Bronze Leaf disease on Swedish Aspen & Tower Poplar.


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BLD is a killer.

What I know:
  • Leaf colour can vary from red, to new copper penny, to dishwater.
  • Leaves can look totally dead, but feel leathery. Veins remain green.
  • One stage of the fungus overwinters on leaves, and produces spores that infect new leaves in spring.
  • Infected leaves can stay on the tree, making infection next year easier.
  • Rain splash spreads the fungus spores too. A policy of limbing up 5 feet reduces spread rate.
  • Newly infected leaves don't show symptoms until late summer early fall. For this reason I tell people to postpone putting in new swedish aspen until after labour day.
  • The fungus enters the vascular system.
  • Fungus can spread to neighbouring susceptible poplar through shared root systems. It's common for poplar to spontaneously root graft to each other.
  • Infection requires the right combination of temperature and humidity.
  • Time from first infection to tree death is about 5 years.
What I don't know:
  • Does the fungus spread on pruners. Dealing with this would be a lot easier if I didn't have to keep a pruner in bleach.
  • Is the fungus always apparent in cut twigs? I think so. There is a brown stain on cut twigs, and when I get far enough down the twig it vanishes. If this is true, and it can be spread by pruners, then a two pruner method is easier to implement: One pruner is 'dirty' one 'clean'. Prune the affected branch progressively until you find no more fungal stain. Then with the clean pruner cut once more 4" closer to the root.
  • How effective is pruning and cleanup? From the life cycle of the beast, leaf cleanup should help a lot with re-infection, and spread. But if a single leaf is producing billions of spores, then cleanup has to be thorough. I don't think this is the case. Infections tend to be very patchy with one branch having 20-30% of the leaves infected. This implies that infecting a new leaf is a big limiter.
  • Is there an agent that will stop the infection at some stage? E.g. A spray that killed the overwintering organism in leaves that cling to the tree? If this were a fairly harmless spray it could be applied to the tree and the ground around the tree to reduce spore counts for the next spring.
  • Is there a systemic fungicide that is economical to inject into a tree to prevent tree death
  • Are there other diseases/conditions that mimic the appearance of this disease? I have several poplar with some twigs with bright red leaves -- but not with green veins. I have ones that have gone a general pink.
  • Would applying a fungicide to infected leaves intertupt the cycle and keep the fungus on those leaves from completeling their cycle.
  • At what point does the fungus enter the tree's vascular system? Does this happen in fall during leaf drop, or does it happen earlier in the season. This affects if there is even any point in trying the previous bullet.
What I've done:
  • I have had it get into one stand in my demo area. I've lost one tree in that circle, and pulled it and it's root system -- as much as I could get. Several other trees in the circle (started with 12 trees on the circumference of a 50 foot diameter circle) have had infected twigs. I've been removing these as I find them. I do no leaf pickup. I've been doing this for several years now, and the amount of infected leaves has declined each year.

Bronze_Leaf_Disease-1.png Bronze_Leaf_Disease-1.png

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