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  Mike's Gardening Tips

"The Cornell Formula"--a natural plant fungicide
(as seen in his weekly column in the Philadelphia Daily News)

Developed at Cornell University, this baking-soda based remedy is a great spray-on plant disease preventative and fighter. It's as good as any chemical fungicide you can buy--the only thing that's stronger is Fermented Compost Tea (details on this SUPER disease fighter can be found elsewhere on this site).

To use The Cornell Formula, get a sprayer (anything from a hand-held one-gallon job to those really cool big-boy backpack styles), and mix this up right inside the container part of the sprayer...

The formula:
In one gallon of water, mix

  • 1 tablespoon baking soda
  • 1 tablespoon oil (see 'note', below)
  • 1 or 2 drops dishwashing liquid
Shake well and then spray on the plant you wish to protect and/or rescue.

Note: the "oil" to be used is ideally horticultural oil (also known to many folks generically as 'dormant oil', although that term, in truth, refers to a specific type of hort oil.) For this recipe you can use any grade of the stuff all the way from true dormant oil (very heavy petroleum product) to the new lighter-weight "summer oils" (vegetable oil based). Horticultural oil is available at all nurseries and garden centers and is an essential tool for tree owners (especially of fruit trees) who use it to smother over-wintering tree pests. If you intend to use this spray a lot (like to protect disease-prone roses), buy some real hort oil--I'd recommend one of the lighter summer oils.

In a pinch you can use regular vegetable oil from the kitchen, but hort oil will work better. And no, 'duct-tape gardeners', you can't be a cowboy and use motor oil or WD-40 or any other such foolish thing. I will find out if you do and make you sorry.

For a plant that is already showing signs of disease, first remove all of the diseased parts that you can reach. Then remove all of the mulch underneath the plant (burn, bury or trash--don't compost this old, diseased mulch!), then spray The Formula liberally on the cleaned-up plant (and apply fresh mulch). Be sure your spray gets to the underside of any leaves you can reach--that's where disease takes hold. To protect a disease-prone plant (like us, its much easier to prevent disease than to cure it!), remove all old mulch as above and then spray the plant well---again, paying particular attention to the undersides of the leaves,

Don't spray in direct sun in the heat of the day. Early morning is good; late evening is fine too.

The gardening information contained in this site is copyright 1999 by Mike McGrath, and is for the use of WHYY-FM/Public Radio listeners only. Such listeners can print it out for their own use and such, but under no circumstances may be it sold in any manner, used in a book or periodical, on another website or for any commercial purposes of any kind without the express written consent of Mike McGrath. To obtain such permission, e-mail Mike McGrath at garden@whyy.org. ALL RIGHTS RESERVED.

©2005 WHYY