Drissa Silué

Glucosinolates in cauliflower as biochemical markers for resistance against downy mildew

Abstract Glucosinolate contents were quantified by HPLC in three susceptible (Billabong, Fanch and Jakavan) and two resistant (C300 and Maudez) cauliflower plants ( Brassica oleracea var. botrytis ) infected by Peronospora parasitica to determine a possible correlation between glucosinolates and resistance against downy mildew. Uninfected plants were used as controls. Three aliphatic (glucoiberin, sinigrin and glucoiberverin) and four indole glucosinolates (glucobrassicin, 4-methoxyglucobrassicin, 4-hydroxyglucobrassicin and neoglucobrassicin) were detected. Six days after inoculation with downy mildew spore suspension, the glucosinolate pattern of susceptible and resistant varieties differed. Sinigrin content was higher in resistant varieties than in susceptible ones. Glucobrassicin and methoxyglucobrassicin amounts expressed as a percentage to total indole glucosinolates were compared in infected and healthy seedlings. The susceptible seedlings exhibited a 12% decrease in glucobrassicin and a 25% increase in methoxyglucobrassicin when compared with healthy ones six days after treatment whereas no difference in glucobrassicin and a 10% increase in methoxyglucobrassicin were observed between healthy and inoculated resistant seedlings. Based on the ratio between methoxyglucobrassicin and glucobrassicin, resistant varieties were distinguished from susceptible ones. The susceptible varieties had a ratio greater than 1.5 but the resistant varieties showed a ratio of less than 1. This feature can be readily utilised as a biochemical inducible marker in screening seedlings resistant/susceptible to P. parasitica .

Phytogard® and DL-β-amino Butyric Acid (BABA) Induce Resistance to Downy Mildew (Bremia Lactucae) in Lettuce (Lactuca sativa L)

Downy mildew of lettuce (Bremia lactucae) is a serious disease. An alternative to chemicals is the application of disease resistance inducers. The aim of this study was to test whether DL-β-amino butyric acid (BABA) and Phytogard® (K2HPO3) could induce resistance in downy mildew susceptible plants. Aqueous solutions of BABA (0, 10, 20, 30, 50, 80, 100 mM) and Phytogard® (0.0, 5.8, 29.0, 40.6, 58.0 and 87.0 ppm) were sprayed on seven-day-old seedlings 0, 3, 7 and 15 days before or 1–3 days after inoculation with B. lactucae. Results obtained showed that Phytogard®- and BABA-induced resistance was dose-dependent. At 40.6 ppm for Phytogard® and 10 mM for BABA, complete protection was obtained. Both compounds had a curative effect and the induced resistance lasted for at least 15 days. It was also shown that both compounds induced systemic resistance in lettuce against downy mildew. Phytogard® at 40.6 ppm completely inhibited spore germination while BABA at 20 mM did not. Pathogenesis related (PR) protein analysis showed that BABA induced weak accumulation of PR-2, but not PR-1, PR-5 and PR-9. Phytogard® induced none of these proteins. The use of these two compounds to protect lettuce from B. lactucae is discussed.

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.