Using Onion Disease Forecasts Effectively

Weather-based disease forecasts are available for three fungal diseases of onions: Botrytis leaf blight (caused by Botrytis squamosa) purple blotch (caused by Alternaria porri) and downy mildew (caused by Peronospora destructor).  There is a separate forecasting program for each disease.  Each of the three diseases needs to be taken into account when using the forecasts to determine fungicide spray programs.

Botrytis Leaf Blight Forecasts

There are three different disease forecasts available for Botrytis leaf blight; two versions of Blight Alert, developed in New York by Dr. Paul Vincelli and Dr. Jim Lorbeer at Cornell University, and the Michigan Botrytis Forecast developed by Dr. Mel Lacey at Michigan State University.  Blight Alert was developed in New York and is the most appropriate forecast to use for New York conditions.  The Michigan forecast can provide additional information to use when making a spray decision.  The decision flow chart shown below is based on the use of Blight Alert for forecasting Botrytis leaf blight

Blight Alert

A combination of scouting thresholds and weather-based forecasts is used to determine fungicide programs for Botrytis leaf blight using Blight Alert.  The first fungicide is applied when scouting determines that there is an average of one Botrytis lesion per leaf.  See the Cornell Vegetable Production guidelines for information on scouting. http://www.nysaes.cornell.edu/recommends/21onions.html#disease 


Fungicide applications are assumed to provide protection for seven days unless the residue is subjected to more that one inch of rain.  If more than one inch of rain has fallen since the last application, fungicide protection has been lost and the weather and disease forecasts should be consulted to determine the need for another application. 


Otherwise, seven days after the first fungicide application another application may be needed if the chance of rain is > 30%, and the inoculum production index from the Blight Alert forecast is 7 or greater.  Note that forecasts are given for four different planting dates on the NEWA website.  Use the one that is closest to the planting date for your field.  The weather and disease forecasts are consulted daily until conditions call for a fungicide application.  Then the decision procedure is repeated after 7 days or if more than one inch of rain has fallen.


Modified Blight Alert:

Modified Blight Alert is identical to Blight Alert except that Modified Blight Alert allows for the possiblity of infection from weather periods that are favorable for spore production early in the season. In many onion-growing areas of New York these early weather events do not result in disease because the Botrytis squamosa organism has not developed to the point of spore production. In some other onion growing areas spore production can sometimes occur at an earlier date. Modified Blight Alert takes this second situation into account.


Forecasts by Modified Blight Alert can potentially call for one or two additional early season fungicide applications. Modified Blight Alert is best used in those locations where very early BLB infections have been observed in past years. For other locations where BLB is first observed in June or July, it is generally not necessary to use Modified Blight Alert.


Michigan Botrytis forecast

The Michigan Botrytis forecast predicts the release of Botrytis spores and doesn’t include scouting or need a daily forecast for the chance of rainfall.  The forecast indicates whether there was a significant inoculum potential (SIP) or an insignificant inoculum potential (IIP) for a given day.  A significant inoculum potential indicates that spores have been released and that there is a risk of infection. This model is a good early season warning system to determine if Blight conditions exists prior to scouting. Also a good indicator to schedule the first scouting of the season.


Downy Mildew Forecast

The downy mildew forecast (DOWNCAST) was developed in Ontario, Canada by Dr. P.D. Hildebrand and Dr. J. C. Sutton and the University of Guelph.  DOWNCAST predicts sporulation of Peronopora destructor,the fungus that causes downy mildew, if it is present.  The forecast indicates whether the weather was favorable or unfavorable for sporulation.  Occasional favorable days in areas where downy mildew has not been reported are not cause for concern or special fungicide applications.  If long periods of favorable weather have occurred maintain excellent coverage with protectant fungicides and scout fields carefully for downy mildew, which can be very destructive.  Finding downy mildew in your field or the immediate vicinity may be cause for specific fungicides targeting downy mildew.


Alternaria Purple Blotch Forecast

The Alternaria forecast was developed in Michigan by Dr. Mel Lacey.  Dr. Brian Cortwright has been working to fine tune the forecast.  The forecast uses temperature and relative humidity information to calculate a predictive index (PRI) that predicts the release of Alternaria porri  (purple blotch) spores.  A PRI of 5 or greater indicates a high risk of purple blotch infection.  A PRI less than 5 indicates a low risk.


*If disease risk is high for any of the three forecasted diseases

Apply appropriate fungicide for high risk disease(s).


Because weather conditions can vary depending on topography and altitude, the forecast information will be most accurate very close to the weather monitor.  For locations that are not close to a weather monitor, forecast information should only be used as a *general indication* of how favorable weather has been for disease.  Knowing the location of the monitor, and thinking about how temperature and humidity conditions at the monitor differ from particular fields on your farm can help you interpret forecast information.  For example, if the weather monitor closest to you is in a valley that tends to dry off slowly compared to your farm, the forecast information would tend to be relatively conservative for your location.  If a weather monitor location is in an open area that would tend to dry off quickly relative to your fields, then the forecast information from that location would indicate that conditions had been less favorable for disease than they were at your farm.  Comparing the amount of rain that fell at the weather monitor site with the amount that fell on your farm can also help you determine how similar weather conditions have been at the two locations.

*Overhead Irrigation* can create disease favorable conditions in a field that a weather monitor will not be taking into account.  Irrigation that starts when the leaves are still wet from dew in the morning, or continues after dew has fallen at night will extend the wetting period for that day and must be taken into consideration when disease forecasts are interpreted.