New study gets to root of ‘rose sick’ soil
A cardboard box has long been a homespun remedy against rose replant disease, but recent research has unravelled the complex soil microbiology which makes it work. By Bunny Guinness
Roses, the nation’s favourite flower, have been grown in beds for hundreds of years – and rose replant disease has been a thorn in the flesh of gardeners for just as long as we have been growing them. However, until now, the cause has never been identified. Rose replant disease (RRD) is so-called because, if new roses are planted in ground where roses have been growing previously, they not only fail to thrive, they look miserable and sulk – big time.
This is a worldwide problem. Many causes have been suggested, including eelworm, nutrient deficiency and pathogenic fungi.
Recently in these pages, I mentioned that the former Gardeners’ Question Time panellist Daphne Ledward had found that, by placing roses in their pots on the ground where she wanted them to grow (having enlarged the drainage hole of the pot to around 3cm diameter), the roses established well and did not appear to suffer from RRD.
This prompted a reader, Nick Previté, to ring me to say that more than 40 years ago he bought some roses from Henry Street (henrystreet.co.uk).
As Nick intended to plant them where roses had been grown before, Henry Street advised that if he planted them in a cardboard box filled with good soil, they would not suffer from RRD. And by the time the rose roots needed to extend into the surrounding soil, the cardboard would have rotted away. Nick followed his instructions and it worked brilliantly.
Intrigued, I then hit the scientific search engine “Google Scholar” and happened upon a scientific paper published last December which appears to have discovered a definite cause of RRD – although there may be others in different parts of the world. It goes a long way to explain exactly why both Daphne and Henry Street’s ideas help to avoid the problem.
WHAT IS ROSE REPLANT DISEASE?
Many gardeners find it difficult to recognise RRD. This is not surprising, since an affected rose shows no marked symptoms above ground, such as leaf spots or yellowing. In addition, it often takes a couple of years for a rose to establish well and get into its stride. So gardeners often assume the rose is “just settling in”.
What you do get with RRD, though, is about 50 per cent depression in growth above and below ground.
The intermodal growth will be shorter (the distance between two leaf nodes), the leaves will be smaller and the stems less stable and more fragile. Underground, the picture is much clearer. The roots are often black and shrivelled, which obviously limits the plant’s ability to thrive.
Over the past week, I have had some fascinating Zoom conversations with Professor Traud Winkelmann, one of the authors of the paper. Prof Winkelmann works at the Institute of Horticultural Production Systems at Leibniz University in Hanover, Germany.
They have been researching RRD since 2010, but received significant funding in 2015 from the Mathias Tantau Foundation (Herr Tantau was a famous German rose breeder).
This painstaking work involved growing Rosa corymbifera ‘Laxa’ (widely used in commercial rose growing as a rootstock) in replant diseaseinfected soils from two different locations, as well as a control where the roses were grown in disease-free gamma-irradiated soils. All soils were fed with a controlled-release fertiliser.
When the team examined the roots of all the roses at a cellular level they found frequent fungal infections and associated cellular disorders in the roses growing in the RRD-infected soil. They then identified the various pathogenic fungi (and bacteria) and found many fungi to be members of the Nectriaceae family.
In contrast, roses grown in the irradiated soil were healthy.
THE WORLD OF SOIL
The soil is a complex microbiome, teeming with many fungi, bacteria as well as other living organisms.
From their recent work, Prof Winkelmann suggests that bacteria of the Streptomyces family act as “door openers” for the pathogenic fungi to attack the roots.
A working hypothesis is that the bacteria are attracted to the phenolic compounds that the roses excrete, and these bacteria then weaken the rose root in some way, which allows fungal entry.
Until recently, many commercial rose growers would disinfect their soil using various fumigants before replanting a crop. But as you can imagine, these chemicals have far-reaching effects on the soil and have been found to permeate the soil groundwater in some cases.
In three years’ time, in 2024, the use of these chemicals will be banned in Germany, so other measures to inhibit replant diseases must be found. Hence, the funding of the research which will benefit rose lovers the world over.
Many other interesting findings have come out. It seems that apple roots seem to sense the pathogenic fungi and actively try to avoid infected soil.
This also applies to small soil organisms such as springtails, which also sense diseased soil and avoid it. Assuming rose roots respond in a similar way, this helps to explain the success of the cardboard box method of replanting – the roots confine themselves to the healthy soil, which is especially critical in the early stages of growth.
Mycorrhiza might have a role to play in combating RRD. Recent research found that mycorrhiza did not protect against severe infections, and cell damage was still caused by Nectriaceae in Rosa ‘Laxa’, but the work was not totally conclusive.
Scientists assume (based on work done on apples) that the rose needs to be established with the mycorrhiza for eight weeks before being exposed to a hostile environment.
Prof Winkelmann suggests that maybe rose growers should sell plants in containers with the mycorrhiza already incorporated.
The Rosa ‘Laxa’ rootstocks that were used in the research were seed-grown and transplanted into larger pots. However, when gardeners plant roses into the ground they are often bare-rooted specimens – and many of the roots will have been cut on lifting.
This might allow easier ingress of the pathogenic fungi, making roses grown in the garden even more vulnerable than those grown in infected soil in pots in the research.
WHAT’S THE ANSWER?
I asked Prof Winkelmann what she would do today, if she were replanting roses into “rose sick” soil.
She said first of all she would sow a crop of marigolds, specifically Tagetes patula Nema Mix (nema short for nematode) in May. These are a type of marigold that many tree nurserymen use when replanting trees, as they affect the soil nematode population and also bulk up mycorrhiza in the soil.
The nematodes apparently eat certain fungi and bacteria and so shape the soil microbiome, thus enriching the beneficial microbes (seed available from marshallsgarden.com).
Then, in October, she would dig the marigolds into the soil. The following year she would plant the roses in a cardboard box filled with good soil, making sure to water well until they were established. This might take a couple of years, but with a good chance of success.
Paper on “Early Fungal Infections with Relevance for Replant Disease in Fine Roots of the Rose rootstock Rosa corymbifera ‘Laxa’ ” published by