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My basil plants have recently started spotting on the leaves with a couple yellow leaf drops and streaking on stems. Is this a disease or some other issues?

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A few details:

  • Hardiness Zone 4a to 3a
  • 6 - 8 hours of direct sunlight.
  • Not a significant amount of leaf drop. Here and there.
  • Some curled and deformed leaves.
  • Plant trying to flower like crazy.
  • Only water when soil feels dry.
  • How humid is it where you live, and how often does it rain? Did it appear after a rain? – Brōtsyorfuzthrāx Jul 27 '15 at 22:02
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    @Shule they're sheltered from rain and its typically very dry here. One sign that makes me think it's deficiency is it seemed to happen after a big growth. The wilting is actually minimal too. – Vian Esterhuizen Jul 27 '15 at 22:03
  • Ah. Well, it doesn't look like a deficiency in and of itself, but it may have a deficiency that made it susceptible to a disease. If you added a lot of nitrogen, then adding extra potassium should be a good idea. Adding extra potassium may increase your need for calcium if you add very much. Extra silica wouldn't hurt either, I'm sure. – Brōtsyorfuzthrāx Jul 27 '15 at 22:08
  • I could be wrong about it not being a deficiency, but usually you don't see stem symptoms like that, unless there's something I don't know about. The curling and deformed leaves could indicate something to do with a deficiency/toxicity, though, as could the spots on the leaves. – Brōtsyorfuzthrāx Jul 27 '15 at 22:17
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In light of your dry climate, it could be copper deficiency. Watering with a blue water soluble fertilizer should help to fix that, though, if true. Kelp is an option for extra copper (and other minerals).

Too much phosphorus could have caused the copper not to be available to your plant. I see that you fertilized with 10-15-10 fertilizer, and that you said your plant is trying hard to bloom (phosphorus is supposed to contribute to blossoms and plant maturity).

However, since you've already fertilized, you might want to go a different route than adding more blue fertilizer, and just balance your N-P-K ratios by adding more potassium and maybe also nitrogen, but not more phosphorus; a soil test should help. Balancing the N-P-K levels might make the copper more available, but I could be wrong—you might have to deplete some phosphorus first (but I don't think so).

I've read someone say that phosphorus should never be higher than potassium in a fertilizer (assuming the soil isn't high in potassium and low in phosphorus). I would have to research this to know how true it is, though.

It could be both copper deficiency and a fungus, because copper is anti-fungal and I postulate that a lack of it might make a plant more susceptible to fungi.


Old answer:

It could be downy mildew. I wouldn't have known to suggest that, but it looks a lot like the picture at the bottom here (they say that's what it is there).

I wouldn't be surprised if it were pythium, either (if that infects leaves; I know it infects stems). It can cause damping off disease in seedlings, too.

Septoria leaf spot is another candidate.

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  • Honestly haven't even considered root rot because I've been quite diligent about watering only when dry and the pot drains really well. I just did a littl check and the roots look white and healthy. Downy Mildew I ruled out due to the lack of grey fuzz described here extension.umn.edu/garden/yard-garden/vegetables/… I'm fairly convinced it's either nutrient deficiency or Fusariam but I really don't know for sure. – Vian Esterhuizen Jul 27 '15 at 21:59
  • @VianEsterhuizen Hmm. Potassium sulfate helps to strengthen my plants against fungal infections, I've found. – Brōtsyorfuzthrāx Jul 27 '15 at 22:03
  • I wouldn't rule out downy mildew on account of the lack of fuzz. Fuzz comes in advanced stages, I hear—tell me if I'm wrong. I don't know that pythium always causes root rot if it's infecting a plant (but perhaps it would; I think my tomatoes that seemed to have it in the stem had fine roots). Fusarium looks like a possibility. – Brōtsyorfuzthrāx Jul 27 '15 at 22:05
  • However, since your area is dry (I mean arid by that; it is arid, right?), I wouldn't think it would be downy mildew or pythium. – Brōtsyorfuzthrāx Jul 27 '15 at 22:10
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    it's very dry here. I guess we fall under semi-arid. I left the soil dry out sufficiently before a thorough watering. I used to be a classic over-waterer so now I probably lean more to the side of under-watering. I cut the plants back significantly and an giving some extra nutrients until someone can tell me with confidence what it is. – Vian Esterhuizen Jul 27 '15 at 22:18

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