Cryptic fungal infections: the hidden agenda of plant pathogens

INTRODUCTION Host-microbe interactions have traditionally been viewed primarily from the perspective of pathogenesis and disease. The principal assumption is that a susceptible host will support microbial growth inside host tissues, leading eventually to the development of symptoms and disease. Inversely, non-susceptible (resistant) hosts will block pathogen infection and the expression of disease (Casadevall and Pirofski, 2001; Hok et al., 2010). Compatible host-microbe interactions, however, do not always result in overt negative effects, and asymptomatic or cryptic fungal infections are increasingly recognized as a common feature of many symbiotic associations between fungi and their hosts (Rodriguez et al., 2009; Porras-Alfaro and Bayman, 2011; Malcolm et al., 2013). It is now generally accepted that extensive intra (Stone, 1987; Bernardi-Wenzel et al., 2010) and intercellular (Schulz and Boyle, 2006) (Gómez-Vidal et al., 2006; Rodriguez et al., 2009) colonization of plants by endophytic fungi and other microorganisms (Thomas and Sekhar, 2014) is the norm, and that it would not be unusual for an individual host to sustain the growth and development of dozens of microbial taxa (Suryanarayanan et al., 2003; Ganley and Newcombe, 2006; Gazis and Chaverri, 2010; De Siqueira et al., 2011). Consequently, it should also not come as a surprise to discover that many of the fungi that we know only as pathogens have in fact a much broader repertoire of ecological interactions with their hosts, which may include the capacity to infect and colonize them without inducing any visible damage (Malcolm et al., 2013). Despite this fact, we still largely regard these relationships as aberrations and not typical of the manner in which fungi interact with their hosts, often excluding them from the study of host-microbe interactions with potentially adverse consequences on disease control (Filipe et al., 2012). Here we use selected examples to emphasize how cryptic associations between fungi and plants can influence our capacity to manage plant diseases, while also providing insights into the origins of plant pathogens.