Inoculum potential of Sclerotinia sclerotiorum sclerotia depends on isolate and host plant

A Taylor,J S West,C Handy,J P Clarkson,C S Young,E Coventry

doi:10.1111/ppa.12843

Abstract

The soilborne fungus Sclerotinia sclerotiorum infects many important crop plants. Central to the success of this pathogen is the production of sclerotia, which enables survival in soil and constitutes the primary inoculum. This study aimed to determine how crop plant type and S. sclerotiorum isolate impact sclerotial production and germination and hence inoculum potential. Three S. sclerotiorum isolates (L6, L17, L44) were used to inoculate plants of bean, carrot, lettuce, oilseed rape (OSR) and potato, and the number and weight of sclerotia per plant quantified. Carpogenic germination of sclerotia collected from different hosts was also assessed for L6. Production of sclerotia was dependent on both crop plant type and S. sclerotiorum isolate, with OSR and lettuce supporting the greatest number (42–122) and weight (1.6–3.0 g) of sclerotia per plant. The largest sclerotia were produced on OSR (33–66 mg). The three S. sclerotiorum isolates exhibited a consistent pattern of sclerotial production irrespective of crop type; L6 produced large numbers of small sclerotia while L44 produced smaller numbers of large sclerotia, with L17 intermediate between the two. Germination rate and percentage was greatest for larger sclerotia (4.0–6.7 mm) and also varied between host plants. Combining sclerotial production data and typical field crop densities suggested that infected carrot and OSR could produce the greatest number (3944 m−2) and weight (73 g m−2) of S. sclerotiorum sclerotia, respectively, suggesting these crops potentially contribute a greater increase in inoculum. This information, once further validated in field trials, could be used to inform future crop rotation decisions.

Highlights

The cosmopolitan necrotrophic fungus Sclerotinia sclerotiorum (Lib) de Bary infects more than 400 species of plants throughout the world (Boland & Hall, 1994) leading to severe economic losses in a wide range of crops, including beans, carrot, lettuce, oilseed rape (OSR) and potatoes
Three S. sclerotiorum isolates were selected for further experiments on plants based on variation in number of sclerotia produced per plate (L6, L17 and L44 producing 44, 36 and 29 sclerotia per plate respectively)
Significant differences were observed in the mean number of sclerotia produced on the different host plants (P < 0.001, Fig. 2a), with the largest number produced on lettuce (122 sclerotia per plant for isolate L6) and the smallest number produced on bean (7 sclerotia per plant for isolate L44)

Summary

Introduction

The cosmopolitan necrotrophic fungus Sclerotinia sclerotiorum (Lib) de Bary infects more than 400 species of plants throughout the world (Boland & Hall, 1994) leading to severe economic losses in a wide range of crops, including beans, carrot, lettuce, oilseed rape (OSR) and potatoes. In OSR, S. sclerotiorum causes a stem rot and results in substantial yield losses in all major growing areas including Australia, Canada, UK and USA (Derbyshire & Denton Giles, 2016). In this case, flower petals are initially infected which fall onto leaves or leaf axils resulting in development of the disease in stems (Derbyshire & Denton Giles, 2016). S. sclerotiorum can be a significant disease in potato where it causes a stem rot, infecting the lower parts of the stems towards the end of the growing season, leading to reduced yields. As for OSR, stem lesions are associated with infected flowers dropping onto stems (Atallah & Johnson, 2004)

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