Genetic and molecular basis of nonhost disease resistance: complex, yes; silver bullet, no.

Nonhost resistance (NHR), in which a successful pathogen on some plants fails to overcome host barriers on others, has attracted much attention owing to its potential for robust crop improvement. Recent advances reveal that a multitude of underlying mechanisms contribute to NHR, ranging from components shared with recognition-based defenses up to recessive susceptibility factors involved in plant primary metabolism. Most NHR appears multi-factorial and quantitative. This implies that there is no single, 'silver bullet' NHR mechanism that can be used to broadly restrict pathogens in many or all crops.