Of all the wilts, blasts, declines, spots, blights (early and late), smuts, fires, and other types of plant maladies that I’ve gotten to tour this semester as a TA for Plant Pathology, it’s the rusts – as boring and creaky as they sound – that have captured my heart. They’re everything you want in a fungus: edgy, shape-shifting, clever, misbehaved, and mysterious. They’re also some of our most important plant pathogens, culturally and economically. It was the coffee rust Hemileia vastatrix and its devastation of coffee plantations in Ceylon (now called Sri Lanka) in the 1870s that pushed the Brits to acquire a taste for tea. A wheat rust, Puccinia graminis, has evaded our best efforts at breeding resistant varieties of wheat, and creeps ever closer to the Middle East and the Indian subcontinent. Here in the Northeast, Cedar-apple rust (Gymnosporangium juniperis-virginae) sprouts bright orange horns and adorns cedar and juniper trees with unmistakeable alien blobs. Do some googling – you won’t be disappointed.
So what makes a rust a rust? Many of them do indeed create a blistery, red, orange or brown spore-producing growth that coats the host plant. Wheat rust, for example, would be hard to describe with any other word.
But part of what makes the whole group of “rusts” so sinister is that most have two separate plant hosts. The organism hops from host to alternate host seasonally and under the guise of five different spore types – making them very hard to pin down.
Why five spore types? For the rust fungi, it’s a matter of movement: something that most fungi lack in any obvious sense. So instead of wings or legs or even flagellae to carry these fungi across from host to host, these fungi delegate these tasks to a number of specialized spores. Each spore performs a different type of movement or storage: there’s genetic movement, of course, which happens in two different spore types (one spore to split the genes up, another to recombine them); but also there’s a spore for waiting/overwintering, a spore to hop from primary to alternate host, a different spore to hop back from the alternate host to the primary host, and yet another spore to re-infect the same primary host plant over and over again. It’s this last spore, called a uredia, that often causes the “rust” effect on the host plants.
Something that will never get old for me is the ability of microscopes to make a flake of a leaf or a speck of carpet dust into a visual Versailles under magnification. As a TA for the Plant Pathology lab, most of what we do is look at structures under high power – needless to say, I’m one happy clam. So today, as my fellow TA Emma and I were conducting the chaos of 40 students trying to trace out the steps of the life cycles of wheat–barberry rust and white pine–currant rust, I took a moment just to view a particularly lovely slide of teliospores – they’re the red blobs, roughly diamond-shaped, with a lateral cross-wall pictured at left– as they emerged from a telium on a wheat plant.
Maybe you don’t have a microscope, but you can still appreciate these creatures in many ways. An easy challenge to you all: go to any nearby apple tree (crabapple will do), and start looking at leaves with blotchy brown spots on them: turn the leaves upside down, and see what you see. If you find yourself starting suddenly at what looks like a mole sprouting thick tufts of wiry hairs – which I bet you will find if you look – you, my friend, are in the dear company of cedar-apple rust aecia. Who cares what exactly that means: you’re a guest in their world, so take a hand lens, get curious, and enjoy the alien beauty of these fungi.