Fungicide Applications for Watermelons

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.eduand Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu

As the season winds down, it is a good time to reflect on how well your fungicide program worked in 2014. Both Nathan and I have received numerous questions about best practices for fungicide application this year, so it is also a good time to review these best practices.

Fungicide applications should begin on watermelons when the vines are about 18 inches long or about when the first flowers open (these will be male flowers). This early spray is very important to prevent disease in becoming established. Following the first spray, fungicides should be applied weekly.

The first fungicide application should include a copper product to manage bacterial fruit blotch (BFB). Additional applications for BFB should be applied in two weeks as the female flowers open and again two weeks later. Protect additional fruit set from BFB with additional copper applications.

Apply the best fungicides for the diseases that are present in your field. Correct identification of the problem is critical. There are many “look-alike” diseases and other diseases that mimic abiotic problems. In addition to identifying what diseases are present in the field, assess what the most prevalent problems are in selecting what fungicide to use.

In selecting fungicides, be sure to stay informed about fungicide resistance (insensitivity) that has developed to watermelon disease pathogens. For example, the gummy stem blight pathogen is resistant to strobilurins, Topsin M and Pristine. Recently resistance to tebuconazole was reported in Georgia. Choose other fungicides such as Fontelis, Luna Experience, Switch, or Inspire Super. Likewise the powdery mildew pathogen is resistant to strobilurins, and Topsin M. Group 4 fungicides such as Procure are still useful for a limited number of your powdery mildew applications, but should be used with excellent resistance management practices.

Timing of the fungicide applications is very important. All fungicides are more effective if they are applied before disease development. This is true even for protectants, which have some “kick-back” activity. Therefore apply fungicides BEFORE rain. Don’t worry about the fungicides being removed during a typical rain, because modern fungicides are formulated to persist on leaves and fruit following rain. In the case of aerial applications, light rainfall may improve coverage by improving distribution of the fungicide within the plant canopy.

Several very effective fungicides have long pre-harvest intervals (7 days). These fungicides should not be used during the harvest period. That brings me to one additional important point. Always read and follow the label. Be aware of pre-harvest intervals, re-entry intervals, correct personal protective equipment. In addition, formulations of fungicides may be improved and the rates and allowed application methods may change from year to year.