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Gardening: White mold hates the heat

Gardening

Inner leaves of a collard plant are affected with white mold. Note the young gray sclerotia forming on the edge of a leaf. Sclerotia turn black as they mature. Provided/Anthony Keinath

I am the authentic “white mold,” Sclerotinia sclerotiorum. You know, I really should have trademarked my name, because so many people think any time they see a white fuzzy growth, it’s me. That’s rather flattering, but I’m not that common, especially not in summer.

When the temperature reaches 85 degrees, it just gets too hot for me. I can’t take the heat, so I make like a bear in winter and hibernate until the weather cools down again. If you want to impress your friends, tell them I “aestivate,” which means I sleep in the summer.

My favorite vegetables to eat are collards and green beans. Vegetables are good for you, so I try to eat a lot of them: broccoli, cabbage and kale; carrots, dill and parsley; lettuce, pumpkins and tomatoes. I’m really glad people are planting more vegetables during the winter.

But you can’t live just on vegetables. Farmers plant a lot of other crops that appeal to me: canola, peanut, soybean and sunflower. As you can tell, I like plenty of oil and protein in my diet, too.

During the winter, when these crops aren’t available, I’ve decided that some flowers, especially snapdragon and stock, are just as tasty. I prefer the blossoms and peduncles (the stalks the flowers are on) instead of the leaves.

Gardening

A young savoy cabbage showing watery soft rot and white mold growing on the base of the leaves. Provided/Anthony Keinath

If you cut open a diseased tomato or sunflower stem, you probably will find my sclerotia, hard black “nuggets” of my dried mold that are my lifeline, literally. (Someone also said they look like mouse droppings, but that’s gross.) My tough sclerotia allow me to survive in the ground for years and years, as I weather the heat and the cold, the wet and the dry.

As they say, don’t put all your eggs in one basket. I don’t rely just on my sclerotia to get me through hard times. I also make ascospores, airborne spores that help me spread around.

Everything’s planned out step by step. Cold weather doesn’t hurt my sclerotia. In fact, it’s a signal that it’s time for me to wake up. About a month after the first cold snap, tiny mushrooms push up from any sclerotia that are just barely under ground level. Unlike toadstools, the caps of my mushrooms are upside down, so the ascospores float away instead of falling to the ground. It looks like this: bit.ly/2NrKlZN. If you want to read more about my floating masses of spores, see this article: bit.ly/35QygUA.

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In the air, my ascospores float around and until they land on leaves or flowers. A trick I learned is that if an ascospore lands on a wild radish flower that fell on a collard leaf, the flower offers a high calorie snack to power me up after traveling, so I’m ready to attack the collards with gusto.

This is one time I literally look like a white mold. I crawl all over the collards, slowly dissolving them with digestive enzymes leaking out of my cottony white “threads.” As the collards turn to mush, some people call this disease “watery soft rot.”

You might think once the cabbage is rotted and disgusting, that’s the end. Nope. Before the food runs out, I make new sclerotia, and then rest all summer.

Farmers and gardeners try to thwart my life cycle by burying the diseased crops and my sclerotia with disking, plowing or double-digging. Since my mushrooms are short and tiny, the mushroom caps can’t poke above ground level if the sclerotia are buried 4 inches deep, so there’s no ascospores to worry about.

But I don’t give up that easily. Sclerotia 4 to 10 inches underground can sprout, grow and attack plants directly. As soil is disked again to get it ready for planting, some of my sclerotia work their way back to the surface, too.

One thing that makes life easy for me is beans, parsley and other crops planted so close together that the inside of the plant canopy takes a long time to dry out after it rains. I hate the heat, but I love moisture.

That about sums up my life story. This mild winter is perfect for me, so I’m sure we’ll see each other soon.

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.

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