Using disease resistance to fight back against black rot in Brassica crops

So what's the problem?

Black rot of crucifers, caused by the bacterium Xanthomonas campestris pv. Campestris (Xcc), is considered the most serious disease of crucifer crops worldwide. This seedborne disease can affect all crucifer crops, which include cabbages, cauliflowers, broccoli and radishes. Yield can be affected in several ways: infected plants may die prematurely, heads may remain small, and quality may be reduced because of symptoms on the marketable part of the plant.

Plant breeding can be used to improve the resistance of crops to diseases. Disease resistance genes protect crops against pathogens that have the corresponding avirulence (avr) genes. Where protection comes from single, generally dominant disease resistance genes, this protection can be short-lived as the pathogen evolves through mutation and selection, favouring strains with altered or missing avr genes that are therefore able to cause disease. Broad spectrum disease resistance that is quantitative, involving several genes, is generally expected to be more durable, but is more difficult to exploit in breeding programmes. This project uses quantitative resistance to Xcc in Brassica rapa (turnip mustard) as a model to examine the basis of such disease resistance. Key practical outcomes of this work will be resistant plant material with tightly linked molecular markers, and information on the extent of natural variation for resistance in Brassica genomes that could be exploited in plant breeding to provide durable resistance to a very significant pathogen.

What is this project doing?

This multidisciplinary project combines expertise in genetics, breeding, genomics and pathology to generate novel information on quantitative resistance to black rot. Exciting new developments in Brassica rapa genomics and emerging information on the close relatedness of the Brassica and Arabidopsis genomes are being used to characterise the resistance and identify the genes involved. Genes can then be cloned and transformed into other Brassica crops.

As there are many strains of Xcc, the broad spectrum of resistance is being examined by testing against at least 50 isolates of the disease from a collection of over 150 currently available. Representative sets of the Brassica genepool will be screened to identify allelic variation and potential new candidates for resistance breeding. Well defined resistant germplasm with tightly linked molecular markers and information on allelic variation in Brassica genomes will be key practical outcomes. Pre-breeding lines of Brassicas resistant to black rot which can be used in cabbage and kale breeding programmes in East Africa will be identified.

Results so far

A survey of the status of black rot was conducted on smallholder farms in East Africa, from which a total of
250 isolates of the bacterium (141, 79 and 30 isolates from Kenya, Tanzania and Uganda respectively) were
isolated and categorized. Race (variety) 1 was observed in Kenya and Tanzania while race 4 was observed in
the three countries. Genomic fingerprinting with repetitive-PCR (a type of polymerase chain reaction that targets
the repetitive sequences in bacterial genomes) revealed clusters that did not depict significant correlations
between isolates and geographical location, isolates and host adaptation or isolates and race. It did however
demonstrate existence of genetic differences within the East African black rot strains indicating that it is not just
a cloned population.

The next activity is to assess field resistance of the third-generation brassicas bred during the project, together
with cabbage and kale varieties commonly grown in East Africa, and pre-breeding cabbage and kale lines for
resistance to black rot. This will involve replicated field trials throughout two seasons in Kenya. And, on-going
work involves fine mapping of the identified genomic regions.

Project reports

Project flyer

Project Manager

Daniel Karanja

Address: ICRAF Complex, United Nations Avenue, Gigiri, P.O. Box 633-00621, Nairobi, Kenya

Tel: +254 (0)20 7224462

Email: d.karanja@cabi.org

Project Team

Duncan Chacha

Address: ICRAF Complex, United Nations Avenue, Gigiri, P.O. Box 633-00621, Nairobi, Kenya

Tel: +254 20 7224462

Email: d.chacha@cabi.org