Evaluations carried out by the Genetic Resources Unit, Horticulture Research International, Wellesbourne, UK as part of the RESGEN project.
Resistance to Albugo candida
solate 7V from Canada was supplied by the Institute of Arable Crops Research, Rothamsted. Cotyledons containing ripe pustules were used to provide the inoculum, these cotyledons being taken from susceptible maintenance plants or the controls of the previous experiment. If this was not possible, dry spores were used directly from the freezer.
The Acem2 isolate was collected from a wild plant of Capsella bursa-pastoris (L.) Medicus, Shepherd’s Purse. This weed plant has long been suspected of acting as a winter host for the White blister fungus. The objective of using the Acem2 isolate was to score the reaction of the B. rapa core collection for susceptibility in order to assess the extent of cross contamination from a common field crucifer weed.
The cotyledons were placed in Coulter pots and 1-2 mls of distilled, filtered and chilled water was added. The pustules released the spores upon agitation. The spore suspension was placed in an incubator at a temperature of 15 ± 1 °C until the sporangia released the zoospores (in about 2 hours). The inoculum concentration was adjusted to 1 x 105 swimming zoospores/ml using a haemocytometer. The spore suspension was kept on ice and the inoculation of seedlings was carried out as soon as possible after inoculum preparation.
15 plants per accession in 1 tray. Seeds were sown in seed trays in a glasshouse at a temperature of 20 ± 2 °C with an 18-hour photoperiod. Seedlings were inoculated 10 days after sowing. Each accession was thinned to 15 individuals at this time. One 5 μl drop of spore suspension was placed on each cotyledon. Once all seedlings were inoculated, the tray was covered with a propagator lid with vents closed. The tray was then immediately transferred to a growth room at a temperature of 15 ±1 °C with a 12-hour photoperiod, where it was kept throughout the experiment. Scoring took place at 10 days and 14 days after inoculation.
The scoring system was developed to take into account the host plant response and the growth of the pathogen as measured by the amount and type of sporulation. The combination of these characters gave six interaction phenotype classes of host/pathogen interaction, which were used to categorise the seedling reactions, as shown below. The most susceptible response on each plant was recorded.
Host plant response
n = no symptoms
(F) = Slight necrotic flecking
F = heavy necrotic flecking
dN = diffuse necrosis
ch = chlorosis
tc = tissue collapse
Pathogen growth
N = No sporulation
pu = small pustules on upper surface of cotyledon
spl = small pustules on lower surface of cotyledon
lpl = large or coalesced pustules on lower surface of cotyledon
Interaction Phenotypes
NN. = No host response, no sporulation
(F)N = Light necrotic flecking, no sporulation
FN = Heavy necrotic flecking, no sporulation
S1 = Any host response, small pustules on upper surface
S2 = Any host response, small pustules on lower surface
S3 = Any host response, large or coalesced pustules on lower surface
The results were scored following the system developed by Leckie et al (1996), whereby plant reaction and pathogen growth were recorded in an Excel table. Based on these observations individual plants were classified into the 6 interaction phenotypes defined above. Leckie et al (1996) judged that plants showing NN, (F) N and FN interaction phenotypes exhibited some resistance to the pathogen, while plants with the S1, S2 and S3 interaction phenotypes were susceptible. Each plant was recorded as not showing (N) or showing (S) sporulation. The total number of plants exhibiting N and S interaction phenotypes for each accession in each trial was recorded and the average calculated.
Leckie, D., Astley, D., Crute, I.R., Ellis, P.R., Pink, D.A.C., Boukema, I.W., Monteiro, A.A. & Dias, J.S. (1996). The location & exploitation of genes for pest & disease resistance in European gene bank collections of horticultural brassicas. Proc. Int. Sym. on Brassicas, Ninth Crucifer Genetics Workshop. Ed. J.S. Dias, I. Crute & A.A. Monteiro. Acta Hort. 407, 95-101, ISHS 1996.