The Pathogen in your Poinsettia

Guest post: Kathie Hodge is a mycology professor in Cornell University's Dept. of Plant Pathology. She likes fungi best, and writes about them at the Cornell Mushroom Blog.

Poinsettia production greenhouse 

Poinsettias are worthless as house plants, Mr. Brown Thumb told us recently. I agree they are vexing things. Soon after Christmas they grow leggy, drop their leaves, and drip white sap on my seed catalogs. Further, they're reputed to be fatal to cats (untrue!). Yet it's hard to pitch them in the trash after Christmas, because there's no truer mark of a gardener than stubborn stick-to-it-iveness.

In fact, poinsettias are only worthwhile because they are intriguingly diseased. Commercially grown poinsettias are all infected with a pathogen called a phytoplasma. It is this phytoplasma that renders them bushy, compact, and temporarily desirable. In their natural habitat in Mexico, healthy poinsettias are unkempt, leggy weeds. That's right, the best-looking poinsettias are all diseased.

The poinsettia industry is huge, especially around Christmas time. In 2010, the US poinsettia crop was worth well over $146 million (wholesale!). They're cheap, perky, and come in several colors. In my experience, it is practically impossible not to obtain one. They owe their success to a phytoplasma.

A phytoplasma is a crummy kind of bacterium, sometimes charmingly called a "mollicute." Lacking walls, it can't survive outside a living host. It's very fussy about hosts, too: your poinsettia phytoplasma will not attack your orchid or cactus or you. It lives in the phloem of the plant--the part of the vascular system that carries nutrient-filled sap. Once inside, phytoplasmas sip sap and multiply; they alter hormone balance and wreak havoc. They may not kill their hosts outright, but often induce aberrations like witch's brooms, stubbiness, and yellowing (a stubby poinsettia is a good poinsettia). But here's their slickest trick: phytoplasmas are transmitted plant-to-plant via leafhoppers.

a leafhopper: Graphocephala sp

After a sip of sap, phytoplasmas in a leafhopper's gut move into its body cells, multiply, go for rides in the circulatory system, and then settle in the salivary glands. Now leafhopper spit is infective! This infective spit spreads phytoplasmas to the next plant the leafhopper probes with its syringe-like mouth. Despite their seeming crumminess, phytoplasmas are impressively co-evolved with their bug hosts. Which leafhopper species transmits poinsettia phytoplasma? We don't know--all of today's cultivars arose from a single mysteriously infected plant in the 1920s.

Phytoplasmas cause about 600 plant diseases, abetted by their insect partners. They include Ash yellows, Hibiscus witch's broom, and X-disease of stone fruits. Many cause real economic damage-- our poinsettia phytoplasma is an anomaly in that the disease symptoms it causes are considered handsome. Incidentally, there are plenty of less handsome poinsettia diseases to make poinsettia-growing difficult-- I especially admire the powdery mildew that blotches them in snowy white. And friends, if you have one of those leggy leftover poinsettias dropping leaves and sap on your windowsill, I hereby give you permission to get rid of it.
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I enjoy Professor Hodge's postings at Cornell Mushroom Blog. I'm no
mycologist. Honestly, my interests in fungi is proportional to how "weird" they are, or how much they appeal to my juvenile sense of humor. Ahem. Recently I commented on the professor's blog and she emailed to mention that she had recently been at this blog to read about houseplants. In our email exchanged she told me part of the story on the pathogen that make poinsettia look so bushy so I asked her to guest post here. Over at the Cornell Mushroom Blog I recommend reading: Evening glow, Puffballs at my mulch, and The Lichens of Tower Road among other entries. Thanks for the informative post.

Mushroom in my yard.