Inter‐individual genetic variation in the temperature response of Leptosphaeria species pathogenic on oilseed rape

Fay Newbery,Faye Ritchie,Peter Gladders,Bruce D L Fitt,Michael W Shaw

doi:10.1111/ppa.13236

Abstract

AbstractIt is important to understand the likely response of plant pathogens to increased temperatures due to anthropogenic climate change. This includes evolutionary change due to selection on genetically based variation in growth rate with temperature. We attempted to quantify this in two ways. First, radial mycelial growth rates in agar culture were determined for a collection of 44 English isolates of Leptosphaeria maculans and 17 isolates of L. biglobosa, at 14 temperatures. For L. maculans the genotypic variances in four parameters were measured: minimum temperature allowing growth, optimum temperature, growth rate at the optimum temperature, and growth rate at the highest usable temperature, 31.8°C. The standard deviations were 0.068°C, 1.28°C, 0.21 mm/day, and 0.31 mm⋅day−1⋅°C−1, respectively. For L. biglobosa, these figures were, respectively: immeasurably small, 1.31°C, 0.053 mm/day, and 0.53 mm⋅day−1⋅°C−1. In addition, the incidence and severity of phoma stem canker in planta over a natural growing cycle at four temperatures (16, 20, 24, and 28°C) around the average culture optimum were determined. There was no correlation between in vitro and in planta growth, and the decrease in pathogen measures either side of the optimum temperature was much less for in planta growth than for in vitro growth. We conclude that both pathogens have the capacity to evolve to adapt to changes in environmental conditions, but that predictions of the effect of this adaptation, or estimates of heritability in natural conditions, cannot be made from measurements in vitro.

Highlights

Long-term predictions of plant disease severity, and prevalence, in response to anthropogenic climate change are needed in order to plan appropriate societal and production responses
Radial mycelial growth rates in agar culture were determined for a collection of 44 English isolates of Leptosphaeria maculans and 17 isolates of L. biglobosa, at 14 temperatures
The “environmental” variance for measured temperature response parameters was often larger than the variance attributable to genetic differences between isolates

Summary

| INTRODUCTION

Long-term predictions of plant disease severity, and prevalence, in response to anthropogenic climate change are needed in order to plan appropriate societal and production responses. In Leptosphaeria spp. on oilseed rape, there has been much research on the relation of environmental variables to pseudothecial maturation, canker development, and ascospore release, again focused on the average population response This is because available fungicides have little effect against Leptosphaeria spp. once the fungi have invaded the host stem, and advice on the most effective time to control the disease when leaf symptoms are apparent is important. The second model predicts the date of onset of stem canker symptoms in spring, relative to the date when 10% of plants have leaf spots This assumes a linear relationship of pathogen growth with thermal time. In order to allow a more robust data set with a manageable number of plants, fungal isolates, and temperatures, in planta growth was represented by canker incidence and severity, the growth measure most strongly correlated with yield loss rather than growth rate in petioles

| MATERIALS AND METHODS

Findings

| DISCUSSION