Camellia plant resistance and susceptibility to petal blight disease are defined by the timing of defence responses.

The family Sclerotiniaceae includes important phytopathogens, such as Botrytis cinerea and Sclerotinia sclerotiorum, which activate plant immune responses to facilitate infection propagation. The mechanisms of plant resistance to these necrotrophic pathogens are still poorly understood. To discover mechanisms of resistance we used the Ciborinia camelliae (Sclerotiniaceae)-Camellia spp. pathosystem. This fungus induces rapid infection of the blooms of susceptible cultivar 'Nicky Crisp' (Camellia japonica x Camellia pitardii var. pitardii), while Camellia lutchuensis is highly resistant. Genome-wide analysis of gene expression in resistant plants revealed fast modulation of host transcriptional activity six hours after ascospore inoculation. Ascospores induced the same defence pathways in the susceptible Camellia cultivar, but much delayed and coinciding with disease development. We next tested the hypothesis that differences in defence timing influences disease outcome. We induced early defence in the susceptible cultivar using methyl jasmonate and this strongly reduced disease development. Conversely, delaying the response in the resistant species, by infecting it with actively growing fungal mycelium, increased susceptibility. The same plant defence pathways therefore contribute to both resistance and susceptibility, suggesting that defence timing is a critical factor in plant health and resistance against necrotrophic pathogens may occur during the initial biotrophy-like stages.