Dahlia - white substance on shoots?

In my greenhouse my two recently planted Dahlia Arabian Knight tubers are just poking through the compost in their pots, but they have both developed a white substance on the shoots.

They dont look like eggs but seeing as I have lots of other different plant seedlings in the greenhouse that arent affected im wondering if its something specific to Dahlias?





  • If you have fungicide I would use it. It also resembles mealybug.Picture below.



  • Leadfarmer scrape one of plant with Q tip if theres a bug inside its mealybug and I think it is.No bug then its fungus.


  • LeadFarmerLeadFarmer Posts: 849

    Thanks. When you say Q tip do you mean a cotton bud? Which fungiside would be best if needed?

  • LeadFarmer I cant recommend a fungiside as Your brands are different than our Canadian brands. Q tip is a cotton ball on match stick sort of.If its mealy bugs then use that cotton tip to put a dab of rubbing acohol on each one.My problem here are earwigs as they eat everthing.



  • LeadFarmerLeadFarmer Posts: 849

    Thanks Bill. Ill do some research on suitable UK fungisides. The problem is getting worse each day, its like someone has sprinkled talcum powder all over the plant. I wipe it off but it returns.

  • LeadFarmerLeadFarmer Posts: 849

    Well despite lots of internet searching and reading about applying fungicides, I cant find reference to any particular type/brand. Any suggestions please?

  • BobTheGardenerBobTheGardener LeicsPosts: 6,357

    Difficult to say what will work as we can't identify exactly what it is, but assuming a fungus, I'd try a copper-based one first with Bordeaux mixture probably being the easiest to get.  If it doesn't work, it will still be useful for other things - see the RHS advice on fungicides for garden use:


    A trowel in the hand is worth a thousand lost under a bush.
  • LeadFarmerLeadFarmer Posts: 849

    Thanks Bob, I shall take more photos tomorrow as the problem is getting worse by the day with the young plant being covered in the white pwdery substance.

    My greenhouse is full of young seedlings and plants but its only affecting my Dahlias. I purchased them as tubers just a few weeks ago from a well known online seed supplier. Im wondering if the tubers were already affected before I got them, seeing as no other plant is affected?

  • Did you check to see if something is alive inside of the white mass/ball? I still feel that its mealy bugs and I have seen them kill a 10 ft Magnolia tree.


  • LeadFarmerLeadFarmer Posts: 849

    I couldnt find anything alive. The white stuff is as fine as talc powder or dust. I'll get some better photos tomorrow.

  • LeadFarmerLeadFarmer Posts: 849

    Heres a couple more photos if it helps (click on them to make them bigger)...








  • LeadFarmerLeadFarmer Posts: 849

    Think i may have found a cure. 2 x tsp baking soda in pint of water, sprayed onto plant, white powder disappeared instantly. Hope it doesnt return.

  • I did a search and this is all I could find that resembles your problem.

    Wooly aphid





    Wooly vine scale


    Hope this helps.


  • As well as spraying with baking soda, keep the compost on the dry-ish side (i.e. don't overwater at this stage) and don't wet the leaves.

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