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Silver Leaf Disease of Apple Trees Spotted in Minnesota Orchards

Author: Annie Klodd, Extension Educator - Fruit and Vegetable Production

Several apple growers have reported increased incidence of "silver leaf" on their trees this season. One question asked around this topic is whether the cold winter could have contributed to silver leaf. Before answering that question, let's talk a bit about what silver leaf is and how it spreads:

Silver leaf is a fungal disease of apple trees and many other deciduous trees that makes the leaves appear "silver" or gray. This is caused by a fungus called Chondrostereum purpureum. In addition to infecting leaves, it also forms conks (fruiting structures) on the trunks and branches of trees.

How Silver Leaf Spreads

During wet conditions in the spring and fall, the conks release spores that can then infect wounds on the trees, like pruning cuts, splits from winter injury, or broken branches from heavy snow.

The fungus then resides in the xylem of the branches, and a toxin from the fungus moves up into the leaves, causing a silver sheen. The wood in infected branches will eventually start to decay, at which point the conks will start to show up.

Considering that this fungus enters wounds on wood during wet conditions in the spring, it makes sense that it could have infected trees this spring during dormant pruning. The polar vortex did not necessarily cause this, but the heavy snowfall in February may have indirectly caused silver leaf:

Because heavy snowfall made it difficult to get into the orchard to prune in February and March, some growers were forced to wait to prune some or all of their trees until the mid-late spring once warmer temperatures melted the snow. As many will recall, our spring was very wet in 2019, as snow continued to melt for several weeks while the sleet and rain were also falling. Therefore, growers may have been pruning during wet, cool spring conditions when the silver leaf fungus was active, easily infecting fresh pruning wounds. Additionally, winter injury (broken branches from heavy snow, splitting from cold injury) could have also produced entry points for the silver leaf disease.

Management Options for Silver Leaf

For a more thorough description of management options for silver leaf, read this article. In brief, silver leaf is rarely an economic concern for orchards, because it typically only impacts a handful of trees. In these cases, it is feasible to prune out the infected branches: those that exhibited silver leaves, and those with conks on the wood. Clean pruners between cuts, and cut 4 inches below the lowest infection point.
If the orchard is large enough that the removal of one tree would not be a significant loss, and if many or all branches on a tree are impacted, then the grower should consider removing the tree so that the fungus does not spread to neighboring trees. If many trees in the orchard are infected, or if you only have a small planting and it would not make financial sense to lose trees, then mark those trees with spray paint or flags and monitor them for the next couple of years to see if they recover or decline. Sometimes, silver leaf symptoms do go away. Be sure that the trees are adequately watered, as silver leaf reduces water movement in the tree.

At this point, there are no fungicides for silver leaf, and painting pruning cuts is not recommended.

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