Abstract
A wild loose smut–summer annual grass interaction was studied to explore the relative importance of some local spatiotemporal patterns of variation for its existence. The prevalence‐related variable measured was the proportion of diseased plants (PDP). The mean annual PDP of nine consecutive seasons (2009–2017) was analysed using a generalized linear model with a binomial distribution considering covariables related to rainfall. During the seasons 2013–2015, the precise location of each sample within the plot was taken into account. The PDP of these seasons was analysed in various ways by means of generalized linear models, searching for its spatial variation with plant density in a given season, and with sorus and seeded inflorescence densities of the previous season. Symptomless plants were estimated as 6.1% of the 2015 population. The mean annual PDP ranged from 0.08 to 0.42 and covaried positively with precipitation. Within the field, two zones could be repeatedly delimited among seasons: one in which high plant densities and high PDP co‐occurred, and another with lower values of both in which PDP depended on the sorus density. The role played by differences in the encounter rate within and among seasons is discussed; lack of encounter could be as necessary as encounter for plant–pathogen coexistence over time.
Highlights
Studies of interactions between plants and organisms that use plants as nutritional resources have progressed under two coevolutionary perspectives over the past 60 years (Wininger & Rank, 2017); while studies on plant–herbivore interactions focus on an escape-and-radiate approach, plant–pathogen interactions focus mainly on a gene-for-gene approach under armsrace dynamics, where host and pathogen genotype frequencies oscillate over time in attacks and counterattacks
With the aim of shedding some empirical light on the causes of a high or low efficiency of infection, this study presents a particular case of a loose smut–summer annual grass interaction, Ustilago syntherismae–Digitaria sanguinalis, in which ustilospores and spikelets overwinter in the soil, infection can take place at an early seedling stage, the fungus is biotrophic and the disease is monocyclic, with only one cycle of both partners each year in a Mediterranean climate (Mas & Verdú, 2014)
If the within-year variation in plant density throughout the study plot was taken into account, the proportion of diseased plants (PDP) was strongly related to density (Fig. 3)
Summary
Studies of interactions between plants and organisms that use plants as nutritional resources have progressed under two coevolutionary perspectives over the past 60 years (Wininger & Rank, 2017); while studies on plant–herbivore interactions focus on an escape-and-radiate approach, plant–pathogen interactions focus mainly on a gene-for-gene approach under armsrace dynamics, where host and pathogen genotype frequencies oscillate over time in attacks and counterattacks.Recent reviews on wild plant–pathogen associations (e.g. Laine et al, 2011) gather empirical and analytical evidence that maintenance of resistance and virulence polymorphisms could be understood in accordance with either of the two abovementioned perspectives, depending on the particular characteristics of the interaction.
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