MOST growers know cobweb only too well. Its rapid, white, dusty to spiderweb like growth across the surface of casing, enveloping and rotting mushrooms of any size it comes across in a mass of mycelium, means this is a fungus that can not be ignored.
Mairead Kilpatrick tells me that advisory cases arriving at Loughgall indicate cobweb has again become a topical problem. Some notes on it may be helpful.
An invasion from the South About six years ago a major outbreak that appeared to start in the southwest of the Republic, spread rapidly and caused major losses. In some houses not even a second flush was obtained. After it was discovered that the strain of fungus involved was resistant to the most widely used ‘control’ fungicide of that time, the outbreak rapidly declined. However by then it had travelled across Ireland, including the ‘North’. The severity of the outbreak helped stimulate North-South co-operative research. In this, apart from confirming its resistance to Benzimidazole fungicides, the main findings, by use of hi-tech molecular methods, indicated that there had most likely been confusion for many years as to the true identity of the fungus causing cobweb outbreaks. Two species of Cladobotryum with various fungicide resistant strains emerged in a confused web of their own as far as the names given to them in cultures from national collections. The site of resistance on the genome was discovered and found to be in an unexpected part.
If resistance to a fungicide builds up, the continued use of it may actually make an outbreak worse. This is because the chemical may suppress natural competitors in the casing but not the target organism. Furthermore, if the grower has taken what is considered to be a positive control measure and is let down, this may preclude use of an effective alternative. Luckily, no Irish strains were found to be more than mildly resistant to the alternative fungicide, ‘Sporgon’, consequently this helped ‘save the day’.
On at least one of the farms where cobweb caused major losses, I suggest the following chain of events probably occurred. The new cobweb strain arrived, possibly by a cross-contamination contact with another farm, maybe carried on a shared worker, or on a shared piece of equipment, or on a farm visitor or even on a single mushroom fly. Over several crops it increased from insignificant levels. Maybe the strain was also less obvious in its growth habit, or humidity’s were a bit lower, so that its expansion over the casing was not so easily noticed. It was then spread by watering, pickers and perhaps by a less than perfect house emptying procedure so that contaminated dust and spores spread widely. In a later crop when it was noticed as having become a ’problem’, Benzimidazole was used for spot control (as recommended). However, this made matters worse. Despite trying to ‘rescue’ a second flush, the crop literally became ‘covered’ in cobweb mycelium, bearing countless spores. With no ‘cook-out facility available, how could such a house be emptied without contamination of the whole farm and surrounding area? Such farms had to close down for weeks of thorough disinfection.
Studying this disease I was impressed by the size of the multicell spores. They were not in sticky clumps like those of Trichoderma, appearing enmass more like a dusty, white carpet at x50 magnification. I expected to be able to easily blow them off the mycelium but was unable to do so. However, if a human hair was touched against such a colony it rapidly became coated. I also found that mushroom flies carried spores on their wings and bodies. Worse, it appeared that when such a fly died, its body acted as an initial nutrient source for the fungus. Otherwise, the fungus appeared to require mushroom mycelium in the casing as a nutrient source for visible colony growth. Experimentally trying to initiate a colony on casing was more difficult than expected, unless patches of white mushroom mycelium were present. However, once started, a colony expanded exceptionally fast. Toadstools are sometimes found in shelter-belts or on grassland with cobweb on them and the fungus can also sometimes be isolated from soil. Spores, once detached, as by watering, can be easily airborne, so advice as to turning off the fan when watering a house that may have cobweb is good. An ex grower also warns me of the danger of cobweb developing on spilled casing, or on mushrooms growing between bags. Such infection points being partially hidden may remain untreated and greatly expand.
Being ‘normally’ a slow starter, unless the inoculum is high, cobweb is usually only a last flush problem. Nevertheless, it may be serious to lose most of a good last flush.
Control Usually cobweb infection does not appear to get started until late in pinning and so it should be easy to control (unless the inoculum level was high at casing). One of the best methods of control is by maintaining top-class hygiene. Infected dust and spores must not be allowed to blow onto casing. Later good house-lighting is required and a routine of careful casing inspections, with generous spot covering using common salt.
It is important to gently cover colonies with a wet tissue to prevent spore dispersal, before pouring on the salt, making sure to well overlap the colony. Preventing spores spreading, as during watering or picking will also avoid mushroom spotting.
Experience suggests few fungicides can be relied on indefinitely so spot salting may be as good a control method as any. In her excellent disease article in The Mushroom Journal no. 626, Helen Grogan mentions off-label approval for Dorado (pyrifenox) for control of Bavistan-resistant isolates.
Cooking out Cooking out has been mentioned above and was one of the topics at Loughgall in a recent multidisciplinary update seminar for advisors. Cathal Ellis and Steven Jess described a wide ranging, but as yet unfinished series of field-experiments, cobweb being one of their test organisms.
When killing by heat, temperature x time has to be thought of as a single factor. A relatively high temperature such as 68ºC for a short time, such as 4h is equivalent to a much longer time at 60ºC. This fact can make analysis quite complex. For example the time to reach a target temperature can be very extended, particularly if the boiler is insufficiently large. Capacity is judged by the number of kg of steam produced per hour. If the boiler is insufficient for the house size a long period may be spent around 60ºC, prior to reaching the 68ºC ‘target’. If the compost is not ‘active’ it may not be able to add to heat production on its own account. Then it may be very difficult to reach a high temperature. Moisture can affect the electrical contacts of the measuring probes, leading to false readings. This has to be guarded against by taping them up very thoroughly. Total kill of all pests and diseases, particularly of Trichoderma is difficult and thus costly. However, a ‘total kill’ is seldom required and cobweb, if it is restricted in the casing area, should pose few problems. Nevertheless, I suspect floor washing plus disinfection will still be advisable after emptying.