Tulip fire

A feature of spring in Wellington is tulip day at the Botanic Garden. According to the Friends of the Wellington Botanic Garden tulip day started in the 1940s. It is possible that the first tulip day was 16 October 1944 from information supplied by the Wellington City Archive however it became a significant event in 1948 following a donation of tulip bulbs by the Netherland’s government as a thank you for sheltering children during WWII. The gardeners plant about 24,000 bulbs for the spring display.

Tulip Sunday 1948 [photo Friends of the Wellington Botanic Garden]

Since moving back to Wellington last year I walk to work through the main garden and have watched the preparation, planting, and the emergence of the tulips over the last few months. At the end of last week, large area of tulips in one bed was removed and the bed mulched over.

Looking across the new mulch [photo Geoff Ridley, 18 September]

Signs have been erected and articles in the local papers explain that this bed has been badly affected by the disease ‘tulip fire’ (Anon, 2014). The disease is caused by a fungus Botrytis tulipae which infects all parts of the plant causing grey to brown lesions which may join up to cause a general dieback of the emerging tulips.

Tulips with tulip fire [photo Geoff Ridley, 18 September]

For a disease to develop there needs to a pathogen (Botrytis tulipae), a susceptible host (tulip), and a favourable environment. This is known as the disease triangle.

The disease triangle [diagram University of Wisconsin-Madison]

In this case, we have had the pathogen since before 1948 (Dingley, 1969) but do we have the other two factors? According to Cornell University’s fact sheet a favourable environment is “cool (15°C), rainy spring and summer weather … can be particularly damaging when rainy, drizzly weather continues over several days”. Well, that pretty much describes Wellington weather most of the time

So, we have two of the factors for disease development. That just leaves a susceptible host? The Botanic Garden has been growing tulips annually in these flower beds for 70 years with a low incidence of disease. The tulip cultivar most affected is the bright pink ‘Carola’ which was planted for the first time last year. So here is our susceptible host.

‘Carola’ [photo Jacques Amand]

Botanic Garden staff members removed many of the worst diseases plants to control the spread of the disease and intend to apply “a one-off fungicide to nip it in the bud” (Anon, 2014). While this will solve the immediate problem what other long term solutions are available? Then most obvious is to break the triangle and not grow susceptible cultivars such as ‘Carola’.

The second is to reduce the presence of the pathogen in the soil. Botrytis tulipae forms black survival bodies or sclerotia which survive in the soil when tulips are not present.

Sclerotia of Botrytis tulipae [photo 5458101, Sandra Jensen, Cornell University, Bugwood.org]

These could be eliminated by treating the soil with fungicide or other chemicals but this is not environmentally friendly. The other approach is to implement crop rotation as you would do in a vegetable garden. Interestingly there is not much written about this and I found only one article about in the Otago Daily Times (2014):

Crop rotation in the flower garden is also possible. Stocks and wallflowers in mass displays exhaust the soil in similar ways and should not follow each other. Likewise, if dahlias are grown in the same soil year after year, they will make excessive demands on some soil elements and plant quality will drop. Ideally, dahlias, gladioli, pansies, violas, tulips, hyacinths and narcissi should be planted in different areas of the garden each year. Rose-planting time has arrived and nurseries soon will be full of this most popular shrub.

References

Amand, J. The tulip gallery. http://www.thetulipgallery.com/view/859

Anon, 2014. Council gardeners fight fire for tulip Sunday. Independent Herald (Wellington, 17 September 2014): 18.

Cornell University, 2013. Botrytis blight of tulip. Plant Disease Diagnostic Clinic. http://plantclinic.cornell.edu/factsheets/botrytisblighttulip.pdf

Dingley, J.M. 1969. Records of Plant Diseases in New Zealand. DSIR Bulletin No. 192. Government Printer: Wellington, New Zealand.

Friends of the Wellington Botanic Garden. Tulips. http://friendswbg.org.nz/newTULIPS.htm

Jensen, S. 2012. Mycelial neck rot. Cornell University, Bugwood.org http://www.insectimages.org/browse/detail.cfm?imgnum=5458101

Otago Daily Times, 2014. Crop rotation important for healthy garden. 27 June 2014. http://www.odt.co.nz/lifestyle/home-garden/307255/crop-rotation-important-healthy-garden

Plant Pathology at the University of Wisconsin-Madison, General Master Gardener Training. The disease triangle. http://www.plantpath.wisc.edu/PDDCEducation/ppt/img1.php

Post script

The photo below was taken 7 October when the last of the disease affected ‘Carola’ tulips had been removed from the infected bed. What was left was the ‘Black Diamond’ cultivar which is obviously has a resistance to the pathogen.

Disease resistant ‘Black Diamond’ tulips [photo Geoff Ridley]