MUSCARDINE

MUSCARDINE

Muscardine is the major fungal disease of silkworms. Of the various types of muscardine disease
like white muscardine (Beauveria bassiana), green muscardine (spicaria bassiana), black muscardine
(Metarrhizium arisopliae), brown muscardine (Aspergillus flavus, A. oryzae, A. ochraccous), red
muscardine (Sorosporella uvella and Spicaria fumosorosea), yellow muscardine (poecilomyces
farinosus), white muscardine is more prevalent in India especially in hill and hilly regions.

White Muscardine

Causal organism : Beauveria bassiana (Bal vuill)
Belonging to fungi impercti, white muscardine is transmitted by the spore formed on the outer
body surface of the diseased catter piller after its death. The spores are globular or oval having a size of 2.5 to 4.5 x 2.3 to 4.0µ with thick and resistant spore wall which enables the spore to remain alive
for two to three years under favourable conditions. The spore, upon contact with the larval skin, germinates outside the body if the conditions of temperature and humidity are favourable.If the spore are taken orally by the silkworm,these pass out as such with the feacal matter. On the body surface of
the worm, the spore pushes out the hyphae that penetrate any part of the larval skin especially the
spiracles by mechanical and enzymatic forces to reach the haemocoel and invade and draw nourishment from the tissues. During the initial stages of the disease, the infected larvae cannot be differentiated from the healthy ones. As the disease progresses,the larvae lose appetite, become shiny and colourless patches appear mostly around spiracles and legs which are difficult to detect. (Aruga, 1994). In the final stage of infection, vomiting diarrhoea and stiffness of body results in the death. The duration from infection to death is generally 10 days in cold weather and 4 to 5 days in hot weather.
Soon after the death, the worm exhibits a characteristic white powdry appearance which results due
to the formation of white mycelia all over the body except the head, bearing numerous spores which
infect the healthy larvae. Due to continuous growth ofthe fungus, the soft dead larva becomes stiff and mumified (Plate 26).

Mode of Infection

The spores infect the healthy larvae by adhering to the skin where they germinate under favourable
humidity and temperature conditions which range from 80 to 100 per cent and 26 to 28°C,
respectively. Under such conditions, germination takes place in 6 to 8 hours. The germinated hyphae
enter the body of the silkworm, grow rapidly and form a network of mycelium. The penetration of
hyphae into body depends upon the thickness of the integument. Since young larvae have
comparatively thinner integument, they are more susceptible to disease.
The mycelium ramifies all through the body of the worm, drawing nourishment form the living
tissues and produces club shaped conidia ‘which again give rise to mycelia as well as spores. The mycelia spread, penetrate in all directions and body becomes full of mycelia and conidia. The time
interval between the germination of spore and the formation of conidia depends on the larval age, being 2 to 3 days in young larvae and 3 to 5 days in advanced. As the mycelia ramify, the haemolymph content gets reduced and becomes acidic. This results in hardning of the body even the midgut, which is alkaline in nature, turns acidic which results in a change in the metabolism and finally in the death of larva, pupa or adult, depending upon the stage of infection.

When the larva dies, the pathogen pushes its hyphae out mainly through spiracles and inter-
segmental membranes. Body becomes stiff due to loss of moisture. Finally, mycelia grow all over the
body except the head and entire body becomes covered with filt of white filaments (Plate 26) bearing
spores which become source of further infection. A worm that has died from muscardine frequently
shows sometime after death a very characteristic production of crystals on the body surface which are
said to be calcium magnesium or ammonium oxalate crystals, the products of the fungal metabolism.
When the host is alive, propagation of mycelium is limited within the body fluid but after several
hours of death, propagation of mycelium is very fast and it attacks first the fat bodies, malpighian
tubules then silkglands, ganglia and other muscles. If the pathogen infects the worms during late
stages, the worms either spin flimsy cocoon (Plate 27) or may get transformed into pupae which later
die within the cocoon.
The resistance of muscardine varies from race to race. While comparing the resistance of
domesticated silkworm to that of wild, the domestic silkworm was found weaker (Aruga, 1994). This
indicates that, like pebrine, domestication has made this disease more serious than it is in wild insects.

Management

Since high humidity and temperature favours the germination of pathogen,it is imperative to keep

the humidity low and ventilation good in the rearing rooms. Regular bed cleaning and burning of infected larvae, litter and unconsumed leaves helps in minimising the infection. As soon as any diseased worm is observed in the rearing bed, it is proper to clean the bed and burn the litter so as to reduce the apprehension of spread of infection.

Disinfection of rearing rooms and appliances with 2 to 4 per cent formaline solution is effective in checking the incidence of disease. Sprinkling Diathane M 45 or Mancozeb and lime over worms

also helps in keeping check over the disease. The recommended concentration for first to third age is

0.8 per cent and 1.8 per cent for advanced ages (Anonymous, 1994). The spread of infection is also

controlled by dusting Reshemkeet oushed over worms (Baig and Kumar, 1987). For first, second,

third, fourth and fifth ages, 70,150,600,1100 and 1800 g/dusting/100 Dfls are, respectively used.

Ceresan lime (a mixture of slaked lime with 5 percent ceresan) spread evenly on the rearing trays of silkworms at the rate of 5 g/0.1 m² also helps in reducing the incidence of disease (Baig and Kumar,1987). However, being highly poisonous, the chemical is not in commonuse. Labex (a mixture of lime and bleaching powder) sprinked directly on the worms once in each instar upto fourth instar and once daily in the fifth instar at the rate of 4 g/0.1m² is also helpful in checking the infestation (Ganga and Chetty,1997).