Fungicide trials show what works

One of the most rewarding research areas for Chase Horticultural Research has been evaluation of new products for disease management on ornamentals. We have been working on as many new diseases and plants as possible including many more woody and perennial crops in 2007. Last year, we made a special effort to work on anthracnose diseases including Euonymus and Mandevilla as well as several trials on cyclamen.

Here are the most current results on disease control of anthracnose on woody ornamentals, Alternaria leaf spot on Pittosporum, powdery mildew on rose and rust on Hypericum.

Differences in production

The fungicides used in nursery production are not always the same as those used for greenhouse production. This may be due to labeling differences, cost of use, longevity under different irrigation regimens and, finally, crop safety. Thus work on greenhouse crops may have minimal use in a nursery setting.

Throughout 2007 we evaluated industry standards and new products. We also tested sets of products with similar chemistry including sterol inhibitors and strobilurins. The sterol inhibitors we tested were Banner MAXX (propiconazole from Syngenta Professional Products), Eagle (myclobutanil from Dow Agro) and Trinity (triticonazole from BASF Corporation -- not labeled on container ornamentals at this time).

The strobilurins included Heritage (azoxystrobin from Syngenta), Compass (trifloxystrobin from Bayer), Cygnus (kresoxim methyl from BASF) and Insignia (pyraclostrobin also from BASF). We also tested Medallion (fludioxinil from Syngenta), Clevis (a combination of myclobutanil and mancozeb from Prokoz) and Pageant (a combination of pyraclostrobin and boscalid under development from BASF). The table summarizes results from these trials. (Click the link below for a PDF of the table.) The rows are colored to indicate chemical class to make resistance management through rotation easier to accomplish.

Click here for table.

Powdery mildew and rust

Overall, the strobilurins (pink rows) including Compass, Cygnus, Heritage and Insignia are very good to excellent on rust and powdery mildew on woody ornamentals (historical results of our trials included). However, as the rose tests showed last year, controlling rose powdery mildew with strobilurins can be unsatisfactory.

Of all the strobilurins we have tested, Compass has worked best for powdery mildew control on a variety of crops including rose. The new strobilurins-boscalid combination Pageant has performed very well in our trials on rose powdery mildew. Rust control has been very good with strobilurins and in most trials Heritage has performed the best. Be sure to add a surfactant when using strobilurins to eradicate rust since penetration of the pustule must occur for the fungicide to work.

Sterol inhibitors (green rows) are also usually very good for rust and powdery mildew on woody ornamentals. Over the past three years, our trials on Hypericum rust showed variable results for control with sterol inhibitors. However, the most consistent control of various rust diseases has occurred with Eagle. Choosing one sterol inhibitor to rotate with one strobilurin is a very effective way to control rust and powdery mildew diseases on woody ornamentals.

Leaf spot and anthracnose

There are many different fungal leaf spots on woody ornamentals including Alternaria, Entomosporium and anthracnose caused by Colletotrichum and Phyllosticta (=Phoma). The anthracnose pathogens cause leaf spot and dieback and must be controlled early in the production cycle. Both of the trials we performed in 2007 were done with active infections and we attempted eradication instead of prevention.

Both sterol inhibitors and strobilurins have proven very effective on some leaf spot diseases like Alternaria on Pittosporum but less so on anthracnose diseases. The Phyllosticta anthracnose was improved after repeated applications of Banner MAXX, Eagle and Clevis (all have sterol inhibitor as an active ingredient). The strobilurins were slightly less effective in this case. The only products that were effective in the Colletotrichum leaf spot on Mandevilla trial were two unlabeled products Pageant (strobilurin and boscalid) and Trinity (sterol inhibitor). Much more work is needed on anthracnose diseases on woody ornamentals to determine the best rotation for control.

More trials are needed

It is important to remember that our results are based on a limited set of trials and that more trials are needed especially on anthracnose diseases. Secondly, please rotate between chemical classes. This is one of the best ways we have available to reduce the possibility of fungal resistance to important fungicides. Finally, be sure to get a diagnosis of new diseases. If you do not know what you are trying to control, you cannot choose the best control strategy. All products, even in the same chemical class, are not created equal.

- A.R. Chase

{sidebar id=3}

A.R. Chase is president, Chase Horticultural Research Inc., (530) 620-1624;archase@chaseresearch.net; www.chasehorticulturalresearch.com.

June 2008