Breeding Brassicas resistant to Black rot: An overview

The objective of this project is to breed Brassicas that can tolerate the gram‐negative bacterium Xanthomonas campestris pv. campestris.

Year of Publication2010

Brassicas especially cabbage (Brassica oleracea ssp. capitata) and kale (B. oleracea ssp. acephala) constitute a major component of the diet in East Africa (EA). However production is constrained by a number of factors especially pests and diseases particularly black rot. This disease is caused by the gram‐negative bacterium Xanthomonas campestris pv. campestris (Xcc). It is a seed‐borne pathogen and can cause severe losses especially in warm and wet environmental conditions. Available methods for control include use of disease‐free planting material (seeds or seedlings) and the elimination of potential sources of inoculum (infected crop debris and cruciferous weeds). However, development of disease resistant Brassicas provides the most cost effective and promising means of control. Previous studies have demonstrated that Xcc is highly pathogenically variable with seven (1‐7) races known to exist to date with races 1 and 4 being the most predominant. Unlike other races, sources of resistance to both race 1 and 4 within the C genome (B. oleracea) seem to be very rare or even non‐existent. However, broad‐spectrum quantitative resistance to both races has been found at a low frequency in other species of the genus Brassica (B. rapa, B. nigra and B. carinata). Recently, quantitative trait loci (QTL) analysis revealed a highly significant QTL for race 1 and 4 on linkage group (LG) 6 of B. rapa with two additional QTLs for resistance to race 4 found on LGs A02 and A09. The overall objective of our project is to breed Brassicas that can tolerate Xcc for the benefit of EA smallholders. A recent survey in which Xcc infected Brassicas were collected from EA (Kenya, Tanzania and Uganda) has revealed a predomination of race 4 in Uganda and races 1 and 4 in Kenya and Tanzania. Hierarchical clustering using profiles generated from genomic fingerprinting using repetitive PCR did not reveal significant correlations between isolates and geographical location, isolates and host adaptation or isolates and race. However it did demonstrate the enormous genetic differences within this population of Xcc, an indication that this is not a similar clonal population of the same genetic background. With races 1 and 4 being the most predominant in EA current studies are aimed at fine mapping previously identified QTLs for resistance to both races 1 and 4 with a view of discovering candidate genes that underlie those QTLs and markers for molecular breeding. We will also screen diverse Brassica accessions for additional sources of resistance.

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