Histological characterization of downy mildew (Hyaloperonospora brassicae) resistance in Portuguese cabbage (Brassica oleracea var. tronchuda)

Abstract

Downy mildew in Brassica crops is caused by the oomycete Hyaloperonospora brassicae. A histological study was conducted with the objective of characterising the first stages of the infection by various H.brassicae isolates on hosts expressing different levels of resistance in order to determine what makes the resistance mechanism present in ‘Couve Algarvia’ so effective in controlling downy mildew. The inoculation of H.brassicae isolates P501, P005 and P006 on cotyledons of Brassica oleracea hosts KB207/00(resistant), KB14/00(partially resistant) and CrGC3.4(susceptible) revealed a range of interaction phenotypes including: KB207/00 - necrotic flecking on the upper surface of the cotyledon restricted to the infection area, no sporulation; KB14/00 similar to the aforementioned host, produced few conidiophores on the lower surface of the cotyledons and CrGC3.4 without necrosis but abundant sporulation on the infected area. For the histological study of cotyledons of the same H.brassicae isolate, x host combinations were sampled at 4, 6, 8, 10, 18, 24, 36 and 48 h after the inoculation (HAI) and prepared using the whole cotyledon clearing and staining technique to study the development of fungal infection and the host responses. The hyphal length and the number of haustoria per infection site were lower in the resistant and partially resistant hosts compared to the susceptible one, from 24 HAI. Thus, the susceptible host was densely colonised without apparent inhibition. The restricted fungal growth of the resistant hosts was associated to the autofluorescence and/or browning of the host cells at sites of pathogen infection and to haustoria encasement.