Wednesday, December 21, 2011

Mistletoe, Past and Present

"Everybody knows, a turkey and some mistletoe…" (1)

Leafy mistletoe in the crown of a willow oak.
Photo by Dr. Larry F. Grand
The age-old tradition of kissing under the mistletoe is well known in our culture, and most people have heard of the importance of mistletoe to the ancient Druids, for whom it served ritual and medicinal purposes (2). Many also know the berry-like fruits to be toxic to people, but according to Poisonous Plants of North Carolina, poisoning will occur only if large quantities are eaten.

Fewer people know the marvelous mistletoe story from Norse Mythology, wherein a conniving Loki arranges for a mistletoe arrow to be used to kill Balder, whom the creatures of earth were sworn not to harm. The mistletoe – which never touches the earth – was not obligated by the oath (3). On a more scientific note, mistletoe was recognized as being a parasite of its host tree as far back as the 13th century by Albertus Magnus (4), a.k.a. St. Albert the Great, who gets my vote for patron saint of plant pathology.

In North Carolina mistletoe is a common sight this time of year in oaks and other hardwoods, since it stays green even after its host tree has dropped its leaves. I've seen mistletoes recently in sycamore and even in ornamental pear. They usually appear as round sprays of leafy stems high in the crown. This makes them inconvenient for gathering as holiday decor, and people have been known to bring them down with a shotgun blast. In some cases – this seems especially common in red maple – mistletoe occurs as a trunk infection, causing a gnarling and roughening of the bark. The mistletoe shoots coming from these trunks are more accessible to would-be collectors, but are rather too short for effective decorating. Unless someone is beating me to the bigger ones.

Leafy or true mistletoes in the New World belong mostly to the genus Phoradendron. They are classified in the Viscaceae, the same family as the famous Eurasian mistletoe, Viscum album. Both are "water parasites" which take up water and dissolved minerals from their host trees but photosynthesize most of their own food. As you might imagine, this arrangement is a more serious burden for trees in the drier parts of the country than here in the Southeast. You would not likely confuse the true mistletoes with their cousins, the dwarf mistletoes (Arceuthobium spp.). Not only are their size and appearance different, but dwarf mistletoes don't occur in North Carolina, even in the seemingly suitable climate of our mountains. They do occur in western and northern U.S. states, where they can be serious pests of conifers. 

Dwarf mistletoe. Photo by David Shew
If you look up mistletoe in a Spanish-English dictionary, you'll be given the word "muérdago", but in my wife's home in southern Mexico, mistletoe is referred to as "injerto". This word means "graft" and is a very apt moniker, given the xylem union between the mistletoe and its host. Interestingly, the mistletoe of my wife’s upbringing has showy flowers, much different from the inconspicuous flowers of our local types.

The mistletoe's whitish berries are eaten by birds, but the seeds pass through the birds’ digestive systems. Some are lucky enough to be deposited on thin-barked tree branches, where they are held by the seeds' sticky coating (Remember the family name Viscaceae?). After germination, instead of roots the young parasite forms structures that penetrate the branch and establish an infection. The mistletoe's shoots develop later, but are very small the first year. Growth continues in subsequent years, and the mistletoe will be with the branch for life. (6)

"Your heart's a dead tomato / splotched with moldy purple spots, Mr. Grinch." (7)

From all of us here at the Plant Disease Clinic, Happy Holidays! Please remember that we'll be closed December 26-30, 2011. We look forward to checking your tomatoes, trees, and the critters that bug you in 2012.

(1) Mel Tormé and Bob Wells. 1944. "The Christmas Song".
(2) Bussing, Arndt, ed. 2000. Mistletoe: The Genus Viscum. Harwood Academic Publishers. p.1
(3) Ibid., p.2
(4) Agrios, G. 1997. Plant Pathology, 4th ed. Academic Press. p.10
(6) Sinclair, W.A., and Lyon, H.H. 2005. Diseases of Trees and Shrubs, 2nd ed. Cornell University Press.
(7) Theodore "Seuss" Geisel. 1966. "You're a Mean One, Mr. Grinch".