Growth of Pyrenophora teres in planta during barley net blotch disease

The two forms of barley net blotch disease, spot form and net form, are caused by Pyrenophora teres f. maculata (Ptm) and P. teres f. teres (Ptt) respectively. While Ptm and Ptt are genetically very similar, their disease pathophysiologies are different. Ptm causes circular or elliptical brown lesions whereas Ptt causes distinctive dark-brown, longitudinal lesions. In this study, we have demonstrated that these distinct disease symptoms may be explained by differences in fungal growth. Ptm appeared to initially grow as a biotroph forming intracellular vesicles within epidermal cells before switching to necrotrophic (intercellular) growth in the mesophyll. In contrast, Ptt appeared to avoid the biotrophic stage and established itself within the mesophyll more quickly. Symptom development was considerably lessened during seedling and adult plant resistance to Ptt, even though growth of the pathogen was still quite similar to that seen on a susceptible cultivar (albeit not as extensively). In a detached leaf system, net blotch symptoms did not develop and extensive Ptt growth occurred on leaves of a resistant barley line, suggesting in planta studies are more reliable. The greater knowledge of both forms of P. teres contributed by this study may enable development of resistance strategies to these pathogens.