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I have a threesome of tomato plants planted in on my porch. All three have some leaves that seem to be a bit white and powdery. One of the leaves has even started to turn a bit brown.

I don't think this is blight, I'm pretty sure I know what that looks like and it doesn't seem to be a watering deficiency/too much water as they are in self watering containers and I really only ever water from the bottom.

My biggest of the 3 plants seems fairly unaffected although a couple of its lower leaves seem to have some of this discoloration.

However the other two (on either side of the first) seem to have the discoloration on the leaves. Its mostly the lower leaves. Is this something to be worried about or is it normal? (pics to come if needed)

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do the plants have good air circulation on the porch? Has it been (very) humid there? Also before the powdery (mildew) appeared on the lower leaves, did those leaves show other symptoms eg Light green or bright yellow spots? – Mike Perry Aug 17 '11 at 19:37
@Mike - they shouldh ave pretty good circulation, the wind has been blowing across the house rather than towards it lately. I didn't notice any light or yellow spots... – wax eagle Aug 17 '11 at 19:41
try this, Tomato Disease Identification Key by Affected Plant Part: Leaf Symptoms. If need be I can write up an answer (let me know if I should), but I would rather wait until I see the photos... – Mike Perry Aug 17 '11 at 19:51
@Mike: Thanks for that link, I've been doing it the hard way... :) – bstpierre Aug 18 '11 at 3:04
up vote 5 down vote accepted

This is almost certainly Powdery Mildew (see Figure 3, here), and a severe attack can weaken a plant considerably. Tomato powdery mildew is apparently caused by two different fungi, Leveillula taurica (Oidiopsis sicula) and by Erysiphe (Oidium lycopersicum), and tomato leaves that are affected by the second variety don't develop any yellow spots; given that yours have not shown this symptom, it's reasonable to assume that they are affected by the Erysiphe type.

Control Measures

  • Removing old leaves from the bottom of plants reduces disease from both fungi, often to the extent that no other control measure is needed.

  • If symptoms develop early, multiple, early applications of wettable sulfur or dusting sulfur will prevent new infections. Sulfur should be used with caution on young foliage at temperatures above 90°F; read and follow label directions.

  • Applications of other contacts such as potassium bicarbonate products and oil sprays will kill the existing mycelium and spores on the leaf surface and are most effective when disease is in early stages.

University of Arizona: Powdery Mildew. This article includes some very useful links.

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