Detection and control of pathogens causing blotch and green mould in mushroom cultivation

This project addresses two of the highest priority diseases in UK mushroom cultivation, namely bacterial blotch disease and green mould, to improve detection and control.

Bacterial blotch disease caused by Pseudomonas spp. is one of the most important disease problems currently faced by the mushroom industry in the UK and elsewhere in Europe. The disease is favoured by high relative air humidity that can lead to condensation on the caps when temperature fluctuates and ventilation is insufficient. Previous work in AHDB project M063  has contributed to our understanding of the phylogenetic relationships between representative strains of blotch causing severe symptoms (P. tolaasii and P. gingeri) however the relationship between the range of Pseudomonas pathogens causing blotch, particularly those causing mild symptoms, is still poorly understood. This project will utilise an existing isolate collection, now held at FERA, in addition to isolates collected within this project and characterise them genotypically and for pathogenicity using bioassays. The early detection of the pathogens is essential for the control of the disease. AHDB projects  M054  and  M063  developed diagnostic tests for Pseudomonas tolaasii and gingeri respectively, however there are currently no methods for the detection of all pathogens that cause the disease, this project addresses this by developing a suite of diagnostic tests and making them available to the industry. It is also important to ensure the discrimination of potential pathogens from related beneficial pseudomonads that stimulate initiation of mushrooms and might also suppress pathogenic populations. This project will examine changes in microbial populations of mushroom casing during the development of disease to identify changes in communities of pathogenic and beneficial bacteria which may be exploited for better disease control. There are still no practical treatments for bacterial blotch and the disease is generally managed by cultural practices that include reducing the humidity in the growing rooms. The manipulation of the environmental conditions through balancing the temperature, irrigation and ventilation helps control bacterial blotch, though these methods have limitations, are dependent on outside conditions, and can lead to cracking and yield reductions due to high temperatures and low humidity. In some cases, environmental regulation is not effective, and disease can spread causing significant loses. This project will evaluate biological (including bacteriophage) treatments to control the spread of disease.

Green mould caused by the fungus Trichoderma aggressivum f. europeum can cause severe mushroom crop losses in the UK and a different form, T. aggressivum f. aggressivum has caused significant losses in North America and remains a threat for the UK. In addition, two other species T. harzianum and T. atroviride are present in the UK, but do not generally cause economic losses. In 2013, it was estimated that the cost of green mould to the UK mushroom industry could be £0.5M – 1.0M annually  (M057). Green mould is generally controlled by compost pasteurisation and sanitation measures including the use of disinfectants, but it is difficult to achieve eradication of the pathogen and some disinfectants leave residues and therefore should be used only where they are unlikely to come in contact with the crop or substrates. Although prevention has to be central to avoid this disease, when the infection occurs in a producing facility, appropriate strategies for control should be in place. This project will develop a detection system for Trichoderma species as a farm hygiene indicator of T. aggressivum and other mushroom pathogens and test novel control strategies for disease control.

Three main aims:

• Improved detection of pathogenic pseudomonads and Trichoderma in mushroom cropping
• Examine changes in microbial populations of mushroom casing during the development of disease to identify changes in communities of pathogenic and beneficial bacteria which may be exploited for better disease control
• Investigate improved control measures for mushroom blotch and green mould

Achieved through the following objectives:

1. Conduct bioassays to determine if Pseudomonas species other than P. tolaasii and ‘P. gingeri’, including other species also containing the tolaasin gene, can produce blotch
2. Exploit newly available genome sequence data for selection of TaqMan assays suitable for diagnostic detection of blotch causing Pseudomonas species including those identified above
3. Develop improved detection of Pseudomonas by incubation of substrate samples in nutrient broth containing ethyl hexanol to selectively enrich populations of Pseudomonas isolates allowing subsequent testing with Taqman PCR
4. Determine if the above analysis accurately predicts the blotch disease incidence resulting from using the substrate samples in mushroom growing conditions conducive to blotch disease development
5. Develop a detection system for Trichoderma species as a farm hygiene indicator of T. aggressivum and other mushroom pathogens
6. Test the developed diagnostic tests and disease control measures on commercial mushroom farms and substrate producing facilities
7. Make diagnostic tests available and disseminate the results to the mushroom industry
8. Carry out ITS and 16S metabarcoding on range of casing samples during mushroom production cycle
9. Investigate control of bacterial blotch and green mould by irrigating with antagonists, bacteriophages or ionic solutions