Claudie Monot

Induction of systemic resistance in broccoli (Brassica oleracea var. botrytis) against downy mildew (Peronospora parasitica) by avirulent isolates

Abstract The aim of this study was to investigate whether pre-inoculation of susceptible brassica plants with avirulent isolates of Peronospora parasitica (downy mildew) could induce resistance against virulent isolates. The accessions Milady (broccoli) and Billabong (cauliflower) susceptible to the P. parasitica isolates FP15 and FP16 were used. Isolates FP06, FP07, FP11, and FP17 are avirulent on Milady but virulent on Billabong. Both accessions are nonhosts of Fusarium oxysporum , P. parasitica from Arabidopsis thaliana , and Bremia lactucae . Seven-day-old seedlings were first inoculated with the inducing organisms and then challenged 0–15 days later with the virulent P. parasitica isolates. Neither F. oxysporum , B. lactucae , the virulent brassica P. parasitica isolates, nor Noco2 induced resistance in Milady and Billabong against the virulent isolates. However, pre-inoculation of the seedlings of Milady with the avirulent isolates induced resistance against the virulent isolates (FP15 and FP16), and disease reduction ranged between 49% and 70%. The induced resistance (IR) was systemic and lasted at least 15 days. Seedlings expressing IR accumulated the pathogenesis-related proteins PR-2 and PR-5 but not PR-1, PR-3, and PR-9.

Evidence that a Leaf-Disk Test Allows Assessment of Isolate-Specific Resistance in Brassica oleracea Crops Against Downy Mildew (Peronospora parasitica)

A rapid resistance/susceptibility test for Peronospora parasitica (downy mildew) was established by inoculating leaf-disks of four Brassica oleracea accessions. Several conditions were tested: disk disinfection or not, agar medium with or without nutrients and with 50 or 100 ppm of benzimidazole. Using disinfected disks placed on agar (no nutrient and benzimidazole at 50 or 100 ppm), the responses of leaf-disks to four isolates were similar to those obtained using the classical cotyledon test, whereas undesired contaminations occurred in all other conditions. The possible effect of the particular leaf used for obtaining the disks was also studied. In each incompatible interaction tested, disks were resistant whatever the leaf used. In compatible interactions, susceptible phenotypes were observed on disks derived from the six lowest leaves, but disks from upper leaves were resistant. The genetic basis of resistance in a F1 hybrid broccoli was assessed, by testing six isolates on an F2 population derived from this hybrid. The cotyledon test only allows inoculation of two isolates per seedling, whereas many isolates can be tested on each plant by using leaf-disks. The segregation of the resistance to each of the six isolates was analysed: two dominant genes (tightly linked) control resistance to all isolates (one to five isolates; the other to only one isolate).

Comparison of the resistance and susceptibility to downy mildew [Hyaloperonospora parasitica, constant. (Pers. Ex Fr)] of nine Brassica olearacea accessions in laboratory, seedbed, and field screens

The relationship between resistance in seedlings, young and adult plants is studied for the pathosystem Brassica oleracea–Hyaloperonospora parasitica. Genotypes identified in the laboratory as resistant or susceptible or exhibiting a differential reaction to a selection of H. parasitica isolates were tested in 1997–1999 in seedbeds and fields under natural infestation. Isolates tested in the laboratory were grouped in five pathotypes, of which four were presented by isolates from Brittany, France. Genotypes susceptible to all pathotypes in the laboratory were also susceptible in the seedbed and field tests, while genotypes expressing a differential response to pathotypes were either resistant or susceptible. Accessions Everest, DEGC, ESPG and RS1105 exhibiting resistance to all pathotypes except I, were resistant in all environments and remained resistant in 2000–2002. Pathotype I was not prevailing in the field and results support the hypothesis that accessions resistant under laboratory conditions will be resistant under field conditions, provided the same pathotypes are present under the laboratory and field conditions.