Comparative Studies on the Disease Prevalence and Population Dynamics of Ginger Blotch and Brown Blotch Pathogens of Button Mushrooms

Abstract

Introduction: Bacterial blotch is one of the most economically important diseases of button mushrooms. Knowledge on mechanisms of disease expression, inoculum thresholds and disease management is limited to the most-well known pathogen, Pseudomonas tolaasii. Recent outbreaks in Western Europe have been attributed to ‘P. gingeri’ and P. salomonii for ginger and brown blotch, respectively, although information on their identity, infection dynamics and pathogenicity is largely lacking. Methods: The disease pressure in an experimental mushroom cultivation facility was evaluated for ‘P. gingeri’ and P. salomonii over varying inoculation densities, casing soil types, environmental humidity and cultivation cycles. The pathogen population structures in the casing soils were simultaneously tracked across the cropping cycle using highly specific and sensitive TaqmanTM-qPCR assays. Results: ‘P. gingeri’ caused disease outbreaks at lower inoculum thresholds (104 cfu/g) in the soil than P. salomonii (105 cfu/g). Ginger blotch generically declined in later harvest cycles, although brown blotch did not. Casing soils were differentially suppressive to blotch diseases, based on their composition and supplementation. Endemic pathogen populations increased across the cultivation cycle although the inoculated pathogen populations were consistent between the 1st and 2nd flush. Conclusion: ‘P. gingeri’ and P. salomonii have unique infection and population dynamics, that vary over soil types. Their endemic populations are also differently abundant in peat-based casing soils. This knowledge is essential to interpret diagnostic results from screening mushroom farms and design localized disease control strategies.

Ginger blotches on Agaricus bisporus due to monoacetylphloroglucinol production by the pathogen Pseudomonas ‘gingeri’

“…‘ gingeri ’, which is the first report of it causing A. bisporus ginger blotch disease in China. This disease has a natural prevalence of approximately 3% in Guangxi, China, similar to that described by Taparia et al 4 . Unlike the brown blotches caused by other pathogenic bacteria, P .…”

“…Unlike the brown blotches caused by other pathogenic bacteria, P . ‘ gingeri ’ causes a ginger discoloration of A. bisporus and is the only causal agent of ginger blotch disease 4,10 . Isolation, purification, and virulence evaluation of P .…”

“…P. tolaasii , causing brown blotches, is the most studied bacterial pathogen. Research has reported on disease thresholds, pathogenic factors and their mechanisms, and disease prevention and management 4 . Studies have also shown that P. tolaasii synthesizes the extracellular toxin tolaasin, an 18 amino acid lipopeptide, which is its most important virulence factor, 5,6 as well as a variety of other virulence factors that cause lesions on the edible fungal fruiting bodies and inhibit the growth of aerial mycelia 5,7,8 .…”

“…However, many bacterial blotch pathogens of A. bisporus are also derived from casing soil 3 . Bacterial blotch is the most economically important disease in A. bisporus and is caused by a diversity of pathogens, mostly belonging to the genus Pseudomonas 4 …”