Resistance Management Guidelines for Strobilurin Use on Vegetables

This article by Alan MacNab appeared in "The Vegetable and Small Fruit Gazette" Vol. 4, No. 4 (April 2000), published by Pennsylvania State University, Department of Horticulture.

Resistance management guidelines were recently developed by FRAC (Fungicide Resistance Action Committee), and were agreed upon by three companies that manufacture and sell these products. The strobilurin products now labeled on some vegetables include Quadris (from Zeneca) and Flint (from Novartis); an additional strobilurin for use on vegetables is being developed by BASF. Guidelines also are available for these additional strobilurin products: Heritage, labeled for ornamentals and turf, and Abound, labeled for various fruit and nut crops.

These guidelines are important because some fungi can develop resistance to the strobilurins; in fact, this has already occurred, but only outside North America.

The reasons for this update are (1) to remind farmers that there is a potential for resistance to develop where these fungicides are used, and (2) to encourage growers to follow the guidelines outlined below. By alternating strobilurin materials with other effective materials, and by limiting the number of strobilurin applications per season, the chance for development of resistance to the materials will be reduced.

Information in the guidelines will be included on all new labels. Even though there will be a transition time before all labels have the new wording, all growers will benefit from following the new guidelines now. Guidelines pertaining to vegetables and potatoes are summarized below. Similar guidelines are available for other crops listed on strobilurin labels.

Cucurbits
(1) No more than one application before alternation to an unrelated fungicide that is effective in controlling the causal fungus.
(2) No more than four total applications per season.

Potatoes
(1) No more than one application before alternation to an unrelated fungicide that is effective in controlling the causal fungus.
(2) No more than six total applications per season.

Tomatoes
(1) No more than one application before alternation to an unrelated fungicide that is effective in controlling the late blight fungus.
(2) For all other diseases, no more than three sequential applications before alternation to an unrelated fungicide that is effective in controlling the causal fungus.
(3) No more than five total applications per seasons.

Note: These sequences and totals reflect total strobilurin applications, i.e., at this time, both Quadris and Flint for vegetables. Therefore, if Flint is used for two applications on cucurbits, only two additional sprays can be made, regardless of whether they be Quadris or Flint. Likewise, if Quadris is used for two applications on cucurbits, on two additional sprays can be made, regardless of whether they are Flint or Quadris.