During different phases of button mushroom production, a large number of nematodes are encountered along with insect-pests and mites. The term nematode (nema = thread + oides = resembling) indicates the external appearance of the organisms (Fig. 20.1). Nematodes are microscopic (upto 1 mm) and can swim easily in the surface film of water in casing and compost. Mycelium of the fungi is favourable source of food for nematodes. Nematodes are one of the most dangerous pests of button mushroom, which once enter the beds cannot be eradicated completely, until and unless crop beds are destroyed and disposed off completely. Their presence in the beds simply means very poor yield or total crop failure. A number of instances of crop failure due to nematodes have been reported during the last two decades. Compost preparation using long method where no pasteurization is done, improper pasteurization in short method, seasonal cultivation under unhygienic conditions and lack of awareness about cultivation practices are some of the factors responsible for easy access of nematodes in cropping beds. In early days wooden trays because of their repeated use and inadequate sterilization were an important source of nematode infestation. Generally three types of nematodes viz., myceliophagous, saprophytic and predatory are encountered in mushroom beds.

Fig. 20.1. Nematode

In all, 21 nematode species have been reported to be harmfully associated with mushroom cultivation from various parts of the world. Among these, twenty species belong to four genera (Aphelenchoides, Aphelenchus, Paraphelenchus and Seinura) of order Aphelenchida and the only Tylenchid is Ditylenchus myceliophagus (Table 20.1). In India, occurrence of eight species of Aphelenchoides and D. myceliophagus have been recorded from mushroom beds.

Table 20.1. Pathogenicity of myceliophagous nematodes associated with mushrooms

* Species reported from India, +++ Highly pathogenic, ++ Pathogenic, + Found to multiply on mushroom, -Pathogenicity not studied

Myceliophagous nematodes have needle like structure (stylet) in their mouth parts. The stylet is hollow inside and can be moved forward and backward by the contraction and relaxation of the muscles. The nematodes secrete variety of strong enzymes. These enzymes act immediately after ejection and help in penetration of stipe and to convert the cell contents in assimiable forms. The nematodes have very fast rate of multiplication (50-100 fold/week). Rate of multiplication is faster during spawn run period (22-280C) than the cropping period (14-180C). Beyond 300C these do not reproduce. It has been found that initial infestation with 3 nematodes of D. myceliophagous/100 g of compost can entirely destroy the mycelium with in a period of 70 days. These nematodes survive in a state of anabiosis (a restoring to life from a death like condition) for up to two years, if the compost is dried gradually but they die if the compost is dried rapidly.

Button mushroom is highly susceptible to nematode attack during entire cultivation process. The common source of nematode contamination are damp wheat straw, manures, FYM, garden soil, spent compost, platform soil, irrigation water and contaminated implements. Sometimes flies, particularly sciarids carry the nematodes from one bed to another.

Since the growers are reluctant to disturb the beds after casing, the early symptoms of nematodes attack are generally overlooked and yield reduction is the first effect noticed by them. Following symptoms of nematode attack appear in infected beds in succession: a. Mycelial growth is sparse, patchy and mycelium turns stingy b. The compost surface sinks c. Whiteness of spawn-run slowly changes to brown d. Mushroom flushes are poor and delayed e. Alternate high and poor yield in successive flushes f. Browning of pinheads g. Decline in yield h. Complete crop failure i. White fungal growth over casing in patch areas

Fig. 20.2. Death of pin heads

These nematodes lack the stylet. Buccal cavity is short and wide. These are generally clumsy looking worms with round caudal or clavate ends.The only nematode order that represents saprophages in mushroom cultivation is Rhabditida. The genera of common prevalence are Acrobeoides, Caenorhabditis, Cephalobus, Diplogaster, Panagrolaimus and Rhabditis.

There are different schools of thoughts regarding the nature of damage caused by saprophytic nematodes. According to some workers, these namatodes invade the mushroom beds only after the invasion of myceliophagous nematodes. These nematodes secrete some enzymes and toxins, which increase the pH of compost and hampers spawn run. These are also known to create unhygienic conditions or sometimes they may carry harmful bacteria on their body surface. Recently it was found that saprophytic nematodes Caenorhabiditis elegans can damage the sporophore of A. bisporus, if the population reaches 300-500 nematodes/g of casing mixture. Some of the typical symptoms produced by these nematodes are:

a. Distorted, notched and kidney shaped mushroom
b. Violet colour of gills
c. Browning of sporophores

Many of the hybrid strains are sensitive to saprophagous nematodes. However, further investigation to prove their exact role in mushroom cultivation is still needed.

Keeping in view the nature of the crop, limited availability of safe and potent nematicides, residue problems and other hazards of nematicides, it is better to prevent entry of nematodes into the beds rather than controlling them afterwards. So far there is no curative measure available, which can be adopted during cropping stage. Hence, only control measure is to follow an integrated approach.

a. Cropping should be done in purposely built mushroom houses with proper ventilation using properly pasteurized compost made by short method
b. Strict hygienic and sanitation measures should be followed throughout the cropping period
c. Composting yard must be cemented to prevent the direct contact of compost with the soil. It must be disinfected with 2% formalin, 24 hours earlier to compost preparation
d. All the instruments, walls, floors and galleries should be disinfected with 4% formalin
e. Composting ingredients should always be stored in clean area
f. Cleanliness should be maintained inside and in surroundings of mushroom farms
g. Casing mixture should be properly pasteurized
h. Manures used for composting should be thoroughly broken and allowed to decay properly
i. In long method of composting, covering compost with double PVC sheet for 24 hours after third turning gives effective control of nematodes
j. Foot dips must be installed in front of each cropping room
k. Irrigation water should be clean
l. No person or worker should be allowed to enter into the farm without proper disinfection of his/her hands and feet
m. All the cropping rooms should be fly proof and only recommended insecticides should be sprayed for the control of flies
n. In long method chicken manure may be avoided. Neem cake can be added

Mushrooms, being indoor crop, provide little scope for pesticidal usage. Short life of the crop and deleterious effect of pesticides on mycelium and residue problems further limits the scope of pesticide application. Many chemicals have been tested but most of these show detectable amount of toxic residue. However, some of the chemicals, which can be used during composting itself are effective in checking nematode population especially long method of composting. Dichlorvos (0.04%) under polythene cover for 3-4 days was found to be most effective for control of A. composticola and Rhabditis sp. Fenamiphos EC at 20 mg/kg incorporated in compost is a practical preventive measure. Thianozin @ 80 ppm (0.008%) in compost and its spray on the bed surface during spawn run effectively controls the nematodes without any detectable residue on mushrooms.

Since the use of chemicals for the management of nematodes has many constraints, only alternative left is to exploite biological means. Various microorganisms, plant-parts and extracts have been exploited for the management of nematodes. Research on the use of bacteria, fungi, mites and nematodes for their control is going on. However, maximum work has been done on nematode trapping fungi such as Arthrobotrys oligospora and A. superbe, which are known to reduce the nematode population and increase the yield. Fungus, A. robusta has been recommended against mycophagus nematodes. This fungus is available commercially as Royal-300 strain Anti polis in France. Candelalretta musiformis isolated from spent compost has been found highly effective in checking nematode multiplication. Fungus A. irregularis, is highly effective against A. composticola.

A large number of plants have been found to possess nematicidal properties. Incorporation of dried leaves of Azadirachta indica, Cannabis sativa, Eucalyptus tereticornis and Ricinus communis at 3 kg/100 kg of dry wheat straw, enhanced the population of thermophilic fungi, mesophilic antibiotic producing fungi and at the same time reduced the population of A. composticola below economic injury level. Karanj leaves mixed in compost also reduces the nematode population. Addition of neem leaf powder @ 2% on w/w basis of compost has been recommended for controlling nematodes. Incorporation of neem cake @ 5% on w/w basis of compost at spawning has been reported to hamper the multiplication of A. compostiocola.

So far there is no strain of A. bisporus, which is resistant against nematodes. However, A. edulis is resistant to the nematode D. myceliphagus and A. bitorquis (K-30) to A. saachari but susceptible for D. myceliophagus.

Use of heat is the most successful method of nematode control in mushroom cultivation. It is recommended that to make compost nematodes free, air and bed temperature in the pasteurization room must be maintained at 600C at least for 2 hours and cook out of mushroom house at 700C for 5-6 hours or 800C for 30-60 minutes is necessary. Used trays and handling tools must be disinfected by dropping in formalin or cresylic acid. Dipping of the appliances in boiling water for 1-2 minutes is sufficient for complete destruction of nematodes. Finally, spent substrate should be disposed off completely. The nematode, D. myceliophagus, can withstand drying for up to 3 years and this makes the disposal of all spent compost very important, as dry infested debris is a potential source of trouble. Lowering down the room temperature to 130C during cropping period retards the pathogencity of these nematodes.

a. Observe strict hygiene throughout the farm
b. Ensure that the temperature during peak heat is satisfactory
c. Make sure that casing ingredients are stored and mixed in clean area and casing is properly pasteurized
d. Make sure that all spent compost is removed from the farm
e. Properly clean the cropping rooms after every crop