Plant Pathology and Microbiology Department, Horticulture Research International, Wellesbourne, Warwick CV35 9EF, UK
At the recent ISMS Congress in Maastricht we presented a poster entitled “Mushroom Bacterial Soft Rots”, which attracted considerable interest. Mushroom soft rot outbreaks are by no means commonplace but, when they do occur, their often dramatic symptoms can cause consternation on the farm.
First indications may be sticky yellowish patches on caps, or discrete pitted lesions. In conditions conducive to the diseases (relative humidity RH >90%) mushrooms may dissolve in situ on the beds within 24-48h.
Fluorescent Pseudomonas species cause the majority of mushroom bacterial diseases (Fermor, 1987; Gill, 1995). However unidentified soft rot pathogens have appeared sporadically in the mushroom industry for many years (Fermor and Lincoln, 2000). Two such pathogens have been identified (Lincoln et al. 1991,1999) and their attributes compared and contrasted. Many pectinolytic bacteria including pseudomonads are found in soil and on leaf or root surfaces and are known to cause soft rots of fruits and vegetables. Until Burkholderia gladioli no bacterium had been associated with the soft rot of mushrooms (Lincoln et al.,1991). On the basis of conventional morphological, nutritional and physiological tests combined with host tests, fatty-acid analysis and serology a new pathovar B. gladioli agaricicola was described.The original mushroom soft rot outbreaks were generally associated with Agaricus bitorquis grown at 25-28° C. Subsequently the pathogen has been isolated from Agaricus bisporuscrops; usually grown at higher than the recommended RH. and temperature. Several years later on, soft rotted mushrooms were seen in crops of A bisporus, growing at much lower temperatures, and a novel bacterial pathogen was suspected. This proved to be the case and the organism has been described as Janthinobacterium agaricidamnosum sp. nov..(Lincoln et al.,1999)
By reducing the RH. in a cropping house to below 85% a loss in total yield may be experienced, but the loss of mushrooms to soft rot disease may be restricted, in the absence of any additional chemical controls.
A potential threat
Soft rot bacteria, if undetected, can cause potentially grave problems in mushroom marketing if serious lapses in post-harvest cold chain handling occur.
Both pathogens can also attack a range of mushrooms, including, Pleurotus species and Lentinula edodes, in controlled cropping experiments. We are unaware of any reports of these pathogens occurring in commercial crops of Shiitake; there are unsubstantiated observations of their presence in Oyster mushroom crops in Asia. B.gladioli pv. agaricicola is a potential threat to mushroom industries in tropical and sub-tropical countries andJ.agaricidamnosum should be recognized in similar light.
Fermor, TR. (1987). Bacterial diseases of edible mushrooms and their control. In: “Cultivating Edible Fungi”, Developments in Crop Science 10, 361-370. Eds. PJ. Wuest, DJ. Royse and RB. Beelman; Elsevier Sci. Pub., Amsterdam.
Fermor, TR. and Lincoln, SP. (2000). Mushroom soft rots. Mushroom News 48(4),16,17,20,21,23,24.
Gill, WM. (1995). Bacterial diseases of Agaricus mushrooms. Report of Tottori Mycological Institute 33, 34-55.
Lincoln,SP., Fermor,TR., Stead,DE. and Sellwood,JE. (1991). Bacterial soft rot of Agaricus bitorquis. Plant Pathology 40, 136-144.
Lincoln, SP., Fermor, TR.. and Tindall, BJ. (1999) Janthinobacterium agaricidamnosum sp. nov. a soft rot pathogen of Agaricus bisporus International Journal of Systematic Bacteriology 49, 1577-1589.