Short Communication: Latent Detection of Downy Mildew (Peronospora pisi) in Bioassays against Pisum sativum

Alexia Marr,Eirian Jones,Seona Casonato

doi:10.3390/pathogens10101312

Abstract

Downy mildew of peas is caused by the obligate parasite Peronospora pisi, which occurs sporadically throughout temperate pea-growing regions across the world. To screen pea lines against this biotrophic pathogen, a suitable and reproducible in vitro method using living plant material is required. Field screening can be influenced by environmental factors, thus giving variable results. The aim of this study was to develop a method that could reliably be used to screen pea cultivars against P. pisi in a laboratory setting. A range of bioassays were used to test various methods of inoculation, utilizing sporangia and naturally infested soil. Latent infection was achieved by planting seeds in soil collected from a site with a known history of P. pisi infection and directly inoculating young pea plants with sporangia. Out of the 108 plants which survived the experimental period, only two plants expressed visible signs of disease; however, through a two-step nested PCR process we detected latent infection in 24 plants. This research highlights the importance of considering the presence of latent infection when screening pea lines against downy mildew.

Highlights

Downy mildew disases are caused by Oomycete parasites that can infect and cause damage to a multitude of horticultural and ornamental plants
The use of fresh sporangia in Experiments 1–4 resulted in irregular asymptomatic infection; plants did not express signs of disease in any of the sporangial assays (Table 1)
Mence and Pegg (1971) [15] examined the effect of the inoculation site on systemic infection and reported that sporangial suspensions are more effective when pipetted into the bud rather than the youngest leaf lamina (60% became systemically infected when inoculated into the bud, compared with 2.5% when inoculated on the lamina), likely due to the increased susceptibility of younger tissues [16]

Summary

Introduction

Downy mildew disases are caused by Oomycete parasites that can infect and cause damage to a multitude of horticultural and ornamental plants. P. pisi) is the causal organism of downy mildew disease on peas (Pisum sativum) [1]. Peronospora pisi will often go undetected, remaining asymptomatic within the plants until a 12-h period of at least 90% relative humidity creates a conducive environment for disease expression [3]. This can result in sporadic outbreaks with the disease severity and expression being dependent on local environmental influences, such as the climate and weather, the mode of initial infection and the agricultural practices in the cropping area [2]

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