October 2007

Ask us About Diseases By A.R. Chase

Q What’s new in ornamental fungicides?

A This is a question I am actually asked quite frequently and one I hate to hear because it makes me think. During the past year, we have seen two new products reach the U.S. market — FenStop and Insignia. FenStop, marketed by OHP (formerly Olympic), is excellent for downy mildew, Phytophthora and Pythium. At this time, it is labeled for greenhouse use only but we are hoping to see the label expanded to container nursery crops too. We have seen excellent control of downy mildew on some field-grown cut flowers in our trials. The chemical group is closely related to strobilurins, so you must not rotate with one of the strobilurins or resistance may develop. It would be terrible to lose a new fungicide quickly because of failure to follow label directions.

The other new fungicide is Insignia, a true strobilurin from BASF Corp. Insignia has a very broad spectrum of activity including many foliar diseases from Botrytis to downy mildew, powdery mildew, rust and fungal leaf spots (including Alternaria, Cercospora, Mycocentrospora) and Sphaceloma leaf spot. It is also effective against Fusarium diseases (wilt, stem rot and crown rot), other crown rot and root rots. Insignia excels for anthracnose and crown rots caused by Fusarium, Cylindrocladium and Myrothecium. These are some of the hardest diseases to control, so I welcome the addition of this excellent fungicide.

Additionally, I want to mention some products that will be available soon, including a combination product called Palladium (Syngenta), a hydrogen peroxide product from Phyton Corp. and a triazole called Trinity from BASF Corp. Many others are under development for downy mildew, Pythium and Phytophthora, and also for rust, powdery mildew and Rhizoctonia.

Finally, there is also quite a bit of activity in the arena of “green” products based on essential oils, phosphonates and true biologicals. We should have many new choices for years to come.

Q Where does Pythium come from?

A Listed below are ways I have seen Pythium introduced into a crop. Some of this information comes from our diagnostic lab where we are seeing a rise in requests to test potting media and water for Pythium and other fungal pathogens.

The most common means of introducing Pythium into a crop is through used pots, flats and on contaminated benches. We worked with one grower to determine the continued presence of a fungicide resistant strain of Pythium in his bedding plants. The problem was that their flats were recycled and the quaternary ammonium treatment had shifted from a very effective dose and exposure time, to one that was not 100-percent effective. Steaming or soaking with quaternary ammoniums are each very effective tools in disinfesting pots and trays.

The next most common means of introduction is through contaminated plant material such as cuttings and plugs. I was really surprised last year when we found Pythium on the cut ends of un-rooted zonal geraniums that appeared to be healthy. Unfortunately, one cannot check all newly arriving plants for all possible pathogens so some judicious use of fungicides on extremely susceptible crops may be needed.

If you are growing plants with recycled water (ebb and flood or otherwise), Pythium is one of the most likely diseases to be carried from one crop to the next. We have found Pythium and Fusarium in some water samples from ponds and storage tanks. It is usually not effective to treat the water with a fungicide although treatment with chlorine, slow sand filtration and a few other methods can be very effective.

The most unfortunate event this past year in our diagnostic lab was finding pathogenic Pythium in un-used potting medium. True, we did find many more cases of Pythium in recycled potting media but we are finding it too often in the components of potting media and even in bagged media. This has led to a research project testing commercial potting media for plant pathogens (mainly Pythium). We will follow up with tests to make sure that the Pythium we find are actually capable of causing disease on ornamentals. Not all Pythium are plant pathogens so this step is critical in evaluating the potential for losses in ornamental production that might occur if the potting media are found with Pythium populations.