ARS Feature Article:
What Are Strobilurins And What Do They Do?


by Dr. Gary A. Ritchie
8026 61st Ave. NE • Olympia, WA 98516

The other day, a rosarian friend asked me what strobilurins are and how they work. All I could tell her was that they are a new class of very effective fungicides that are finding wide use in rose culture, but beyond that I knew remarkably little about them. So her question prompted me to probe more deeply into this subject to see what I could learn. Here's what I found out.

The strobilurins were discovered in 1977 by German scientists. They are fungicides produced by fungi to protect themselves from attack by other fungi. I'm not making this up. They are named for the fungus Strobilurus tenacellus, which inhabits the cones of white pine trees. Strobilurus produces strobilurin to keep other fungi away from the cones.

It is now known that this chemical has a unique mode of action not present in any other fungicide. It disrupts the electron flow pathway in the mitochondria of the antagonistic fungus. What does this mean? Well, mitochondria are small membrane-enclosed organelles that inhabit the cells of all plants, animals and fungi. Although mitochondria have many functions, their main job is to provide energy to the cell. They do this using an enzyme called ATP synthase, that produces ATP – an important energy-storing chemical. Some people have called ATP the "energy currency" of the cell. So, by blocking the formation of mitochondrial ATP in the invading fungus, strobilurin basically causes the invader to starve to death. Yes, it's a real jungle out there.

When strobilurin was discovered, scientists immediately recognized its potential as an agricultural fungicide, but much research had to be accomplished to develop it for commercial use. The main problem was that the natural strobilurin breaks down rapidly when exposed to sunlight. Not a good property for a commercial fungicide that would be applied to field-grown plants. To solve this problem, plant biochemists were able to alter the molecular structure of strobilurin in such as way as to increase its light stability, while maintaining its fungicidal properties. No small task.

Several companies participated in development of different types of strobilurins. Zeneca developed Heritage® and registered it in 1997. In 1998, Novartis introduced Compass®.

But then mergermania took hold and Zeneca merged with Novartis forming Syngenta, which retained Heritage®, but spun off Compass® to Bayer. Got it? Several other strobilurins have been developed and widely used throughout agriculture, but Heritage® and Compass® are perhaps the most often mentioned in the rose literature.

Now let's take a closer look at these new fungicides. Although the mode of action of all strobilurins is essentially the same, they move around in the plant in different ways. Heritage® is "upwardly mobile", meaning that it moves up the plant from the point where it is applied. In that sense it is systemic, but it does not move downward. Compass®, in contrast, becomes locked up in the waxy cuticle that covers the surface of the leaf. A major advantage is that it is not washed away by rain. While it doesn't move around in the plant very much, it can move through to the other side of the leaf. So when applied to, say, the top of the leaf, it can move across to the underside of the leaf. So, while not systemic, it is "trans-laminar."

One major advantage of the strobilurins is that they are broad spectrum fungicides – they work on all three major classes of fungal diseases. They are widely used on turf grass, corn, cucurbits, potato, tomato, beets, carrots, radishes and many other crops, including ornamentals such as roses.

Since the strobilurins are "single site" chemicals, there was early concern that fungi may easily develop resistance to them. But since the mode of action in mitochondrial respiration was believed to be resistance resistant, it was hoped that resistance would be a long time in coming. But, alas, this has not been born out, and we are seeing more resistance to strobilurins developing earlier than had been anticipated. Therefore, they should be alternated with other fungicides, such as Banner Maxx®. It's important that they not be alternated with other strobilurins, however, because all have the same mode of action.

Another disadvantage of these very effective products is their high initial cost. However, when you consider the low usage rate of 1/4 to 1/2 teaspoon per gallon applied every 14 to 21 days (e.g., Compass®) their long-term cost becomes more affordable.

This article was provided to the TVRS as a courtesy by the American Rose Society.