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Causes of nematode infestation.

When a grower sees nematodes on the bed, his first thought is that something went wrong during Phase II pasteurization. This could very well be the case. As we discussed in our previous column, Phase II peak heat is a critical step in eliminating pests and pathogens from the compost so they do not create problems later in the cropping cycle. However, it may be possible that during the pasteurization process of Phase II nematodes survived in wet pockets of the compost or in the sideboards of your beds in areas that didn't achieve adequate temperatures during pasteurization. If nematodes did survive these areas, chances are that they weren't adult nematodes.

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Many nematodes have a survival mechanism in which they enter into a stage known as Dauer larvae, which allow them to survive extreme environmental conditions. Dauer larvae enter into a state of developmental rest until the environmental conditions are favorable and allow the nematode to resume normal metabolism. Nematodes in the Dauer larvae stage may be more resistant to extreme temperatures present during pasteurization. Nematodes are frequently found in late breaks if any other stress conditions arise in the room (dead spots from pathogen pressure, compost overheating during spawn run or case hold, overly wet compost or overwatering of the peat).

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A more probable scenario to account for nematodes found in a mushroom crop is that the nematodes were introduced into the room. Nematodes may be brought in on contaminated equipment, peat moss (pallets and plastic wrap included), or even in the water that is used on the farm. A nematologist working on his thesis once tracked a nematode contamination problem to the tap water that he was using to water his plants (personal communication). Through casual observations it would appear that the nematodes may always be present at low numbers and are opportunistic when conditions lend themselves for their populations to flourish. Under ideal growth conditions (moderate temperatures with adequate moisture), nematode populations can increase up to 100 fold in as short as a few days to a couple of weeks depending on the species present.

Two distinctive groups of nematodes are of primary concern during mushroom cultivation. Parasitic nematodes (ex. Aphelenchoides composticola and Ditylenchus myceliophagus) may be present in the compost and feed directly on mushroom mycelium. Saprophytic (free-living) nematodes (ex. Rhabditis species) may be present and feed on decaying organic matter such as plant matter, bacteria and protozoa. Because of the different feeding characteristics, these different types of nematodes have different mouthparts that are adapted accordingly (Figure 1). Parasitic nematodes have a specialized mouthpart called a stylet that allows them to pierce their prey in order to extract nutrients (though the size of the stylet varies between the different parasitic nematode genera). The stylet is absent in saprophytic, free-living, nematodes.

Saprophytic nematodes are by far the most common groups found on mushroom farms. If present, parasitic nematodes are normally first noticed through the observance of nonproductive areas that turn black and may have a malodor. On the other hand, saprophytic nematodes are often observed swarming on the casing surface and can be easily seen by shining a flashlight across the casing. A gentle swaying and waving or "winking" of tiny hair-like structures can be seen. If populations increase to a high enough level, small masses may project upward from the casing layer (Figure 2). When populations are present on the casing surface, they can be spread throughout the farm through normal disease vectors such as pickers and flies.

As is the case with many pest and disease problems on the farm, sanitation and good hygiene should be a starting point to address nematode problems. This not only includes the equipment used in filling, spawning and casing, but it also includes a thorough post crop steamoff to ensure pasteurization of the beds and the house itself. If sanitation appears to be efficient, you need to look closely at your pasteurization process during Phase II to make sure that there are no cool areas in your house or in your tunnel that would allow unwanted pests and pathogens, such as nematodes, to survive.

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John Pecchia, Ph.D.

Manager, Mushroom Research Facilities Penn State jap281@psu.edu

David M. Beyer, Ph.D.

Professor--Mushroom Extension Specialist Department of Plant Pathology Penn State dmb8@psu.edu

The Penn State Lines is a regular column. Authors Beyer and Pecchia will share their expertise on mushroom growing topics in the coming months.
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Title Annotation:the penn state lines
Author:Pecchia, John; Beyer, David M.
Publication:Mushroom News
Date:Jul 1, 2010
Words:747
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