Mistletoe Un-Update

oklahomaThere is still no news about why mistletoe disappeared this last year in our area. No one really noticed it missing until late in summer. The absence of mistletoe became more apparent as deciduous trees that had been infested with it last year defoliated in autumn. What is even more strange is that the dead mistletoe plants deteriorated so quickly and efficiently that they are completely absent, as if something ate all the mistletoe, or took it away. The only evidence of former infestation in some trees are the swollen portions of stems where mistletoe had been attached. An article about this mysterious absence of mistletoe can be found here; https://tonytomeo.wordpress.com/2017/09/24/where-has-all-the-mistletoe-gone/ .

Someone who harvests mistletoe from local trees, both to eradicate it from a few trees, and also to sell it in local markets, made an interesting observation about the absence of mistletoe just within the past few day. He found that some mistletoe survives, but only in the upper extremities of tall trees. Most of the trees are locust trees, perhaps because they happen to be taller than most of the other trees that had been infested. One infested tree is a black walnut. The viable mistletoe plants are somewhat young and small. Larger plants or colonies that were as high as the smaller surviving plants are gone. Specie of mistletoe that infest coast live oak and some conifers have not been observed.

There are several variables that could account for the survival of relatively small mistletoe plants high in the canopies of host trees. Some pathogens that could affect mistletoe might proliferate in congested growth that limits air circulation, but not where air circulates efficiently through sparse and exposed growth higher up. Some pathogens that proliferate in cool and damp situations are inhibited by drier and sunnier situations. Some pathogens are more likely to infect hosts that are closer to the ground. If rodents are taking mistletoe vegetation, they prefer the shelter of more congested lower growth, and avoid the vulnerability of more exposed higher growth.


Where Has All the Mistletoe Gone?


Locust and alder trees in my region are commonly infested with mistletoe. Some were severely infested. A few of the locust trees downtown were actually unsightly because they are so stressed and sparse from mistletoe infestation, and also because they were so full of big shaggy mistletoe bushes that obviously do not belong there. Now, viable mistletoe can not be found. Even the carcases of the big dead mistletoe bushes are hard to find. They seem to have died and deteriorated before anyone noticed. The formerly infested trees are noticeably healthier, and producing more healthy foliage than they normally do, partly hiding any remnants of dead mistletoe.

No one here seems to know what happened. Is this part of their natural life cycle? Is there something that sets them off to die all in the same season like some specie of bamboo do? Is there a disease or insect that we should know about? Is this isolated to our region, or is it happening elsewhere as well?

I know I should be pleased that the mistletoe is gone, even if it is only temporary. It is such a destructive parasitic weed! The concern is that we just do not know what happened.

Years ago, Phytophthora ramorum started killing tan oaks. At first, not many of us minded. After all, tan oaks are considered to be trashy trees that clutter otherwise pure redwood stands, or compete with more desirable oaks. Dead tan oaks were better than live ones, and only needed to be cut and split to be sold as seasoned firewood.

The following year, the same disease started to kill coast live oaks. Then we had a problem. Coast live oak is one of the two most majestic oaks in our region (and the most majestic in the Santa Cruz Mountains where the valley oak is uncommon). It took a long time to identify the disease because the ambrosia beetle, which is a secondary pathogen, was initially blamed for the widespread death, which became known as Sudden Oak Death Syndrome, or SODS. It took a few years for the worst of the SODS epidemic to subside, and it continues to kill oaks sporadically.

That is why it is hard to ignore what is happening with the mistletoe, even though we really should be pleased to see it go.