Many gardeners and tree shoppers that come into the nursery to purchase a poplar/cottonwood or willow tree have patiently listened while we talked about the diseases and bugs that attack them. After explaining why we don't stock them in the nursery, we make every attempt to steer tree shoppers to a long lived and mostly bug resistant tree that will do well in our area.

Yesterday I was out in our back yard looking at the damage that a large cottonwood tree had done after the last big wind storm. We have a very tall pergola that barely missed becoming firewood when this branch fell. As I was examining it, and trying my hardest to move it away from the patio area under the pergola, I noticed little orange dots on one side of the smaller branches. AHA! Cytospora Canker !!! I didn't think that the fungal disease regularly attached the older cottonwoods, just the newer hybrids. This tree is probably 50 - 60 years old judging by the trunk diameter. A little more technical info from the University of Arizona Extension office:

Cytospora canker :Cytospora canker is caused by the fungus Cytospora chrysosperma, the conidial stage of Valsa sordida. This fungus infects twigs, limbs and trunks of cottonwoods, and other poplars and willows. Cytospora canker causes death of weakened bark in either localized annual cankers or slowly spreading perennial cankers. Dead bark becomes loosened, and numerous small black spots are visible in the cankers (photo 1). These are pycnidia, the reproductive structures of the fungus. They become sticky and orange-yellow when it rains and spores are produced, then they dry to a hard reddish-orange mass.New infections occur in wounds such as sunburned bark, broken branches or pruning wounds. Infected sites are girdled, resulting in death of infected twigs, limbs and trunks. The only controls are to prevent wounding and sunburn, prune correctly and cut out infected branches. Tools should be disinfected between cuts and after pruning to avoid spreading the fungus to new sites. Infected wood that has been cut to use for fuel should be stored dry since spores can be produced on dead bark.

Some photos of the culprit in my own yard:

spores - as they appear on the fallen branch