COBWEB/MILDEW DISEASE FACT SHEET
David M. Beyer
Associate Professor- Mushroom Extension Specialist
Disease Name: Cobweb or Mildew
Pathogen: Cladobotryum mycophilum or C. dendroides. (AKA, Dactylium dendroides; Hypomyces rosellus)
Signs and Symptoms
White, webby, fluffy or sometimes granular mycelia grows on the casing, dead mushroom tissue or over healthy mushrooms (Figure 1). On occasion, the mycelium and infected mushrooms might have some reddish/orange color (Figure 2) and on agar some isolates have a “camphor-like” odor. Colonies growing on the casing are usually circular and rapidly overwhelm mushrooms, causing rapid decay.
| Figure 1. | Figure 2. |
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Spotting on the mushroom tissue, is usual light fleshy in color but can be yellowish brown (blotch-like) in color, Figure 1 (circle). The pathogen grows very rapidly across the beds, especially under moist and humid conditions. A small spot can expand rapidly overnight.
Pathogen Characteristics
Microscopically the spores of the fungus are large and 2-4 celled, most often 2-3 celled produced on vertically branched conidiophores. The colonies grow as fast as 15-20 mm per day on agar plates. Optimal spore germination temperature is 25 C (73 F) and an RH of >97%. Under these conditions, the pathogen can germinate and grow in 7 days. The fungus profusely sporulates in contact with mushroom tissue and the dry spores are easily dislodged when watering or salting an infected area. It has been reported that the thermal death point of spores in soil is 30 minutes at 122 oF (50 C), Wuest and Moore, 1972.
Disease History
Until the late 1980’s the disease was rarely seen on mushroom farms worldwide, and was easily controlled by a variety of fungicides used in the industry (Wuest et al, 1983, Fletcher et al., 1989). Some fungicide (Mertect and Topsin) resistance was reported by Lockley and Gay, 1983. The disease it was often associated with wet seasonal weather. In the early 1990’s outbreak on British mushroom farms became more frequent and reached epidemic proportions in the mid 1990’s. This epidemic was found to be caused by a built up of fungicide resistance in one particular isolate C. dendroides, Type II, (Grogan and Gaze, 2000). Benzimidazole fungicides, like Benlate and Mertect, were found to be less effective in controlling the disease and increased pathogen resistance was demonstrated.
In 2000, samples from a few Pennsylvania mushroom farms were identified as C. dendroides, Type II, and showed a little resistance to Benzimidazole fungicides, Figure 3 (Beyer, 2001). In 2003, samples from a farm with a severe outbreak of Cobweb, showed more resistance to Benlate than an isolate from a farm where the disease was being control with Benlate, (Figure 4). Isolates are being collected and tested for resistance to the registered fungicides used in the US.
Disease Epidemiology
Cobweb spores are easily dislodged by air and are carried considerable distances by air. Flies, people and equipment are also vectors of this disease. Pieces of mycelium can also be disturbed and spread by harvesters and other personnel. It has been reported that spores entering the crop at casing time may not cause symptoms until later breaks, however mycelium carried in with casing will cause severe outbreaks at first break.
The pathogen thrives under warm moist conditions and grows rapidly under ideal mushroom growing conditions. Casing moisture and evaporation rate are closely related to disease development (Fletcher, 2002). Higher casing moistures and/or lower evaporation rates will provide conditions more conducive to disease development.
Changes in ventilation that would influence the evaporation rates in areas of the room or increase air speed/volume could promote spore dispersal. Changes in casing management or materials may also influence the disease occurrence or development. Heavier casing materials change evaporation and casing moistures, which could influence the disease development. A low casing pH does not encourage disease development.
Heavy use of Benlate and/or Mertect may cause more fungicide resistance in the pathogen isolates on a farm. Bravo the other registered (non-Benzimidazole) fungicide can be used, but it has been reported that early applications after casing slows the surface (CAC/CI) mycelium and delays pinning.
Control
· Identify disease symptoms early, not only the obvious web/mildew but also cap spotting.
o Know you enemy!
· Treat spotty infections with a alcohol drenched paper towel before covering with salt.
· Cover infected areas as described above before watering the crop.
· Change from light peats to heavy peat casing may encourage disease development, but heavy black peats are not responsible for initial infections.
o Heavier casing may require increased water applications, therefore may encourage the spread and development the disease.
· Heavily infected 2nd or early 3rd breaks should be steamed off to reduce the spore load on the farm.
· Increase hygiene of the harvesting and watering department.
· Judicious applications of Benzimidazole fungicides should be made
o Long-term use of Benzimidazole fungicides may lead to fungicide resistance.
o Bravo (Chlorothalonil) should be included in the fungicide application program.
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Figure 3. Average effective dosages (ED) by fungicide, isolates collected in 2000-2001 were compared to DC26 isolate collected in 1970, before the registration of Benlate. All isolates were sensitive to Benlate, but the ED for the other fungicides is higher (ED >2.0 ppm). |
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Figure 4. Growth of 3 Cladobotryum sp. isolates on Potato-Dextrose Agar amended with various rates of Benlate after 5 days. Notice the effective dosage (ED) for T03 isolate increased to ~ 8.0 ppm. The T03 isolate is from a farm where Benlate did not appear to be effective in controlling the disease, whereas P03 isolate was from a farm where Benlate did offer effective control. |
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References
Beyer, D.M. and Kremser, J.J. 2001. Possible development of fungicide resistance by cobweb diseases on mushrooms. Phytopathology 91: (6) 58.
Fletcher, J.T. , White, P.F. and Gaze, R.H. 1986. Mushroom-Pest and Disease Control. Inytercept Limited, Ponteland, Newcastle-upon-Tyne, England
Fletcher, J.T. 2002. Cobweb Disease, A New Challenge. Mushroom News 50 (4): 20-25.
Grogan, H. M . and Gaze, R.H. 2000. Fungicide resistance among Cladobotryum spp. - causal agent of cobweb disease of the edible mushroom Agaricus bisporus, Mycological Research 104 (3): 357-364.
Harvey, C.L., Wuest. P.J. and Schisler L.C. 1982. in Penn State handbook for Commercial Mushroom Growers edited by P.J. Wuest p 20-21.
Lockeley, K.D. and Gay, C. 1983. Differential sensitivity to Benzimidazole fungicides in strains of Hypomyces rosellus. Plant Pathologists Technical Conference 1983, PPT/798. Ministry of Agricultural Fisheries and Food, ADAS, Bristol, England.
Sinden, J.W. 1971. Ecological control of pathogens and weed moulds in mushroom culture. Annual Review of Phytopathology 9: 411-432.