Model Description

To control anthracnose, fungicides are often applied to watermelon according to a calendar-based schedule (e.g., every 7 to 14 days). However, outbreaks of anthracnose are regulated by weather factors, primarily moisture on leaves (e.g., rain, relative humidity) and air temperature. Anthracnose increases rapidly under wet, warm conditions, but is inhibited when leaves remain dry for extended periods or when temperatures remain cool. Using detailed weather information, growers may be able to better time their fungicide applications and possibly eliminate unnecessary sprays.

The Oklahoma Watermelon Anthracnose Model is a tool that has been developed to aid growers in proper timing of fungicide application for anthracnose. Using the Oklahoma Mesonet , the state's automated weather station network, the model calculates daily "infection hours" for each Mesonet site. An infection hour is defined as one hour with relative humidity greater than or equal to 80% and air temperature between 68F and 86F. At the beginning of the season before any fungicide has been applied, the model recommends a first spray when 80 infection hours have accumulated since the date of first flowering of male watermelon flowers. After a fungicide has been applied, the model recommends an additional spray when 80 additional more infection hours have accumulated since the date of the last fungicide application.

Growers are encouraged to use the Site-Specific Interactive Model, which provides a spray or no spray recommendation. After the customer clicks on a nearby Mesonet site, the model asks for either the date of the last fungicide application (if it has occurred) or the date of the first flowering of male watermelon flowers (if it has occurred). This information is then entered and the model comes back with the recommendation, including the number of infection hours that have occurred since first flowering date (if no fungicide has yet been applied) or since the last fungicide application. If a number of watermelon fields are involved, each with various first flowering dates or fungicide application dates, the model should be run separately for each field. The grower can try various nearby Mesonet sites as well, as a conservative approach.

Rules for the watermelon anthracnose advisory are as follows:

1. Delay application of a first fungicide until symptoms of anthracnose appear or until the advisory recommends a first spray, whichever event occurs first. If a grower applies a first fungicide before the advisory recommends one, that is acceptable ; the advisory will still work from that point on. Given no symptoms of anthracnose, the model does not recommend a first spray until 80 infection hours after male flowers first appear.

2. If no anthracnose symptoms are present, once the watermelon plants have reached first flowering, the Oklahoma Watermelon Anthracnose Model should be consulted on a regular, if not daily, basis. In addition, after any fungicide application, the model should be consulted regularly beginning 5 days after the spray date.

3. If a given field cannot be sprayed within 3 days of a model spray recommendation, then spray on a 14-day schedule.

4. Use an effective systemic fungicide (e.g., Benlate or Topsion) in combination with an effective protectant fungicide (such as Dithane, Pencozeb, or Bravo).

5. If more than 10% of leaves become diseased or defoliated, disregard the advisory and apply fungicide every 7 days.

6. If downy mildew appears in the field, disregard the advisory and apply an effective fungicide (e.g., Bravo) every 7 days.

7. If more than 60 infection hours have occurred (but less than the 80 hours that the advisory uses to recommend a spray), apply a fungicide if rain is likely and if no fungicide has been applied in the last 7 days. Note that the advisory will tell you the number of infection hours.

8. Apply fungicide up to 7 days before the first harvest.

9. Follow the instructions on all pesticide labels.

In addition to the Site-Specific Interactive Model section, the Oklahoma Watermelon Anthracnose Model Web page features other sections. The Current Model Output section displays the latest model output for every Mesonet site. This includes the number of infection hours that have occurred during the last 24-hour period (ending at 5 a.m. CDT of the date listed), the accumulation of infection hours since May 1, and the max/min temperature and max/min relative humidity during the 24-hour period. For a time history of a specific Mesonet site, go to the Seasonal Output section to see daily model output since May 1 for that specific site. The Images section contains images of anthracnose lesions on watermelon leaves, stems, and fruit. Finally, Related Links contains links to the "Watermelon World" Web page at Lane, OK.

Finally, the Oklahoma Watermelon Anthracnose Model has been designed to be used as a tool in scheduling fungicide applications. Other factors, however, should also be considered in making such decisions, such as the market value of the crop, method of irrigation, growth stage of the crop, density of the foilage, history of anthracnose in the field, and time of year. For example, the likelihood of an outbreak may be underestimated by the advisory when overhead irrigation is applied or when foilage is very dense. Thus, the Oklahoma Watermelon Anthracnose Model is an addition to, but not a replacement for, a well-conceived foliar disease management program.