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Old May 17, 2015   #1
Stainless
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Default Is this Downy Mildew on my cucumber plants?

My cucumber plants have just started to really take off in the last few weeks due to it finally heating up. I went out to check on them the middle of last week and I had to pull five plants out. Now the other side is doing the same thing. They are developing brown spots and wilting. I did some searching and "Downy Mildew" looks to be the culprit but I'm not 100% sure.

Anyone else have any explanations? We have had lots and lots of rain here lately. I'm surprised my tomato plants are still alive.
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Old May 17, 2015   #2
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Old May 17, 2015   #3
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Yes it appears to be Downy Mildew. If it were Powdery Mildew it would start with a white powdery growth on the top and bottom of the leaves. I always have to deal with Powdery Mildew every year up here which is a true fungus. Downy Mildew is a oomycete pathogen, a water mold which is worse than a fungus. Rainy conditions and overcast skies are perfect conditions for Downy Mildew to thrive.
A phosphorous acid fungicide is usually very effective against Downy Mildews. Look for ORGANOCIDE Plant Doctor or AGRI-FOS Systemic Fungicide

This Downy Mildew (Pseudoperonospora cubensis) only infects Curcubits, it's no danger to tomatoes or any other crop.
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Old May 17, 2015   #4
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Default Is this Downy Mildew on my cucumber plants?

So am I correct in reading that the mildew can only be controlled at this point and not eliminated? I'm thinking that I may be better off pulling these plants up and starting new seeds or will this stuff reside in the soil and spread to the new plants as well?

Last edited by Stainless; May 17, 2015 at 02:10 PM.
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Old May 17, 2015   #5
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The phosphorous acid fungicide will kill the pathogen infecting the leaves but you got the spores from somewhere in the environment which means you are likely to get it again when planting new seedlings. Either they were wind blown or they were already in the soil and got splashed up onto the plant from the rain. If you plant new seedlings I would treat them with the fungicide before planting them. That will give them a strong defense from being infected. The phosphite ion becomes systemic in the plant and even though it's a phosphorous based molecule the plant can't use it a P source of nutrition. It's harmless to the plant, it's just a foreign molecule it doesn't know what to do with. When the pathogen encounters the molecule it totally messes up its metabolism and it dies.
There are clear directions for how much to use and the maximum number of treatments per season.


Are Phosphorous and Phosphoric Acids Equal Phosphorous Sources for Plant Growth?
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Old May 17, 2015   #6
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Thank you, RayR.

Hopefully I can salvage what I have left and plant a few more seeds. I'll head to the local store and see if I can pick something up for the dew. I guess I'll update this thread on whether it works or not.
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Old May 17, 2015   #7
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What about spraying with Monopotassium Phosphate 0-52-34?
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