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Downy mildew
One of our watermelons has these yellow spots
Tags: disease
August 20, 2015
12:38 am
Near Meadview, between Lake Mead and Grand Canyon West (Skywalk)
Forum Posts: 177
Member Since:
May 6, 2015
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I've read conflicting info on how to treat downy mildew.  Some recommend copper fungicides, other say it doesn't work.  Either way, I wouldn't want to use copper anyway as it can be rather toxic if too much is applied and I don't have copper.

Unlike powdery mildew, downy mildew requires moisture and humidity and relatively cool temperatures.   While it's been so hot and dry the for several days, we also had a few days of very light rain every few hours, so that could have caused problems.

Excellent pics of infected leaves as well as the scientific explanation at Managing Downy Mildew in Organic and Conventional Vine Crops:

For organic growers, there are several alternative fungicides labeled for cucurbit downy mildew, including copper-based fungicides. Growers should be cautious in applying copper, as it can be phytotoxic to cucurbits, and high levels in soil are toxic to earthworms and other beneficial organisms. Phytotoxicity is most common during cool, moist conditions, which are also the most favorable for downy mildew. A list of the other alternative fungicides can be found at the National Sustainable Agriculture Information Services “Downy Mildew in Cucurbits” web page at Make sure to check with OMRI and your organic certifier to determine if the suggested products are currently considered acceptable for organic production.

In our dry desert climate we really shouldn't have to worry about it.


Biological Control

Biological control of downy mildew includes the use of a microbial pesticide, such as Bacillus subtilis. Available for home garden use, this control is created from strains of bacteria. The product stops the growth of fungal infections and keeps active fungi from attaching themselves to host plants, according to the Michigan State University Organic Farming Exchange. The bacteria is found in soil and is valued for its nontoxic status toward humans, animals and plants.

Several sites recommended baking soda. Wiki on baking soda:

As a biopesticide

Sodium bicarbonate can be an effective way of controlling fungus growth,[54] and in the United States is registered by the Environmental Protection Agency as a biopesticide.[55]

Testimonials and recipes:

There's a bad link to ATTRA and I decided to look there and I found a rather comprehensive pdf:

Some warnings about neem oil and they also mention peroxide.  Concise and comprehensive, wish they'd updated it since publication in 2003.

Since many posts mention the addition of oil and soap, I might just add a little coconut oil and some yucca extract (as surfactant instead of soap.)

Also, for powdery mildew diluted milk and especially raw milk is often recommended.

I haven't spent a lot of time researching downy mildew and if you have more info, please share!
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