Home & Garden Columns

Garden Variety: Attack of The Mildew Kingdom

By Ron Sullivan
Friday November 17, 2006

I thrashed myself but good last weekend, just doing a little lightweight gardening. 

I was just cutting back some cabbage and weeding out a lot of nastygrass and blackberry, but it did rain just a bit before that, and much of the bed we were working in is in shade.  

So I’m whacking away merrily and what the heck? 

Where’d the air go? And I’m turning blue and wheezing like a punctured accordion and coughing like a whole TB ward and never mind the more unpleasant specifics: I realized I’d got nose-deep in some allergen and it’s a nasty surprise indeed.  

That allergen was almost certainly a mildew growing on the leaves of some hapless plant. There are a slew of mildews that affect garden plants, generally a particular mildew species (or more!) for each plant species. They find happy homes here because we have damp and fog so consistently in the warm growing seasons and, being mildew, they thrive in moisture and find lots of tender sweet cells to feed on then.  

Mildews don’t feed only on those live cells, as we know. They show up indoors in our north-facing rooms, on dampish walls and windowframes, on books and clothing. 

I lost my favorite pair of boots a couple of decades ago—knee-high purple suede with fringe around the tops, and my mother, of all people, had picked them out for me—to some evil-smelling white mildew, though I’d hung them from the closet ceiling for ventilation. Sometimes this California indoor/outdoor living thing goes too far.  

Mildews aren’t plants themselves. Some of them are fungi, members of a whole different kingdom. They have cell walls like plants, but composed of chitin, like shrimp shells. Mushrooms are fungi, of course, and so are lots of less tasty and more annoying things like athlete’s foot. Strange to think of eating something related to athlete’s foot. In fact, let’s stop thinking about it right now. 

Powdery mildews are fungi; they turn up on roses, grapes, apples, oh, lots of plants. 

They don’t need moisture to grow, but they need it to reproduce, and often spread their spores via water splashes or droplets from garden “debris”—which in happier circumstances is nice nutritious mulch from last year’s fallen leaves.  

Downy mildews (or water molds) are not fungi. They’re oomycetes, as are the Phytophthora that cause sudden oak death and potato blight, and the Pythium types that cause damping-off of plant seedlings. They have cell walls too, but composed of cellulose like plants. They’re “primitive” single-celled protists.  

Downy mildews show up on grapes too—poor grapes; it’s a wonder we still have wine—and other plants, and also use water to get their young distributed, though their sexual cells are more survivalist-oriented, tough enough to stand heat and drying. 

Some downy mildews make their living from fungi and from other oomycetes. It’s a dog-eat-dog world even if you’re not a dog.  

All these kingdoms picking on me in one garden foray! So much for being the crown of creation.