The mysterious ‘rose replant disease’ or ‘rose sickness’ has puzzled rose growers for years. Nothing specific has been identified as the cause of this phenomenon. When new roses are planted where old roses used to be, they may not grow as well as they would if they were planted in soil never planted with roses. Many suspect that key nutrients may be depleted in soils where roses have been grown for a long time, and as a result, the new roses do not get all the nutrition they need. Additionally, where roses have been grown for years without attention to good soil health, the soil’s structure and general physical properties may have declined — due to compaction and reduced organic matter. The soil may not effectively deliver oxygen, nutrition and water to the roots to support the vigorous growth of new roses.

We are fortunate that Traud Winkelmann and her colleagues (Bunlong Yim and Kornelia Smalla) are working to better understand rose replant disease. Winkelmann spoke on their research at the International Symposium on Rose Research and Cultivation this past August (RHA members Jim Sproul, David Byrne and I attended). Her talk was titled “Rose Replant Problems — Detection by bio-tests and investigation of bulk soil and rhizosphere microbial communities using DGGE fingerprints.”

Winkelmann described the challenge commercial nurseries face when growing roses year after year on the same land, and notes that reduced growth in rose crops on that land is typical over time. Many commercial German nurseries proactively address this problem by rotating rose crops with other crops on their land. For instance, when we toured W. Kordes’ Söhne we learned they rent neighboring agricultural land to have enough space to grow their roses with multiple years between rose crops on any given piece of land. The reason for the decline in growth of new roses on land where other roses once grew is still unclear. Winkelmann suggested the decline has been thought to result from nematodes, nutritional deficiencies, toxins and/or microbes.

To better document the cause or causes of rose replant disease, Winkelmann and her colleagues worked with three different German nurseries to acquire soil samples. Soil of different types (sandy loam and others) was collected at a depth of 5- to 20-cm, and the samples were also chosen based on how recently roses had been grown in the soil, and if there had been any other crop grown in the soil since roses were grown there. The soil samples were then each divided into three treatment groups: untreated, heat-treated (one hour at 50°C) and gamma-irradiated. In 3-liter pots containing all these soil variations, seedlings of Rosa corymbifera ‘Laxa’ and apple rootstocks (apples are also in the rose family) were planted and their growth documented. It was hoped that different growth of seedlings across the three soil treatment groups (untreated, heat-treated and irradiated), would shed some light on the cause(s) of reduced growth associated with rose replant disease. Heat can neutralize or weaken some toxins, and both heat and radiation can negatively impact soil organisms.

Across most soil samples, seedlings grew better in the heat-treated or irradiated soil than in the untreated soil. This result supports the hypothesis that an inhibitory factor is present in the soil and may be destroyed or neutralized by these two treatments. Winkelmann and her colleagues conducted denaturing gradient gel electrophoresis (DGGE) on a specific gene (16S rRNA) to begin to understand microbial diversity and how it differed between soils. Winkelmann was excited that, by analyzing all their soil samples and data, it appeared that the crop grown before roses were replanted had a stronger positive impact on rose replant issues than was gained through the heat and irradiation treatments on soils where roses had been grown most recently.

It sounds very promising that crop rotation and careful selection of the right non-rose species as an intercrop can dramatically reduce rose replant issues for commercial nurseries and avoid costly and toxic soil sterilization treatments. Winkelmann and her colleagues plan to continue their work by characterizing microbial species present in the soil and trying to understand how they vary in abundance across rose nursery soils managed in different ways. Learning which microbes are contributing the most to rose replant issues may suggest some ways to decrease rose replant issues.

Crop rotation (using other species between crops of roses) is practiced in Germany by some rose nurseries, and is also used in some of the German rose trials. We toured the German Variety Protection Office in Hannover. There, new roses are compared with established cultivars as part of the documentation necessary for securing European plant breeders rights. That location also hosts one of the Allgemeine Deutsche Rosen — neuheitenprüfung (ADR) trial sites. In the space allocated for the ADR trials, a portion of the field was planted in tall growing marigolds (Tagetes erecta; commonly called Mexican or African marigold) to prepare the soil for future rose plantings.

Hearing Winkelmann’s presentation and observing how rose growers handled rose replant issues at different tour stops helped me to realize how important this issue is. I wonder now to what extent rose replant issues may be affecting the rose seedling evaluations we rose breeders make.

I have very limited space and quickly reclaim land where I had rose seedlings to make room for the new ones. Perhaps some very nice roses are being overlooked due to mediocre performance just because of being planted in a pocket of soil with extra-strong residual effects from roses previously grown in that location.

After returning from the rose research symposium, I looked closely at areas of my garden in light of rose replant challenges and noticed something very interesting. In 2012, I expanded part of my rose garden, taking several feet of sod out to have enough room for the crop of new rose seedlings. This past summer the nicest looking seedlings typically have been in the part of the bed that was most recently in sod. As I look how I planted my family rows, part of each family is in the new bed and part of each family row extends into the older bed that has had roses growing continually since 2008.

Perhaps having the nicest looking rose selections in the newest section of the rosebed is a fluke, but it might also result from rose replant challenges making some of the roses less competitive. Hopefully someday I can solve this challenge by just having more land to rotate rose seedlings and having enough space to grow all I ever hope to, but I’m not sure if and when that dream can come true. In the meantime, I may just be a little more willing to select and grow on those seedlings which are better than average and are in pockets of the garden where plants seem to be struggling a bit more.

Field of marigolds adjacent to the current ADR rose trials in Hannover that will be planted in roses again in the future.

David Zlesak (zlesak[at]rocketmail[dot]com), ‘Advances in Understanding Rose Replant Disease’, Winter 2014. Buckeye Rose Bulletin, Mark Miller (tmille3[at]columbus[dor]rr[dot]com), ed. Buckeye District Rose Society. Reprinted from the Fall 2013 Rose Hybridizer Association Newsletter.