Resistance to Phosphite Fungicide

There are a number of turf fungicides the activity of which is based on the phosphite (=phosphonate) ion (P03-). Chipco Signature fungicide (active ingredient fosetyl-Al) is the most common, and has become a "keystone" product for disease control programs on many golf courses across the country. This is in part because of the general enhancement in quality on stressed turfgrass that has been reported in some studies, including selected tests at the University of Kentucky. The product is also known to provide control of Pythium cottony blight in many circumstances, particularly when sequential applications are made. Other phosphite fungicides marketed for turf include Magellan, Resyst, Alude, Prodigy Signature and Vital. These fungicides are unusual in that they control diseases in two very different ways. They do have some direct toxicity to the fungus, much like all conventional fungicides which somehow poison the target fungus. However, phosphite fungicides also enhance the plant's natural defense mechanisms, which is not true for conventional fungicides. Because these fungicides attack fungi on "multiple fronts/' the general belief among plant pathologists has been that they are less subject to fungicide resistance than most other systemic fungicides, and in fact field experience has been consistent with this. Cases of failure of disease control by phosphite fungicides due to fungicide resistance have been rare.