Xanthomonas campestris pv campestris black rot

This pathogen is one of the few afflicting Brassica crops where the pathogen is seed borne. Because this pathogen can cause devastating epidemics in seedlings being propagated under the intensive conditions used to raise transplants for eventual field crops, it is essential to increase the sensitivity and accuracy of test methods used by seed analysts. Application of prolonged extraction procedures and semi-selective agar media is recommended as a result of internationally coordinated tests (Koenraadt et al., 2005). Once seed

Table 7.4. Survival of sclerotia of Sclerotinia sclerotiorum in the field following covering with a plastic sheet and amendment with white mustard residues.

Treatment

Viable sclerotia (%)

Control

93

Mustard residues

82

Plastic sheeting

66

Plastic sheeting + mustard residues

39

Mean of three field experiments (adapted from the data presented by Thaning and Gerhardson, 2001).

infestation is established, then hot water treatment of fresh plump seed at 50°C for 25-30 min for cabbage, plus dressing with organomercurial dusts can provide effective control. The use of organomercurial seed dressings is being phased out, however, for reasons related to environmental protection. Treatment of infested seed with calcium hypochlorite for 16 h in sealed containers reduces infection but only below the threshold established for detection in seed intended for direct field drilling. Hence this technique has only limited value for seed destined to be used by transplant propagators. Other primary control measures include: phytosanitary inspections of seed crops, accompanied by roguing and seed certification; crop rotation; and avoidance of excessive irrigation. Wide spatial separation of seed and ware crops is essential for the production of healthy seed.

Periodic epidemics of black rot disease follow the introduction of susceptible cultivars, careless use of contaminated seeds and seedlings, and weather conditions favourable to the disease. Research suggests there are new highly aggressive variants of the pathogen, and breeding previously has been carried through in the absence of recognition of their existence. The pathogen can survive in soil even on plant debris for only 1-2 growing seasons. Survival on contaminated seeds and on weed crucifers is considered to be essential for the cycle of disease. Plant morphology and life cycle play important roles in the degree of black rot development in the field.

The rate of guttation is important in determining the susceptibility of cultivars. The ability of a pathogen to multiply in the vascular system is also an important factor in determining the eventual level of black rot symptoms. Vein plugging is due to accumulation of fibrillar material in the vessels, which plugs the veins and prevents spread. Resistance reactions develop at the gateway to entry in the hydathodes and in the vascular system to stop pathogen spread. Hence leaf resistance and stem resistance are governed by different genes. Race-specific resistance is seen at the site of inoculation as a hypersentive response (HR). Resistance in B. oleracea that is severely damaged by black rot is very low, with no true resistance in many botanical varieties and land races tested.

By comparison, a resistance developed in Japan was related to the use of heading Mediterranean kale of the 'Penca de Mirandella' type. Since the early 1990s, diseases caused by X. campestris have been spreading on to new hosts and into new regions. Brassica oleracea appears to be the most susceptible host; resistance genes have been identified and the gene-for-gene studies indicate that there are different physiological races. The relationship between X. campestris pv. campestris and the Brassica described in the triangle of U (1932) is shown in Fig. 7.20. This identifies sources of monogenic and multigenic resistance to the pathogen correlated with the six known physiological races of X. campestris pv. campestris.

Unrelated resistance has been identified in southeast Asian cabbage and in Portuguese Penca kale (Ignatov et al., 1998). The origin of the Asian cabbage was traced to the Flat Dutch group of varieties and heading Mediterranean kale. Some forms of resistance may be available in Chinese kale, broccoli and cabbage. Where pathogen-tolerant cabbage lines are available, the mechanism of resistance appears to be present in the hydathode. Co-inoculation of cabbages with X. campestris pv. carotae and X. campestris pv. campestris increased resistance (Cook and Robeson, 1986), hence some forms of cross-protective biological control might be developed. Determinants that trigger host resistance responses may reside on the bacterial cell surfaces (Roberts and Summerfield, 1987). Sources of resistance have been found in two accessions of B. carinata (PIs 199947 and 199949); these are thought to result from a single dominant gene. In B. rapa, resistance is quantitative and of moderate effect. Resistance to black rot was found initially in the Japanese cabbage cvs Early Fuji and Hugenot in the early 1950s. Later, PI 436606 from China, another cabbage, provided further sources of resistance. Williams et al. (1972) demonstrated that resistance in Early Fuji was due to one dominant major gene f, modified by one dominant and one recessive gene where f is in the heterozygous condition. There is, however, almost a continuous variation in the level of resistance from high resistance, but far from immune, to extreme susceptibility. There is also a form of seedling resistance regulated by an additional recessive gene (Vincente et al., 2000).

Screening can best be done at temperatures of 25-30°C. The source of resistance in B. carinata PI 199947 and also 1999949 gives quasi-immunity. This has been transferred into broccoli by fusion (Hansen and Earle, 1995). Following fusion, several generations of rigorous selection were necessary to obtain a true breeding line with the immunity level of resistance found in the B. carinata parent. Broccoli line 9811B was identified as the best and, following further selection, a true breeding subselection with high seedling and mature or field resistance was obtained.

The B. carinata resistance is dominant, but back-crosses still segregated. Guo, Dickson and Hunter (1991) transferred resistance from B. carinata to B. rapa by classical breeding. Ignatov et al. (1998) collected isolates from the UK, Japan and Russia, and identified five races whose inheritance for resistance

Blackrot Sheda

Fig. 7.20. Resistance to Xanthomonas campestris pv. campestris in Brassica spp., related to their position in the triangle of U (1935).

Fig. 7.20. Resistance to Xanthomonas campestris pv. campestris in Brassica spp., related to their position in the triangle of U (1935).

was controlled by dominant genes in races 1-4, and by a recessive gene r5 in race 5. A gene-for-gene relationship best explained the data. Kamoun et al. (1992) identified five races (0-4). Vincente et al. (2000) modified the numbering of races. They withdrew race 3 and separated Kamoun's race 1 into three new races (1, 3 and 5), and also proposed a gene-for-gene model to explain interactions between B. rapa and Brassica differentials. In the UK, race 1 predominates, and worldwide races 1 and 4 are of most significance.

Recent intensive screening of the USA Department of Agriculture (USDA) B. carinata (Ethiopian mustard) collection identified several potentially important sources of resistance to X. campestris pv. campestris. These are thought to offer dominant single gene resistance for use in cabbage and cauliflower (Tongug and Griffiths, 2004b). Interspecific hybrids of two B. carinata lines with B. oleracea are resistant to races 1 and 4 of X. campestris pv. campestris and were developed using embryo rescue. These offer a potentially valuable source of breeding material (Tongug and Griffiths, 2004c).

No line, so far, has been found to be resistant to all six races. The cv. Wrosa (B. oleracea) was susceptible to all six races, cv. Cobra (B. napus) and cv. Just Right (B. nigra) were resistant to race 4, cv. Seven Top Turnip (B. rapa) to races 2 and 4, PI199947 (B. carinata) to races 1, 3 and 4, cv. Florida Broad Leaved Mustard (B. juncea) to races 1, 3 and 4, and cv. Miracle (B. oleracea) to races 2, 3 and 5. Resistance in cauliflower is reputedly governed by dominant polygenes. The US PI 436606 originating from China is resistant in both juvenile and adult stages, these effects apparently being controlled by a single recessive allele (Dickson and Hunter, 1987; Hunter et al., 1987). Two ecotypes of A. thaliana have demonstrated a differential response to X. campestris pv. campestris, suggesting that ancestral resistance genes may be present outside the genus Brassica which could be of value in breeding programmes for crop brassicas.

Camargo et al. (1995) mapped quantitiative trait loci (QTLs) for juvenile and adult resistance in B. oleracea. Two regions on linkage groups 1 and 9 were associated with resistance at both juvenile and adult plant stages. Malvas et al. (1999) mapped 900 F2 individuals from a line 'Badger 16' X 'LC201' cross, and six randomly amplified polymorphic DNA (RAPD) markers were associated with resistance linked to petal colour. Alleles from the susceptible 'LC201' contributed towards resistance. The research field of mapping markers as an aid to developing resistance to black rot is developing rapidly.

A novel molecular approach was made by Morra and Earle (2001) utilizing chitinase genes that have been implicated in plant defence against fungal pathogens because of their chitinolytic activity. Chitin is a linear homopolymer of (3-1,4-linked N-acetylglucosamine. It is an important component of cell walls of fungi, constituting 3-60% of the wall. Chitinase genes cloned from plants and microorganisms have been inserted in the genome of a number of plant species including brassicas in order to achieve resistance against several fungal pathogens.

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