Endophyte-pathogen continuum in Alternaria alternata

Endophytes are microorganisms that live within healthy plant foliage for at least a part of their life cycle without causing any sign of disease.Thus endophytes are defined by their niche, rather than their function.This means that symptomless and unobtrusive behavior of endophytes may be a transient phenomenon, and that the interacting partners may shift among a wide range of interactions from mutualism to parasitism to saprophytism or exploitation.Here, we aimed at clarifying whether Alternaria alternata isolates, formerly introduced by our lab as endophytic fungi of cupressaceous trees, could induce disease in other hosts.Here it is shown that such endophytic A. alternata isolates could induce disease on potato leaves.Indeed, by spraying the conidial suspensions of six A. alternata isolates on target potato, tomato and wheat seedlings, the symptoms of leaf spot were emerged on leaves of Marfona and Agria potato cultivars, but not on tomato and wheat foliage.The percent of disease intensity (PDI) on potato leaves ranged from 9% to 70%, depending on the fungal isolate.Assessment of disease intensity on the potato cultivars indicated that there were differences among pathogenicity of fungal isolates at Pd"0.01.Also, the host cultivars responded differently to the isolates.In conclusion, host-microbe interactions are often dynamic and one organism can change from one life style to another given the appropriate trigger, in this case: the host plant.Thus, our finding provides new insight into such complexity in relation to endophyte-pathogen continuum.