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Sick soil syndrome can wreak havoc in your garden

  • Barbara Robertson
  • “Oh rose, thou are sick. The invisible worm … has found out thy bed of crimson joy: and his dark secret love does thy life destroy,” William Blake wrote.

    When I was in England recently on a garden tour, my horticulturist guide said, as we admired Sissinghurst’s roses, “You know you can’t plant a new rose where an old rose was planted. You must replace the soil.”

    “Really? Why?” I asked.

    “The soil can be toxic. The new plant will fail.”

    This was shocking news to me, but not, I’ve since learned, to rose enthusiasts and growers of various stone fruits and nuts.

    When I returned, I consulted the Royal Horticultural Society (RHS) and Marin Rose Society websites, and the voluminous reference material Master Gardeners have available through the University of California. Sure enough, “replant disease” — aka “sick soil syndrome” or “replant syndrome”— is a known problem.

    Unfortunately, there is no soil test for replant disease; the only way to diagnose the problem is to watch a newly planted species fail to grow and rule out other causes. Moreover, no one knows exactly what causes replant disease, so there’s no cure. An early theory was that substances secreted from old roots impacted new roots. More recent theories posit that the syndrome is due to a lethal mix of fungi, bacteria, nematodes, viruses and other organisms that survive in the soil.

    The Washington State University College of Agriculture website notes further that “replant disease’s complex of organisms can vary from site to site, region to region, and crop to crop. In ornamental roses, for example, roots of one rose need only to be growing a few months for the condition to occur, and if that plant were removed and another planted in the same place, the new plant would not grow well.”

    Thus, even though a rose or other affected plant is the right plant for the right spot in terms of microclimate, water needs and soil type, underground organisms might have compromised the spot. A young plant with an immature root system already traumatized by replanting can’t cope.

    The WSU website notes that apples, cherry and pear trees also experience replant disease, and that planting one type of fruit after another — a cherry after an apple — doesn’t help. RHS adds peach, plum and quince to the list, and occasionally, pine, raspberry, spruce, strawberry and grape vines. In fact, according to California Agriculture magazine, replant disease has affected more than one-third of California’s almond and stone fruit acreage.

    In years past, commercial growers would fumigate the land, and trees grown in fumigated soil were significantly larger than those in non-fumigated soil. The fumigant is now banned, so research on alternatives has begun, and hopefully this research will trickle down to home gardeners.

    One area of research is the application of arbuscular mycorrhizal fungi (AMF) to help plants withstand replant stress. A study in Poland determined that AMF had a positive impact on apple seedlings grown in soil taken from an orchard with a specific apple replant disease. But a similar study in Belgium reported that more research was needed. For their apple trees, a combination of two specific AMF strains was effective, but applying a general mycorrhizal strain alone did not help.

    Since replant disease is invisible and impossible to test, what can a home gardener do? If you suspect a newly planted species is affected, remove it, shake the soil off the roots, replant it elsewhere and it may recover. You can replace a plant in suspect soil with a different species. Or, as my guide in England suggested, remove the soil and plant the same species using new soil. Dig a hole large enough to spread the roots. Consider treating the roots with an appropriate mycorrhizal solution. And, top dress with compost.

    What about the soil you remove? Put it elsewhere. Fortunately, the devil’s brew doesn’t affect dissimilar families of plants.