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I've noticed these moldy, fuzzy spots on many of my tomato plants:

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I figured it is some sort of a mold problem, but I couldn't find which one exactly and what to do with it. The plants are on the balcony. Due to an early mistake of planting them in small pots, I now water them every morning. I also try to avoid putting water onto leaves. The fruiting seems to be going fine though.

Any known treatment?

Thank you.

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Welcome to Powdery Mildew. It is the only fungus I know, that can be treated after infection.

Get a spray bottle and mix 9:1 Water and milk. Spray on your leaves and stems once a day. This worked for me quite well

Cut off yellowed leaves. They aren't working for the plant anyway. This will also promote some air flow.

You most certainly should transplant those plants into 5 gallon pots. Use potting soil, sterilized plain potting soil. Nothing else. No added fertilizer or water holding gimmicks. No compost. Lay the plant and pot on newspaper sideways. Cut off roots coming out of the drain holes, slide pot off root ball.

You might have to give the pot a couple of good wacks if the pot doesn't slide off easily. Fill the new pot with 1/3 of fresh potting soil, firm the soil, water the soil and put your plants root ball in the pot and while you've got the root ball exposed go ahead and scratch up the roots on the outside of the root ball. You could take a knife and slice the outside roots a 1/4 inch a few slices, one on each side? This stops the circling of roots and promotes new root growth.

You'll need to fuss with the stake a bit. I'd leave it in place and plant with plant. Fill up the rest of the pot, firming gently around the root ball. Leave 1", no more no less, of room between the surface of soil and rim of pot for proper watering.

Move the grouping of your plants around; put more space between pots and leaves for air flow. Try turning pots upside down to serve as a stage. This really helps not only aesthetics but great air flow.

Fertilize with a balanced fertilizer of N-P-K. Make sure that the N is the lowest number or equal to the percentages of P and K. 5-5-5 or 4-5-5...the percentages are important if you want reproductive growth; tomatoes.

Powdery mildew is normal for this time of season; late summer and into fall. It will not hurt your tomatoes unless it stops photosynthesis of healthy leaves. Whole milk, skim milk...NOT soy milk. You do not want to be using a fungicide any stronger at this point in tomato production. During the day when you water you should also hose the plant down as well. Powdery mildew doesn't like free water. You'll find this on lots of other plants that are under an eave or roof. If you aren't eating the plant Serenade is a great fungicide to use. Read the directions closely to include the little booklet that comes with pesticides (fungicide is included as is herbicide, miticide and other 'cides' under the label pesticide)

If you've no wind at all I would think about a fan blowing on your tomatoes. Water in the mornings or during the day so that your plants are dry by evening. After you transplant, water then feel the weight of your plant, pot and soil. Pick it up to get a sense of the weight with plenty of water. Check the weight the next day. If it doesn't feel lighter don't water. At first the roots won't be enough to suck up the bulk of the water. Don't water until that pot is obviously lighter. Stick your fingers into the soil and check out the moisture that way as well. When the pot is obviously lighter, water thoroughly. Feel the heft again. You'll soon be watering once a day most likely.

You shouldn't need any more than one application of fertilizer, this late into the season. Use half of the recommended amount. Remove tomatoes as soon as they start to color. Allow them to finish ripening in an airy, dark spot in your kitchen. Not in the frig. Removing tomatoes encourages the plant to make more tomatoes. Best way to preserve excess tomatoes is dehydration. Sun dried tomatoes!

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    The milk treatment needs to be given on a sunny day for it to work. – Graham Chiu Aug 14 '18 at 22:10
  • Why? Why would direct sunlight be advantageous over a cloudy day? I do this in my green house. Ugh. Fungus among us! Not in direct sunlight...I allow time for the moisture to dry before nightfall. I run big big fans 24/7. Do you know the chemistry of why and how milk alleviates this fungus? – stormy Aug 15 '18 at 0:13
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    Yes, I do know, and that's why I said sunlight is needed ! – Graham Chiu Aug 15 '18 at 0:15
  • I was just hoping you could share the chemistry! It is always 'sunny' in my greenhouse. Perhaps better described as a 'cloudy day'? – stormy Aug 15 '18 at 0:17
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Couple of suggestions:

First apply a wettable sulfur fungicide to the tomatoes following label instructions for the plant type and size.

Second, make sure the tomatoes get a full days worth of sunlight if possible.

Third, when watering water at the base of tomato plant and lightly mist the leaves rather than water the whole plant. Do this early in the morning (if possible) the idea being we want the surface of the plant to be completely dry by nightfall.

Fourth, prune the areas where the mildew is most prevalent.

  • Great answer Rob. One thing, free water is a deterrent to Powdery Mildew. Hosing off the leaves is actually a good thing. Misting is no help. I am sure that you have seen this on ornamentals, right? Rarely do you find Powdery Mildew on plants that get rained on and get air flow. Under eaves you will find the same plants full of powdery mildew where they get less air flow and no free water. I had to learn the hard way about pruning tomatoes. Those leaves shade the fruit, cut too many off and your tomatoes get sunburned easily. – stormy Aug 14 '18 at 20:49
  • @stormy A light misting should rinse away the powdery mildew without drenching the leaves; was my reasoning. – Rob Aug 14 '18 at 21:15
  • It doesn't. A hard spray is needed and usually that amount of pressure will damage the leaves. Just free water, a drenching once in a while helps mitigate powdery mildew. This dog gone milk and water thing was from Bamboo. And it seems to work! I wonder if it is just the free water but I have as yet to find out how milk deters fungus. I like that you are 'reasoning'...seriously an art that is getting lost! Have you dealt with PM? Grins? – stormy Aug 14 '18 at 21:33

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