STROBILURIN FUNGICIDES: Nature's Cleanup Crew

It's a jungle out there. At least, that's what it must seem like to a fungus. Maybe that's why fungi have evolved some of the most potent biological weapons known — antibiotics. The most famous example is penicillin, though several others are medically notable. It would be easy to underestimate the potency of fungal antibiotics simply based on the medicines we use. Scientists have found a remarkable number of strong fungal antibiotics that are, unfortunately, equally lethal to humans.

But now comes another fungal cure — strobilurin. Don't remember seeing it on the pharmacy shelf lately? That's because this fungal antibiotic fights infections of the plants, not animals. However, like penicillin, strobilurins are produced by fungi to inhibit other organisms (other fungi, in this case) from competing for resources.

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After German scientists first discovered strobilurins in 1977, it didn't take long for people to realize its potential for use as a fungicide. Thus, the development of what would become one of the most important classes of fungicides began.

Improving on nature

The original, natural strobilurin, dubbed strobilurin A, is produced by the pine cone fungus — Strobilurus tenacellus — a mushroom-producing fungus that is otherwise of no particular note. According to Dr. Henry Wetzel III, Biology Project Leader for BASF, this fungus actually colonizes pine cones and produces strobilurins “to keep the pine cone to itself, basically.”

The potential of strobilurins as fungicides was easy to see, but the molecule needed some improvements. The main obstacle was instability in the presence of light. If you're going to spray something on turf, crops or anything else growing out in the open, it must be able to tolerate sunlight without immediately breaking down. “That was the main impetus to find synthetic analogs… molecules that were more stable,” explains Wetzel.

Several synthetic strobilurins are now available commercially, and the bulk are used agriculturally. The first turf registration was obtained by Zeneca in 1997 for azoxystrobin, better known as Heritage fungicide. Then came trifloxystrobin, Novartis' Compass, in 1998. (If this doesn't sound quite up to date, it's because Zeneca merged with Novartis to form Syngenta, which then had to divest one of its strobilurins. Thus, Compass ended up in Bayer's hands, while Syngenta retained Heritage.) Pyraclostrobin, BASF's Insignia, is expected to gain EPA registration soon. For the next few years, at least, this trio will constitute the strobilurin presence in the turf market.

The same, but different

According to Wetzel, the synthetic products “have the same basic mode of action…they block electron transport through the mitochondrial system.” In layman's terms, “The fungus can no longer produce energy and sort of runs out of gas.”

To create their own unique strobilurins, manufacturers have devised different molecules that still exhibit the same basic mode of action. However, despite a common mode of action, strobilurins exhibit definite practical differences. For example, “They have different mobility in the plant,” says Wetzel.

Heritage is upwardly mobile in plants. “It moves up from wherever it's applied,” says Dr. Dave Ross, technical manager of Syngenta's turf and ornamental team. “Some call this an ‘acropetal systemic’,” says Ross, explaining that by strict definition, it does not qualify as a true systemic, which would also move downward in the plant (Aliette is the only truly systemic fungicide, by this definition).

Michael Daly, turf and ornamental brand manager for Bayer Professional Care, tells a somewhat different story about Compass. “It imbeds itself in the waxy cuticle. It's very rainfast and doesn't move to the growing point,” so it's not a systemic. It's not a contact either, however, because “it can move to the other side of the leaf, and can even affect surrounding foliage.” So what do you do when your product doesn't fit the existing terminology? You create a new word. Thus, Compass is “mesostemic,” reflecting its definite, but limited movement within and around leaf tissues.

Insignia displays similar acivity, according to Wetzel, who refers to the pyraclostrobin as a “localized penetrant.”

The movement of the chemical can have practical consequences. For example, Heritage can be effectively taken up by grass plants through the roots because of its upward mobility. That's partly why, according to Ross, “It's especially strong with soil-borne diseases, such as summer patch and take-all patch and necrotic ring spot.”

Mobility is not the only thing that varies with chemical structure, as a comparison of product labels would show. “They are all a little different in their activity on pathogens,” says Wetzel, explaining that Insignia is especially effective against gray leaf spot, pythium blight and brown patch (but will be labeled for several other diseases as well).

Daly explains that Compass was “always designed as a fairway product. It's very effective on brown patch, anthracnose and gray leaf spot. We've widened that [label] to include pink snow mold, and we'll probably follow up later with summer patch.” Daly adds that, “It's a very broad spectrum turf fungicide, but it also has good ornamental uses, like rust, apple scab and powdery mildew. All of this is on one label.”

Ross echoes the broad-spectrum theme. Regarding Heritage, he says, “We've always positioned it as a broad-spectrum material. It controls diseases in all four classes of fungi. It controls brown patch and pythium,” in addition to summer patch, take-all patch and necrotic ring spot, as mentioned above.

Dollar spot control, a continual nemesis of golf superintendents, is a weakness of the current strobilurins. In fact, Heritage developed something of a reputation for exacerbating dollar spot problems, though it hasn't been clear why this is so. Suppression of other fungi that might be antagonistic or competitive with the dollar spot fungus is the accepted explanation. This isn't necessarily the problem it first appears to be, however. In areas where dollar spot is prevalent, treatment for it often is required anyway.

Insignia and Compass do offer a degree of dollar spot suppression, but not enough to warrant use on that basis alone. According to Wetzel, “We may label Insignia for dollar spot, but only for suppression, because we can't guarantee control. You might be able to manage dollar spot with this in lower pressure areas.” And Bayer does not even put dollar spot on the label, even for suppression. “It does have some suppression,” explains Daly, “but I don't think it should be used on dollar spot at all. That's a recipe for resistance.”

Going even further, Dr. Michael Agnew, a technical support manager for Syngenta, manages to put a positive spin on it. “Dollar spot is a built-in insurance against resistance because you have to rotate out [to a non-strobilurin fungicide] to get the dollar spot,” he says.

Resisting resistance

Manufacturers have shown particular concern for managing resistance to strobilurins. So-called “single site” chemicals that work through just one mode of action are understood to be relatively prone to resistance. Strobilurins, like several other fungicide classes, have single-site activity and, given that they've become one of the predominant classes of fungicides, no one has any illusions about the possibility that pathogens may become resistant.

However, proper use strategies can delay resistance, so it was imperative to label strobilurin products in ways that called attention to the need to rotate with other chemical classes.

Recognizing their mutual interests, fungicide manufacturers have worked together in what's known as FRAC — the Fungicide Resistance Action Committee. (The group even has its own Web page at http://www.gcpf.org/frac/frac.html.) FRAC has working groups that deal with several of the fungicide classes, including one for strobilurins. The result is a degree of labeling consistency that should result in more prudent use of the chemicals.

As Daly explains, “Labels generally address this in their language. There are FRAC guidelines that make these recommendations to make sure people don't overuse one, or rotate among [only strobilurins], thinking they're doing some good …”

Worldwide, strobilurins have been registered for agricultural, greenhouse and turf use for several years now (the first registrations were in 1996). According to Ross, there have been “a few reports of resistance in Europe. We believe it was because rotations were not used. And it was in cereal grains, not turf.”

Heritage has the longest track record of turf use in the United States. Has resistance shown up in turf? According to Ross, “In the U.S., there have been a couple of cases we know of with gray leaf spot resistance. These were on golf courses in Kentucky and Illinois.” In at least one case, too many consecutive applications may have been made. Adds Ross, “We have worked closely with these people to manage resistance. We're also working on mode-of-resistance to learn more about these cases to prevent it from happening in other locations.”

Some have alleged that anthracnose also has developed resistance in certain locations. However, Ross disputes this. “We have not been able to confirm that there has been resistance. We have tested samples [of allegedly resistant anthracnose strains] and we could not find resistance in the isolates.”

Instead, Ross attributes the lack of control in these instances to other factors, including letting the disease become too advanced. “If anthracnose gets into the basal rotting stage, it's pretty difficult to control no matter what you use. Anthracnose is an opportunistic disease — it attacks stressed turf. When superintendents get out and apply early enough in the season to prevent it, however, we haven't seen those problems.”

The confirmed cases of resistance only serve to highlight the importance of prudent application strategies, including proper rotation. “There are only so many applications you can get out of a product,” states Agnew. “There are fewer and fewer classes in the marketplace, so you have to take care of what's out there. We're very proactive in preventing resistance.”

Future developments

Given that we'll soon have three strobilurins in the turf market, is there a need for more? Probably not, unless the new chemical offers some significant characteristic not available from existing products.

BASF thinks it has just such a product. Though much farther off in the registration “pipeline,” dimoxystrobin will likely be the next strobilurin in the turf market. According to Wetzel, dimoxystrobin exhibits a broad spectrum of control, with good activity on diseases such as brown patch and take-all patch. However, “The outstanding thing is that its No. 1 disease is dollar spot.”

The arrival of the strobilurins may be fortuitous. Says Daly, “This is an exciting new class that is coming along just in time. EPA is restricting the use of some of the conventional fungicides and if the product isn't being banned outright, then the amount you can apply is being reduced. Superintendents are looking for more answers.”

And they're finding them in strobilurins.

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© 2014 Penton Media Inc.

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