that these three products are from the same chemical grouping and rotations between them are not true rotations.

You will note that the amount of product per unit volume (dilution) varies greatly. We found that 1 oz. of Heritage gave 100-percent preventive control of certain diseases such as powdery mildew, rust and downy mildew. Similarly, 0.5 oz. of Compass prevented downy mildew on snapdragons. Higher rates (4 oz.) were needed for Heritage to give good control of Fusarium wilt of cyclamen or Pythium root rot on geranium. Be sure to use the labeled rates of each product for the target disease you are attempting to control. The use of lower or higher rates may lead to failure to control, fungal resistance, and (in the case of over-application) excess costs.

We have mentioned that these three products are in the same chemical family, which may lead some growers to assume that the products are more or less interchangeable. In fact, these products are not the same and should not be considered as such. They do have the same mode of action as fungal respiration inhibitors, but they differ in the actual active ingredients, movement within the plant, and degree of control.

Side-by-side tests of Heritage and Compass have shown differences in the amount of product needed to obtain 100-percent control, margin of safety among certain plants, and interval of use. For instance, Myrothecium leaf spot on New Guinea impatiens can be very difficult to control (see Figure 1 and photo on pg. 39). Comparisons with differing rates of Heritage and Compass indicate that Heritage is slightly more effective at lower rates than Compass. In contrast, Compass has been shown to be more effective than Heritage for control of Cylindrocladium cutting rot on azalea (Figure 2).

Testing the products yourself is the best way to determine which to use for each disease on each plant.

Conclusions

We are excited by the introduction of Cygnus, Heritage and Compass into the ornamentals market. The strobilurins are broad-spectrum in their activity, effective at low rates on many diseases, environmentally friendly, and very safe to our crops. They should be considered by most producers as an alternative to the products they rely upon presently. By adding one or more of these new fungicides to (appropriate) control programs, growers should see superior overall control.

Without taking away from the undeniable excitement of these new products, we do want to stress that growers should not forget the benefits of their longtime, reliable standards. Next month we will examine and rediscover the coppers – a real workhorse in disease control on ornamentals.

You no doubt have noticed the references in this article to “we.” I would like to thank the following individuals for kindly reporting results of their excellent work on disease control on ornamentals: Margery Daughtrey (Cornell University), Austin Hagan (Auburn University), John Hartman (University of Kentucky), Bob McGovern (University of Florida), and Gary Moorman (Pennsylvania State University).