Debbie Symes
This article originally appeared on my Facebook blog ‘Rose of the Day’ in August 2018. Now revised with additional information from recent research papers.
Most rose growers want to remove an elderly, or poor performing rose at some point but worry about replanting in the same area. The general advice is to not replant in the same place until a year has passed as roses don’t like putting their roots into the soil where another rose has grown. If you have a small garden and love your roses you don’t want to look at a bare patch for the whole summer, do you? Today I will explain some research on the replant disease, and then consider strategies to overcome the problem.
Replant disease (RD) occurs predominantly in the Rosaceae family, affecting apples and cherries, as well as roses. The extant research predominantly considers apples due to the high commercial crop value, but the symptoms and causative agents are the same for the rose.
The symptoms of RD are poor top growth, small leaves, reduced and delayed blooming – size and number of blooms. The root growth is severely restricted, necrotic, and lacks the fine root hairs that take up both water and nutrients from the soil. This is what is known as a negative soil–plant disease.
Just a small point here. Rose Replant Disease which I refer to as RRD in this article is not the same condition as Rose Rosette Disease also known by the same initials RRD.
Research into Apple Replant Disease
The main body of research looks at apple replant disease (ARD) but there are similarities with rose replant disease (RRD). In a rather old paper Yim et al, (2013) ran a two season study looking at ARD, 2010 & 2011, where the experimental design took soil from two sites – one with replant problems – ‘sick’ and the other clear of problems ‘unsick’. These two types were divided into four categories – untreated, heat treated to 50°C, 100°C, and soil disinfectant- Basamid.
Apple stock was planted in these eight categories and evaluated over 10 weeks. At the end of the trial, the top growth was measured for height, the dry matter mass, and the dry matter mass of the roots. In 2010 the top growth dry mass was higher in the 50°C,100°C, and Basamid treated ‘sick’ soil, compared to the untreated group. These results matched the healthy ‘unsick’ soil group. Root dry mass results were similar. Roots in untreated apple ‘sick’ soil showed destroyed epidermal and cortical layers. The researchers concluded that heat and Basamid treatment removed a ‘problem’ in the soil.
What is the ‘problem’ in the soil? Included in this study were the genetic testing of the composition, and abundance of total soil bacteria, Bacillus, Pseudomonas, and total fungi. The researchers found clear differences between the untreated ‘sick’ soil and the ‘unsick’ soil. Heat and disinfectant treatment on the ‘sick’ soil samples results were similar to the ‘unsick’ soil, the treatment removed the ‘problem’.
This study shows there is a difference in the soil microbial makeup where apple trees have been grown in the recent past – the ‘sick’ soil and the clean ‘unsick’ soil where no apple trees grow. The specific soil microbes in the ‘sick’ had a negative effect on root growth. Soil science is vastly complex and the specific microbe in these replant ‘diseases’ has yet to be identified.
However, the following year (2011) trial showed a much smaller difference, hardly any in some of the groups. The entire study both years ran under controlled temperature, humidity, and light conditions in a greenhouse. Therefore, as well as the unidentified soil microbe destroying the roots, there is another unidentified agent at work. This may explain why sometimes you can get away with replanting a rose into ‘sick’ soil and sometimes you can’t.
Rose Replant Disease
The same research group, Yim et al, (2020) looked more closely at RRD analysing growth, root morphology, and metabolites, together with the microbial community in the rhizosphere – the soil around the roots, of the rootstock Rosa corymbifera ‘Laxa’. This rootstock is the predominant one used worldwide for rose propagation.
The ‘Laxa’ roses were grown in soil samples affected with RRD from two sites. The soil was either untreated or underwent γ-irradiation. As one would expect the roses grown in the irradiated soil showed better growth than those grown in the untreated soil. There were some soil site and treatment differences in the root metabolites. DNA and microscope analysis of the rhizosphere microbes discovered higher levels of the bacterial genus Streptomyces and the fungal genus Nectria in the untreated soil. This suggests that these are possible causes of RRD. Additionally, there were high levels of oomycetes belonging to the genus Pythiogeton also in the untreated soil. Oomycetes are one of the deadliest plant pathogens. Once believed to be fungi they are more closely related to algae. The presence of these microorganisms is similar to those found in ARD.
However, ARD is a complex disease, not fully understood, and less is known about RRD. Both diseases cause severe economic loss. Changing commercial growth sites is not always possible and soil treatment is both costly and environmentally damaging. Until more research is carried out these two options are the only methods of mitigating RRD.
How to get around RRD in your garden
You have a new rose that you are itching to plant in a place where an older rose has been removed. This is what to do. Get a large cardboard box, around 50cm in depth, height, and width, the dimensions are not precise as long as the rose fits comfortably into the box. I have also used strong paper horse feed sacks cut to size. Excavate all the ‘old soil’ from the planting site until your box will fit into the hole. This old soil can be lost somewhere else in your garden. You now need a box worth of ‘new’ soil – from a site that has not had roses growing in it. I realise with small gardens this isn’t so easy. You can use bagged compost and some garden centres sell topsoil. Or chat up someone with an allotment and do a soil swap. I usually add some well rotted manure to the ‘new’ soil. Now you can plant your new rose into the box with the ‘clean’ soil. The box will rot into the soil and by the time the rose roots are out of the box area into the surrounding soil a year will have passed so the levels of the rose ‘hating’ soil microorganisms will have dropped.
Should you use mycorrhizal fungi widely available online or from the garden centre? These are symbiotic fungi that partner up with plant roots. There is no scientific evidence that supports the use of mycorrhizal fungi as a cure for RRD. I used to use these fungi but having read Dr. Merlin Sheldrake’s book ‘This Entangled Life’ my ideas changed, This excellent book reveals the highly complex, organised world of mycorrhizal fungi but also shows our lack of knowledge in this critical area of soil science. The cheap mass produced mycorrhizal fungi sold to gardeners is likely to be incompatible with the particular soil biome in your garden. Do they help or harm? Read the book based on Merlin’s doctoral research on underground fungal networks in tropical forests in Panama.
You can see that RRD is an extremely complex problem. It would seem that changing the topsoil, planting into a cardboard box sunk into the ground and filled with ‘clean’ soil is a good strategy and at the moment the only feasible option. Or you can hope for the unidentified conditions in the 2011 trial where you do nothing and get away with it.
References.
Yim, B., Smalla, K. & Winkelmann, T. Evaluation of apple replant problems based on different soil disinfection treatments—links to soil microbial community structure? Plant Soil 366, 617–631 (2013). https://doi.org/10.1007/s11104-012-1454-6
B Yim, A Baumann, G Grunewaldt-Stöcker, B Liu, L Beerhues, S Zühlke, M Sapp, J Nesme, S J Sørensen, K Smalla, T Winkelmann, Rhizosphere microbial communities associated to rose replant disease: links to plant growth and root metabolites, Horticulture Research, Volume 7, 2020, 144, https://doi.org/10.1038/s41438-020-00365-2
Sheldrake, M. (2020). Entangled Life: how fungi make our worlds, change our minds & shape our futures. New York: Random House.
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