Downy mildew, techniques and organic treatments to avoid it

Downy mildew

Famous for being vineyard owners’ #1 enemy, downy mildew is a constant threat to grape harvests. This blight also threatens certain other vegetables from the vegetable patch such as tomato and potato, too.

Regular preventive treatment should help you produce a beautiful harvest without any trace of downy mildew.

How to recognize downy mildew

Symptoms of downy mildew start with the appearance of several spots on the leaves that encroach on neighboring leaves.

  • This is a fungal disease that mainly impacts grape, tomato and potato.
  • Yellow spots appear on and under the leaves.
  • They turn brown and spread to cover the entire leaf which then falls off.
  • On the underside, whitish patches appear that match topside spots. These white spots look a lot like powdery mildew.

Conditions in which downy mildew appears and spreads

Downy mildew particularly likes warm and moist settings, as well as dense foliage that keeps air and light away.

  • Downy mildew often kills plants off in fallMoisture is the main factor that triggers its spread, especially on leaves.
  • Like most fungal attacks, downy mildew will spread fastest after rain showers.
  • Mildew is hindered when plants are spread far apart and air circulates well.

Preventing downy mildew means ensuring the opposite of these factors:

  • Water from below, with a long hose nozzle, long-spout watering can, drip irrigation or ollas.
  • Avoid performing any other tasks during this phase: brushing against sick plants will spread the fungus fast when wet.
  • Water early morning so plants only get one “wet time” in the day (synchronize with dew).
  • Space plants well and thin them so that air circulates well.
  • Layering mulch helps keep fungus trapped underneath: it won’t splash up to leaves as easily.

Dispose of plants infected with downy mildew:

  • Handle sick plants carefully: spores fly around easily.
  • Place trimmings near immune plants instead of vulnerable ones.
  • Or burn or compost them in a hot compost.

Organic downy mildew treatments

It’s possible to use very effective organic downy mildew treatments.

  • Treating downy mildewBordeaux mixture is the only effective preventive treatment against downy mildew.
  • Apply regularly at the beginning of spring and in fall.
  • Treat during the entire vegetative phase.
  • Treat regularly, on average every two weeks and after every rainfall.
  • In the case that downy mildew appears, only a curative treatment will have any effect.

Bordeaux mixture contains copper. Though it’s relatively safe and organic, you shouldn’t apply it too often. Indeed, if used for years, copper builds up and kills bacterial and insect life. Different countries have regulations to follow if you’re a professional grower.

Downy mildew and baking soda

Downy mildew baking sodaBaking soda (sodium bicarbonate) is an excellent solution to treat downy mildew, though a similar product is more effective: potassium bicarbonate.

  • Dissolve 1 table spoon baking soda in 1 quart (1 liter) water (ideally rainwater or mineral water, for the water to be softer).
  • In dry weather, spray this mix on all the leaves, spraying both on top and on the underside of leaves.

Other diseases and treatments

Images: dreamstime: Evgeniy Muhortov, Leisan Rakhimova; Public Domain: Scot Nelson