Lessons learnt: white mold in the garden

It’s liberating and humbling to write a lesson-learnt blog. Last year’s lesson-learnt evolved from my high-density plantings in a raised bed.

I divided my four ft. by four ft. raised bed into four quadrants. In one quadrant I planted three types of kale; the next quadrant beans; the third quadrant, beets; and the fourth quadrant, cucumbers and zucchini.

 

The plan was to rotate the vegetables according to nutrient needs: leafy crops, fruits, root vegetables and soil boosters.

 

Leafy crops like salad greens and members of the broccoli family (cabbage, kohlrabi, kale and mustard) are heavy feeders and use plenty of nitrogen.

Fruits, like tomatoes, peppers, cucumbers and squash need less nitrogen and more phosphorus and potassium.

Root vegetables, like carrots, beets, radishes and turnips need less nitrogen and more phosphorous.

Soil builders, like beans and peas add back nitrogen to the soil, which is why leafy greens are planted the year after!

 

The diagram below shows my planned crop rotation.

crop rotation diagram

 

I added tall sunflowers in the middle primarily to form a screen from the neighbours. The cheerful sunflowers didn’t go amiss either! So much for good intentions.

 

Since I was late in the season starting, I used seedlings for all my vegetables and started my sunflowers from seed. Everything took off, and before long the sunflowers were blooming and the cucumbers and zucchinis were popping off the vines.

 

So what when wrong? Unbeknownst to me, I had brought kale seedlings infected with white mold (Sclerotinia sclerotiorum). When one variety of kale started to rot, I didn’t pay it much mind. I just thought I had picked a bad variety for close plantings. I didn’t realize I had a problem until the sunflowers topped with their big sunny disk flowers suddenly wilted.

diseased sunflower

 

My laissez-faire approach to gardening, combined with close planting, abundant water, and a plethora of susceptible plant hosts created a white mold paradise.

 

I removed the diseased plant material out of the raised bed, but I had already contaminated the soil. Time to pour over textbooks and Google the disease on the Internet.

 

The news wasn’t good.

 

The fungus favours moist conditions and dense plant canopies. Bingo! Matches my growing conditions to a tee.

 

Not only does the fungus resting bodies (sclerotia) overwinter in plant debris and soil, but they can also remain dormant for 5 to 10 years!

resting bodies of white mold, Sclerotinia sclerotiorum

Sunflower stem filled with white mold resting bodies (sclerotia)

And if that wasn’t bad enough news, I discovered white mold has a huge host range. White mold can infect more than 370 ornamental plant species, field crops, weeds, and vegetables in 64 plant families

 

Susceptible plants other than kale and sunflowers include vegetables like tomato, squash, bean, beet and carrot; perennials like zinnia, chrysanthemum, columbine, delphinium and peony; and herbs like dill, chicory and sage.

 

So what should I do next year?

Although cultural controls won’t control white mold, it will reduce its severity. Any practice that reduces humidity and helps plants dry off quickly will help since white mold fungus needs moisture on the plant tissue to start an infection. This includes:

 

Mulch. Mulch the garden with organic mulch like woodchips or bark. Mulch keeps the plants away from the moist soil.

Manage Water. Use drip irrigation or soaker hose instead of overhead watering systems. I need to avoid saturating the soil or let it dry out completely. Plantings probably require about a 1-inch of water a week. And of course, keep watering the soil, not the foliage. And make sure the water drains quickly.

Move the air. Space plants apart from one another so air moves between them and dries them quickly. So, I need to avoid dense planting. Chose plants with an upright and open form because they will dry more quickly than plants that lie along the ground or grow in dense clumps.

 

And there are other practices that deal with the disease directly:

Sanitize. Of course, it’s important to get rid of the diseased plants, which I’ve already done by placing them in black plastic bags and sending them to the landfill. I can’t burn plants in my neighbourhood.

 Add compost. Adding clean healthy compost to the soil is another must-do practice. Make sure you add organic matter consistently to keep the levels of good organisms up. Adding lots of organic matter introduces more beneficial organisms into the soil, and shifts the balance away from the scalawags!

Sounds like whoo-whoo magic doesn’t it? But they are scientific studies backing up the use of compost in diseased soil. EPA lists the scientific studies supporting use of compost to control/suppress soil disease.

Crop rotation.  Crop rotation is of limited value because so many hosts exist, but you could use non susceptible corn and grasses for the next three to four years.

 

Elizabeth Stell’s  Secrets to Great Soil recommends a three-prong approach to manage/control sick soil. Stell says, the combo of crop rotation, high levels of organic matter & mulching allows gardeners to manage/control their sick soil. The multiple approach provides the best, longest-lasting control.

 

And if the disease is really bad… no, I’m not going to say “use chemicals”! Chemicals aren’t a good choice for many soil diseases because the strong chemicals kill off beneficial microorganisms as well as the pathogens. Strong chemicals only provide short-term control and require frequent use.

 

The answer to “what do you do if the disease is really bad” is solarize the soil.

Solarize. In hot sunny climes (like California) heat the soil by covering it with a clear plastic tarp (1.5 to 2mm) for 4 to 6 weeks during July/August.

 

But in my neck of the woods in southern Ontario or any cloudy, cool or humid climates, spread a second layer of plastic. Leave a little space between the plastic layers by using cans to separate the plastic layers.

 

The plastic sheets traps the sun’s radiant energy in the soil, heating the top 12 -18.“ Kills a wide range of soilborne pests. When properly done, the top 6 inches of the soil will heat up to as high as 140°F. But soil temps of 114 F are effective too.

 

Fortunately, many beneficial soil organisms are able to either survive solarization or recolonize the soil very quickly afterwards.

 

Lesson learnt this year: inspect plant carefully before buying them and only use good quality seeds. And most importantly, I need to pay attention to my garden. The garden was letting me know that there was a problem long before it blossomed into a full-scale fungal takeover. If only I had listened.

 

References

Deardorff, David and Kathryn Wadsworth. What’s Wrong with my Vegetable Garden? Timber Press. 2011

 

Environmental Protection Agency. Innovative uses of compost. Disease control for plants and animals

 

Jesiolowski Cebenko, Jill and Deborah L. Martin, editors. Insect, Disease & Weed ID Guide. Fin-it-fast organic solutions for your garden. Royale Organic Living Books. 2001

 

North Dakota State University Extension. Cheryl R. Biller & Martin A. Draper. White Mold of Vegetables and Ornamentals in the Home Garden. 2001

 

Royal Horticultural Society Sclerotinia disease

 

Stell, Elizabeth.  Secrets to Great Soil. Storey Publishing. 1998.

 

University of California Agricultural and Natural Resources. Soil Solarization for Gardens & Landscapes

 

University of Illinois Extension. Sclerotinia disease, white mold or watery soft rot. 2000

 

University of Minnesota Extension. Michelle Grabowski. White mold in the flower garden. 2007

 

 


Written by Cristina da Silva
Thursday, February 26, 2015 in Plants & Soils

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Comments

  1. This was quite the adventure. I hope you enjoyed it.


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