A year in life of powdery mildew Plant pathogen grows on outside of leaves

Severe powdery mildew on leaves of a young crape myrtle plant.

It’s late winter, and I’ve been enjoying a nice long rest. You could call it hibernating, like the Lowcountry alligators are doing right now. I’m alive, of course, but sleeping or resting, not doing anything except waiting for the warm weather of spring.

You might be surprised to learn where I am. I’m right here in Charleston, but not in view. I’m hidden on the buds of a crape myrtle tree.

This tree is not one of those new varieties bred at the U.S. National Arboretum in Maryland. Frankly, I hate those. Every time I try to approach one of them, they repel me. (Scientists call them resistant.)

This tree might be one of the older crape myrtles around the area. On the other hand, a builder who didn’t know what type of crape myrtle it was might have planted it not long ago in the yard of a newly built house.

Anyway, I should have properly introduced myself before now. I’m crape myrtle powdery mildew. Scientists give me the official name of Erysiphe australiana, but since I don’t know Latin (it wasn’t taught in my high school, even though technically it was on the books), I don’t care much for that name. How about if you call me Mildy for short?

I am proud to say that I am a unique individual. I know that every organism likes to think it is unique, but I really am. I grow on the outside of leaves, not in the wet, humid inside like most other plant pathogens.

I know (eye roll), this makes me more vulnerable to UV light, fungicides, biocontrol fungi and even biopesticides. I’ve heard the list over and over, but I still prefer being outside with fresh air and room to stretch and grow.

Once spring arrives, I hope we can get properly acquainted. Unfortunately, I can’t say for sure when that will be, since I have to wait until my gracious host tree is ready to open its buds, which depends on the air temperature. Once it stays around 50 degrees at night, the buds swell, and the new leaves pop out. That’s when it is time for me to wake up and start growing.

Growing means my tiny, oval spores swell and push out a small bud, a germ tube. This tube grows across the surface of a leaf, on either the top or the bottom. The tube branches and branches to make a nice round patch of white threads.

Getting all the food I need is easy. All I have to do is gently work some of my arms into cracks in the leaf and grab some fructose and amino acids. I try to be careful not to eat too much. I wouldn’t want to annoy my host so it starts resisting me. That would be a problem.

Soon it’s time to start planning for the future. You know, re-pro-du-cing. Making sure there’s more of me to go around after I’m gone. Eating and growing is fine and fun, but I have to focus on ensuring my legacy. The way I see it, it’s the reason I’m here.

Reproducing means making a whole bunch of new spores. They just pop out of specialized cells one after another in a short chain. When the wind blows, or an insect walks by, or those crazy squirrels chase each other up and down the tree, my spores bid adieu and fly away.

One of my favorite spots is on those sprouts that keep coming up at the base of the tree. They are nice and succulent, and it’s shady and a bit cooler down there.

Summer is not my favorite time of year. Day by day, as it gets hotter and hotter, I start to slow down. Once it’s July, I really don’t do much until the weather cools down again in the fall.

When my dear crape myrtle host makes new buds for next year, it’s time for me to make sure I once again have a nice, cozy spot to spend the winter.

For more, see “Crape Myrtle Diseases & Insect Pests” at http://bit.ly/1QIOCzT.

Anthony Keinath is professor of plant pathology at the Clemson Coastal Research & Education Center in Charleston. His expertise is in diseases of vegetables. He is also an avid gardener. Contact him at tknth@clemson.edu