Have you ever wondered about the mistletoe you see growing on trees in our area? What about the mistletoe hanging in doorways during the holiday season? If you get caught under the latter mistletoe you might garner a kiss. The former mistletoe, a parasite, might be the kiss of death to your tree.
Today I am going to address the dwarf mistletoes of the genus, Arceuthobium, that attack various species of conifers. The other group, true or leafy mistletoes are in the genus Phoradendron, which can grow on both conifers and hardwoods.
Different species of dwarf mistletoes can attack Lodgepole pines, Douglas firs, Ponderosa pines, Limber pines and Pinyon pines. It is this last one that we deal with today.
Dwarf mistletoes can spread short distances by means of explosive fruit that shoot out sticky seeds at almost 60 mph. Birds can also spread these seeds. These mistletoes tap into the vascular system of their host, extracting water and nutrients.
The growth of these mistletoes produces a "witch's broom" and can cause loss of vigor, dieback and even death of their host tree in the most severe cases. Pinyons displaying several "witch's brooms" produced through infections by the dwarf mistletoe, A. divaricatum, will be, at the very least, stressed and more susceptible to drought-induced death, and attractive to attacks by wood boring beetles, twig and bark beetles.
If you first observe mistletoe in your pinyon tree, don't expect the tree to die overnight. It may take years and multiple infections to cause the death of your tree.
So what can you do to control dwarf mistletoe on your pinyon trees. First, heavily infested trees should probably be cut down and removed. Pruning infected branches back all the way to their branch collars can work on more limited infections. Less drastic measures would involve cutting back the leaves and stems of the parasite to the wood and then wrap that area with black polyethylene sheeting. This is to block light and prevent the mistletoe from re-sprouting and producing flowers and fruiting, creating seeds that will spread the mistletoe. Removing just the visible mistletoe growth will not kill the parasite but repeated removal will slow down the progression of the infection and subsequent tree death.
One last caveat. Dwarf mistletoe is poisonous to dogs and cats and can even cause illness in humans. So kissing under true mistletoe may bring you love, but dwarf mistletoe will only bring your conifers grief.
Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.