Preventing and Treating Powdery Mildew

Powdery mildew on stem

Powdery mildew can prematurely end a great tomato season, especially in Hawaii where conditions are ideal for the fungus. Powdery mildew thrives in warmth (between 60-90 F) and as the season here changes from dry to rainy the spores germinate en mass, travel through the air, and begin to take hold on plant surfaces. Powdery mildew is recognizable by whitish or light-gray fuzzy spots on leaves, stems, and even fruits. It prefers the youngest leaves and it will often start on them but it can take hold at any stage of growth. Leaves appear distorted, stunted and fall off earlier than healthy ones. Below are some preventative actions to help deter powdery mildew from taking hold as well as recipes to control it.

1. Keep tomato plants off the ground. Being in contact with the ground places the plants in an environment where the mildew thrives by decreasing air circulation and light penetration. In addition, bugs have easier access to the tomatoes. A trellis works great.
2. Plant tomatoes in a spot where they will receive direct sun all day long, if possible. The sunniest side of the house or garden is best.
3. Make sure there is plenty of air flow around and between plants. Planting tomatoes too densely or in too close proximity to other plants can encourage the powdery mildew. Also regularly prune the plants to keep them open so air can flow between branches and leaves.
4. Water plants at the roots (try to avoid wetting the leaves) during the day so that any wet foliage dries quickly and don’t water at night.
5. Check plants frequently for signs of the mildew.

Powdery mildew on leaves

If the fungus is discovered on your plants there are many recipes that can be applied to bring the situation under control but a speedy response is necessary since the mildew can quickly overwhelm and kill tomatoes. First check the plant thoroughly and pick off all leaves and stems showing any signs of powdery mildew. Destroy all diseased plants and definitely don’t put them in a compost pile which will return the spores to your garden. If they are really infected the whole plant should be removed. Some plants never recover once infected and a lot of time can be spent on trying to rehabilitate them. I really enjoy making my own  garden remedies.  Below are some recipes to try both as preventatives and eradication.

Baking Soda

• 1 tablespoon baking soda
• 1 teaspoon insecticidal soap or liquid dish soap
• 1 gallon water

This mixture can be poured into a spray bottle or a larger agricultural sprayer, if you have many plants. The amount of baking soda can be increased but test on a small area of the plant first and observe for 2 days as it can “burn” the leaves. During application be sure to shake up the bottle every few minutes since the baking soda tends to settle on the bottom. Baking soda kills the powdery mildew within minutes but may need to be re-applied every 5 days or so.

Soap acts as a surfactant spreading the baking soda evenly over the plant and causing it to stick. In this next recipe oil replaces the soap and serves a similar function.

• 1 tablespoon baking soda
• 1 tablespoon horticultural oil
• 1 gallon water

Milk

Amazingly, milk is a great remedy for powdery mildew as evidenced by several studies. It also boosts the immunity of the plant. Apply once a week.

• 1 part skim milk
• 9 parts water

Sulfur

An alternative is powdered agricultural sulfur which acts mainly as a preventative. Do not use sulfur within 2 weeks of using oil since they can react and cause damage to the plant. The sulfur can also be mixed with a small amount of  insecticidal soap (not dish washing liquid). A commercially available source of sulfur made specifically for the treatment of powdery mildew is ‘Safer Garden Fungicide’ which is listed as safe for organic farming. Or choose to make your own.

• 2 tablespoons sulfur
• 1 gallon water
• 1/2 teaspoon insecticidal soap (Safer Soap)

Biological Control

‘Serenade’ is a biological control available on the market that helps prevent mildew with its active ingredient bacillus subtilis, a bacteria. It is certified for organic farming.  Apply according to directions.

Natural Oils

Jojoba oil or neem oil can be used in mild to moderate cases of powdery mildew.

• 1 ounce neem oil
• 3 drops liquid soap
• 4 cups  (1 quart) water

Cornmeal

Cornmeal is another great preventative and fungus suppressant that works by attracting a member of the Trichoderma fungus family that kills off disease causing fungi within weeks.

• For every 100 square feet, work 2 pounds of cornmeal into the soil.

Horticultural cornmeal is used mostly for feeding livestock and it is cheaper than food grade cornmeal. Cornmeal “juice” can also be made.

• Soak 1 cup cornmeal in 1 gallon of water overnight
• Spray on plant surfaces

Spraying Tips

When using a spray, be sure to spray all surfaces of the plant including the undersides of leaves. These recipes should give the plants good protection for about 1 week but if it rains, most solutions will be washed off and the application will need to be repeated. Spot apply to new plant growth as well.

 

Tomato Christmas, In October

Tomatoes have extremely hardy seeds and they are always making it through the compost pile and into my homemade potting mixes.  When I plant a flat of bush beans, I may get a dozen or more tomato plants along with them.  This irritates some people but I don’t mind.  I call them “volunteers” when they just happen to show up to help out in the garden.  The majority I pull out but every now and then I save a few plants and find corners of the garden for them.  Then I wait.

As they grow I muse about what kind of tomato each plant will produce.  The feeling is reminiscent of when I was a kid waiting for Christmas to arrive.  What is inside each box?  I hope it’s just what I wanted.   The flowers appear, we are getting closer.  The anticipation increases.

Tomato plants can have different leaf shapes, sizes, and growth habits depending on the variety.  I try to guess based on some subtle differences in each plant.  It’s like squeezing the present to feel what is inside.  I make some more guesses.

Ah, finally the fruits are forming.  How big will they get?  The big ones are fun (or frustrating) to watch growing but I have really come to appreciate small tomatoes in Hawaii.  They hide from bugs and birds much more reliably and the flavor is frequently more intense than most big ones.   What shape and what color will they be?  I have gotten everything from grape and cherry sizes all the way up to big ‘beefsteak’ ones and colors from purple to many shades of red, orange, and yellow.

Finally it is time to begin harvesting and as I gather the tomatoes  it feels a little like Christmas.

I like to group each variety and compare  them. What did I get this time?

The small and medium yellow ones are more mild in flavor and less acidic.  The small dark red ones are bursting with flavor and everyone wants to eat those first.  The pale whitish-green-yellow variety is very unusual.

Being an artist I have an impulse to arrange them on a plate.

It’s just what I wanted and the most successful tomato season I have had.

Now I am waiting on these guys, something-big and something-roma.