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Hollyhock rust & Mallows/Hibiscus worries:

Discussion in 'Pests, Diseases and Cures' started by Caelius, Feb 19, 2021.

  1. Caelius

    Caelius Apprentice Gardener

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    Hello,

    Apologies if this is completely wrong, but my limited understanding of Hollyhock rust is that it affects all plants of the Malvaceae family including Malva and Hibiscus?

    I started some Musk mallows (Malva moschata) from seed last year which were growing well until the last week or two when rust appeared on at least two. I have pulled off the affected leaves at this point, but I assume it is most likely in all the apparently unaffected leaves as well it just hasn't become visible yet.

    Anyway for some background: the only (and I mean only) plant from the entire Malvaceae family we have ever had in over 30 years prior to this is a mature Hibiscus syriacus. Now I must confess I have never looked particularly closely at the leaves of this plant since it seems (at least superficially) healthy and strong, always flowers well, no obvious mass early dropping of leaves etc.

    Now I know that the rust is spread by airborne spores, and is essentially everywhere (hence the problems with the more susceptible Hollyhocks?). However almost everything I have been able to find about this rust is talking specifically about Hollyhocks, and at most mentions it can affect Mallows, Hibiscus etc, but no real specific guidance regarding those species. Given the limited information about how badly it affects Hibiscus or Malva, I have no idea if getting a bit of rust is as potentially disastrous for them as it is for Hollyhocks.

    So I wanted to ask some opinions/advice about how to proceed, with a mind to ensuring our Hibiscus continues to thrive, so a few things on my mind are:

    1) Since the rust is everywhere anyway, is it likely that either our Hibiscus is more resistant to it or that it just gets it every year anyway but never badly enough to actually be really noticeable, so this is all a worry about nothing.

    or should I:
    2) Pull off the affected mallow leaves as and when I see them.
    3) Cut all of the mallows to the ground (removing all leaves) now.
    4) dig up and bin the lot.

    Thanks for any advice you can offer.
     
  2. NigelJ

    NigelJ Total Gardener

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    @Caelius
    I've grown Hibiscus syriacus for as long as you have and also lavatera, hollyhocks and a couple of other members of the mallow family and the only ones that have suffered from rust have been the hollyhocks.
    Are you sure it was rust on the musk mallow; as (assuming you are in the UK) given the winter and the time of year then dodgy looking leaves can be expected.
    I would remove any dead leaves that occur, then wait and see how they grow away when spring happens.
     
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    • pete

      pete Growing a bit of this and a bit of that....

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      I have a hedge of hibiscus syriacus and it has never had rust.
      I do get self sown plants of some kind of mallow and most years they do get rust.
      My thinking is that the hibiscus is not seceptable to that rust in the same way, other wise I'm sure it would be showing signs.
       
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      • luis_pr

        luis_pr Gardener

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        I have not seen rust on hibiscus but I have seen rust in hydrangea. This disease normally appears during the spring so seeing a case now is rare. Symptoms include tiny yellow pustules on leaf undersides, sometimes followed by rust-hued spots on the tops of leaves. Is that what you are seeing? Can you post pictures, if it re-appears, so someone can confirm? I hope it is something else...

        When watering the plants, water the soil directly, not the plant. Neem oil, sulfur-based fungicides or a fungicide with an active ingredient like chlorothalonil (over here/U.S.), applied in cool weather (below 28 or 29 degrees in the summer), can help provide some preventive control if applied to the new leaves.
         
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          Last edited: Feb 19, 2021
        • NigelJ

          NigelJ Total Gardener

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          Agree with Luis regarding timing being out. I normally find my hollyhocks get it towards the end of the summer. Most fungi prefer warmer, damper conditions.
          Note chlorothalonil was banned in the EU in 2019, it's really not a nice chemical to spray around in a domestic setting.
           
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          • pete

            pete Growing a bit of this and a bit of that....

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            Agree usually late summer that I notice it.
             
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            • Caelius

              Caelius Apprentice Gardener

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              Thanks for all the replies.

              I unfortunately cannot post a picture as I already disposed of all the foliage with the spots on. But yes, yellow-ish spots on underside of leaves and yellow/orangey-brown spots on top. I also thought it would be more of a spring/summer thing but to me it certainly looked like it. If it's not I'll not be complaining.

              The fact that some of you have had mallows with rust in the same garden with or even next to Hibiscus which didn't get it is encouraging, as I must admit part of my confusion (again stemming from advice for Hollyhocks) is the idea that the rust spores are practically everywhere, yet the advice for growing Hollyhocks is to avoid growing mallows in case they harbour it, when if it really will just come in from anywhere and everywhere what difference does it make since surely unless you have an enormous garden they'd both get hit regardless?
               
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