The depicted Deutzia Gracilis has apparently begun suffering from some kind of invasion.

Who is bugging it (probably aphids) and how can this be fought off (preferably without "strong" pesticides)?

UPDATE So I was able to catch the/a likely suspect at night (see 4th picture), probably a Vine weevil. RIP.

UPDATE On reflection, the plant of concern my be Forsythia intermedia, not Deutzia gracilis.

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Yes you've got visitors. They come at night so if you've a flashlight, go out at night to check on these visitors.

Not at all a big deal, Drux. Your plants are not healthy for two reasons that I can suss out; one is light and the other is fertilizer. The internodes are extremely long (too little light) and your leaves show that the photosynthetic factories are DOWN (too little nitrogen).

Without the right amount of chemistry (fertilizer) and light and water and air and proper soil...you will get anemic looking plants. To be healthy and the right color of green you need to add a balanced fertilizer. Don't lump fertilizer into 'bad chemicals'...please.

Plants that are susceptible to insects and disease are usually plants with little or no chemistry added to the soil with which they they are able to compete, survive because they need to make their own food...Any problems you see are most likely because this plant has had no added balanced fertilizer. Photosynthesis doesn't work without the proper ingredients at the proper amount. Without a human who understands the chemistry, just the basics, no added fertilizer, one will have plants 'on their last legs'.

Weakened plants put out neon signs saying, "I am weak and easy to eat"...what have you added for fertilizer? If you've added a balanced fertilizer then we all need to go back to the drawing board!

Looks like your potting soil came with fertilizer, balanced? Found 5-4-6 on one of the fertilizer products. So this is in favor of too much fertilizer? They say you need to add your own fertilizer after 5 months? How long has it been since you transplanted this plant? Looks like it has used up what little fertilizer was added to the potting soil and you need to start your own program of fertilizer. Osmocote 14-14-14 would be perfect. Use half the amount specified in the directions. Extended release so you only need to do this once per year.

Look beneath the leaves with a magnifying glass. Look for tiny little spider things, possibly gold in color. Fine fine webbing. The are sucking insects. Aphids are sucking insects. Have you seen one aphid? Most likely white? If you did not notice aphids then I doubt you had an aphid problem. Spider mite is more likely. Your plants need fertilizer but wait until we figure this out for sure.Deutzia care and maintenance

  • +1 Well, the plant receives full sunlight from approx. 7 AM to noon, is that too little? I got it 1-2 months ago, and the it grew quite quickly since then: all of the depicted parts are new. I am also applying a fertilizer (it says "rich in potassium") in the prescribed dose and approx. 1-2 a week. Do you think I should increase over that dose? (I'll do so once right now :-) – Drux Aug 15 '18 at 6:56
  • Is this full sunlight behind windows, Drux? What exactly are you using for fertilizer? "rich in potassium" means nothing other than it has potassium? What are the numbers for your fertilizer? When was the last application? Do not add anymore fertilizer until we sort this out. 1 to 2X per week for applications? Drux, don't do anything right now, grins, we need to understand much more. An indoor plant next to southern windows gets just enough light to manufacture its own food. Forget flowers and fruit...in any abundance! This is more chemistry than any disease going on... – stormy Aug 15 '18 at 7:15
  • Does this article make sense to you? missouribotanicalgarden.org/PlantFinder/… – stormy Aug 15 '18 at 7:19
  • The plant is outdoors (on a terrace). What numbers are we looking for for the fertilizer? Last application was this morning. – Drux Aug 15 '18 at 7:57
  • What's the NPK readout on the fertilizer you're using - and is this plant in a pot or the ground? If its in a pot, can we see a photo of it please? And what part of the world are you in? – Bamboo Aug 15 '18 at 10:23

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