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The attached photographs show a sick basil leaf (which is turning black) compared to a healthy basil leaf from the same plant. The black leaves need to be trimmed to keep the plant healthy. What can I do to prevent such leaves from turning black without using anything chemically poisonous to humans such as pesticides?

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EDIT: My Italian dictionary sais peronospora can affect tomatoes as well. I'm not sure if the same fungus/algae can spread to both basil and tomato tomato 🍅 plants. Thanks.

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EDIT: In response to a comment, here is a better picture of the soil from the garden in approximately March. Not sure what "white mud" is but I guess soil can be quite different based on the types of fertilizer used etc... Thanks.

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    can you post a photograph of the whole plant please, including the pot if ithat's what its growing in? – Bamboo Jul 8 '16 at 11:58
  • Ok, as soon as I get to the garden where I planned the basil plants I will upload a picture. Thanks. – Jack Maddington Jul 8 '16 at 13:17
  • Ah, well if they're in the ground, a photo might not be quite so useful, but its better to add one than not... – Bamboo Jul 8 '16 at 13:58
  • I've updated my post with images of the basil plants (some leaves are beginning to turn black underneath but I've removed most black leaves already from the plants). Thank you for your advice. – Jack Maddington Jul 8 '16 at 17:39
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The images of the individual leaves above are not at a high enough resolution for me to detect the presence of spores or fuzz from Basil Downy Mildew, and whether it could be that or not is dependent, to some extent, on where you are. Link below with information and images of affected plant material for you to compare - you may need to inspect the underside of leaves with a magnifying glass

http://vegetablemdonline.ppath.cornell.edu/NewsArticles/BasilDowny.html

Other common problems with Basil are Bacterial Leaf Spot, an infection spread by water splash, and it could be that, or Fusarium disease, but your plants are not looking as if they're suffering from the latter. Info here regarding both these infections

http://www.gardeningknowhow.com/edible/herbs/basil/basil-diseases.htm

  • Thank you for your reply. My plants are in northern Italy. I think my basil plants might be suffering from a form of Downey Mildew as you said, because my neighbors told me tomatoes in the area suffer from peronospora, although perhaps I am remembering wrong since I don't see aperonospora for tomatoes on Wikipedia. I see a peronospora belbahrii for basil which is Downey Mildew. Coincidentally I watered the plants with a house from above, so that might be the reason. I now water the plants from below to avoid the problem. – Jack Maddington Jul 8 '16 at 20:25
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    One other thing - your soil looks like white mud which is panned on top, presumably from frequent irrigation or rain - what sort of soil is it? Heavy, light, sandy, shaley? And your extra pic of the Italian information at least proves Basil downy mildew could be a problem in your area (thank heavens for Latin names, Peronospora being easily recognisable)! – Bamboo Jul 8 '16 at 20:46
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    Just had a scour of the net - there is mention of Downy Mildew in tomatoes in Western Europe on an internet site based in France - Hypermedia in Plant Protection, so your neighbours may well be right....unless they're confusing Tomato blight and peronospora... It'll likely be a different variety of Peronospora though, they are quite plant specific. – Bamboo Jul 8 '16 at 21:10
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    1. No, biologists in different countries would not use different Latin names (and the official names will always be Lstin) 2. You don't need to worry whether tomato downy mildew is the same as basil downy mildew - you only need to decide if your plants have basil downy mildew and behave accordingly 3. Fungus/algae/cyanobacteria - most are distinct from one another, but the lines are blurred between those for some organisms. Again, not something you need to know - you just need to work out whether you've got a case of bacterial leaf spot or Basil Downy Mildew. – Bamboo Jul 8 '16 at 22:37
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    Sorry, couldn't squeeze it all in to my last comment - when I said maybe they're confusing tomato downy mildew (peronospora) with blight, I meant your neighbours, not biologists... perhaps they (your neighbours) are seeing blight and thinking its peronospora, not sure. – Bamboo Jul 8 '16 at 22:44

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