PyriculariaPopulations are Mostly Host-Specialized with Limited Reciprocal Cross-Infection Between Wheat and Endemic Grasses in Minas Gerais, Brazil

Abstract

Wheat blast, caused byPyricularia oryzaeTriticum (PoT), is an emergent threat to wheat production. Current understanding of the evolution and population biology of the pathogen and epidemiology of the disease has been based on phylogenomic studies that compared the wheat blast pathogen with isolates collected from grasses that were invasive to Brazilian wheat fields. Genetic similarity between isolates from wheat and grasses lead to the conclusion that significant cross-infection occurs, especially on signalgrass (Urochloaspp.); and this in turn prompted speculation that its widespread use as forage is a key driver of the disease epidemiology. We re-analyzed data from those studies and found that all but one of the isolates from non-wheat hosts were members of PoT and the relatedLolium-adapted lineage (PoL1), which meant that thePyriculariapopulations typically found on endemic grasses had not yet been sampled. To address this shortcoming, we performed a comprehensive sampling of blast lesions in wheat crops and endemic grasses found in and away from wheat fields in Minas Gerais. A total 1,368 diseased samples were collected (976 leaves of wheat and grasses and 392 wheat heads) which yielded a working collection of 564Pyriculariaisolates. We show that, contrary to earlier implications, PoT was rarely found on endemic grasses and, conversely, members of grass-adapted populations were rarely found on wheat. Instead, most populations were host-specialized with constituent isolates usually grouping according to their host-of-origin. With regard to the dominant role proposed for signalgrass in wheat blast epidemiology, we found only one PoT member in 67 isolates collected from signalgrass grown away from wheat fields, and only three members ofUrochloa-adapted populations among hundreds of isolates from wheat. Cross-inoculation assays on wheat and a signalgrass used in pastures (U. brizantha) suggested that the limited cross-infection observed in the field may be due to innate compatibility differences. Whether or not the observed level of cross-infection would be sufficient to provide an inoculum reservoir, or serve as a bridge between wheat growing regions, is questionable and, therefore, deserves further investigation.

Pyricularia oryzae: Lab star and field scourge

“…The different host‐specific lineages within P. oryzae were connected by significant and relatively recent genetic exchanges and, therefore, correspond to a single species. More recently, the narrow host range and lineage affiliation of the wheat‐infecting isolates of P. oryzae was further confirmed (Ascari et al., 2024).…”