All Mold Is Not Alike: The Importance of Intraspecific Diversity in Necrotrophic Plant Pathogens

Pathogens commonly possess naturally occurring intraspecific variation for traits associated with pathogenicity or virulence. Studies of host–pathogen interactions frequently fail to acknowledge this variation, particularly in studies of necrotrophic plant pathogens, where the molecular bases of defense are largely unknown. Necrotrophic plant pathogens, in contrast to obligate parasites of living plant cells known as biotrophs, kill plant cells before consuming them and may survive in the absence of living host cells in dormant or saprophytic states [1]–[4]. Necrotrophs may kill host cells using an array of toxins, although it is also proposed that these pathogens may activate plant immune responses designed to work against biotrophic pathogens, thus encouraging plant cells to kill themselves [5]–[9]. While many pathogen species cannot be clearly classified as either biotrophic or necrotrophic, as they shift lifestyles over the course of interactions with their hosts, commonly recognized necrotrophic plant pathogens include various species of Botrytis and Alternaria, as well as Sclerotinia sclerotiorum, Pythium irregulare, and Plectosphaerella cucurmerina [2],[10]. Of these, Botrytis cinerea, a highly generalist pathogen, and Alternaria brassicicola, a specialist pathogen of Brassica, dominate research on molecular mechanisms of plant defense against necrotrophic pathogens.