ISSUE 5 June 12, 2008
ASH ANTHRACNOSE FINALLY OBSERVED
I normally receive calls in mid-to-late May regarding ash trees that are losing their leaves. The cause is ash anthracnose, a fungus that is common during cool, wet weather. This past week, we finally observed it in Fargo and it is likely being seen through much of eastern North Dakota. Besides leaf loss, additional symptoms include black leaf margins, distorted leaves and/or small brown dots in the middle of leaves (see photos). The brown dots within the leaves are where the fungus has entered wounds created by ash plant bugs feeding on the leaves.
Is treatment necessary? Usually, ash anthracnose does not cause enough damage to stress trees. Trees can lose up to 25% of their foliage without major consequences. If trees are heavily defoliated consecutively for 3 or more years, then they will be stressed and susceptible to other pests that could kill them. Fungicides are only effective as preventative treatments as leaves are expanding. However, fertilizing trees that have lost a large amount of leaf tissue could help them refoliate, reducing stress. Since the fungus overwinters in leaves and seeds, raking and destroying these tissues in the fall helps to reduce disease pressure the following spring.
Note the dead leaf margins and distorted growth in the first photo. The second photo shows a dead leaf margin plus small dots where the fungus has entered the leaves through wounds created by feeding of the ash plant bug.
SOME OBSERVATIONS ON ELMS
1 – I have received two calls this week regarding American elm trees losing leaves. In both cases, twigs with a small cluster of 6-10 leaves have been found lying on the ground. The ends of the twigs usually appear to be cut at a sharp angle, though occasionally they are more jagged. The cause? We believe that the culprits are squirrels. Yes, squirrels. Elm trees produce seeds in the spring and in many parts of the state this year, the seed crop is heavy. In the process of eating and gathering seeds, the squirrels are cutting the twigs and causing a mess. Tree health is likely unaffected. Trees can lose up to 25% of their leaves without much loss in photosynthesis.
2 – Siberian elm trees in Bismarck and elsewhere in central North Dakota appear to have a very heavy seed crop this year. Accompanying this prolific seed crop are trees with fewer leaves and smaller-than-normal leaves. It seems that the trees have placed most of their energy reserves into reproductive growth – seeds – leaving very little for vegetative growth – leaves.
Why would trees grow this way? Our hypothesis is that these trees are highly drought-stressed, and they’re putting most of their energy into reproduction before they presumably die. However, a heavy seed crop by itself doesn’t necessarily mean that trees are doomed. For example, in 2003, Colorado blue spruce trees through much of North Dakota had heavy seed crops, but this was not followed by a massive die-off of spruce. Recent rainfall through much of the state will likely limit drought-related mortality, though it will be interesting to observe the Siberian elm trees this year.