Clubroot in the cole crops : the interaction between Plasmodiophora brassicae and Brassica oleracea

The clubroot disease of the cole crops ( Brassica oleracea ) and other crucifers is caused by the fungus Plasmodiophora brassicae . It is an important disease, affecting an estimated 10 % of the total cultured area world-wide. The potential of cultural practices to reduce crop losses due to clubroot are limited, and chemical treatments to control the fungus are either banned due to environmental regulations or are not cost effective. Breeding of resistant cultivars therefore is an interesting alternative. This thesis addresses some aspects of the P. brassicae - B. oleracea interaction associated with resistance breeding.
A seedling test for clubroot resistance was developed. Symptom development in this test was shown to correlate well with symptom development in the field situation. The seedling test was used to identify B. oleracea accessions resistant to a Dutch field isolate of clubroot.
Resistance to the earliest stages of development of P. brassicae occurring in root hairs was shown not to be correlated with resistance to the development of clubroot symptoms. Presumably, only an absolute resistance to root hair stages of P. brassicae would prevent further symptom development, but no such absolute resistance was found.
The probability of infection of a plant by individual spores of P. brassicae was studied in a series of experiments with varying inoculum densities. The probability could be well described by a one-hit model, involving no between-spore interactions. A large betweentest variation in the infection probability was shown to occur, even between tests performed in the same climate room.
Two single-spore isolates (SSIs) obtained from one field isolate of P. brassicae proved to be indistinguishable from each other by testing their interactions with a series of 21 differential accessions, but to differ from the field isolate in their interaction with four of those hosts. The pathotype of those two SSIs was shown to occur at a low frequency in the field isolate. The fact that both SSIs contained the same pathotype indicated that pathotypes may differ in their likelihood to be isolated as SSIs. Results of inoculation experiments with mixtures of field isolate and one of the SSIs. suggested that in some hosts resistance to clubroot may be induced by certain pathotypes present in the field isolate.
From four clubroot-resistant accessions doubled haploid lines (DH-lines) were obtained through microspore culture and resistant DH-lines were selected. Progenies (F ₁ , F ₂ , backcrosses and a DH-population derived from one F ₁ ) of crosses of each of these four resistant DH-lines with a susceptible DH-line were obtained. Segregation for resistance in these progenies was studied and several genetic models were fitted to the observations. Classical genetic studies indicated evidence for one or two major resistance genes in three of the four resistant parental DH-lines. For the fourth resistant x susceptible cross, a genomic map was constructed based on RFLP and AFLP markers scored in a population of DH-lines obtained from the F ₁ . Two major resistance genes were mapped in this population. Evidence for the presence of at least one more resistance gene in the resistant parent was also found.
This study indicates that markers are a very useful tool to optimize breeding programmes for resistance to clubroot. The value of each gene has to be evaluated for specific growing regions as populations of P. brassicae may differ in their pathogenicity.