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Grassy muscardine, a fungal disease affecting silkworms, is caused by the parasitic fungus Hirsutella patouillard. This disease gets its name from the characteristic grass-like appearance of the fungal growth on the silkworm carcass. The hardened body of the infected silkworm is covered with hyphal bundles resembling small onions or grass.
1. The Causal Agent: Hirsutella patouillard
Hirsutella patouillard is a filamentous fungus with distinct features:
| Feature | Description |
|---|---|
| Mycelium | Filamentous, septate (separated by cross-walls), and twisted into bundles. |
| Hyphal Bundles | Branching or unbranched; if branched, the base is often at a near right angle. |
| Conidiophores | Develop on hyphae or as free hyphae. They have a small lateral stalk that is swollen at the base and divides into two smaller stalks, each bearing a single conidium. |
| Conidia | Colorless, single-celled (monospore), and shaped like a wheat grain. These spores are responsible for the spread of the disease. |
2. Infection and Progression
The infection process of grassy muscardine is similar to other fungal diseases:
- Spore Germination: Conidia of Hirsutella patouillard germinate and penetrate the silkworm’s body wall.
- Internal Invasion: The fungus then proliferates inside the silkworm’s body, and short hyphae can be found in the hemolymph (blood).
- Lack of Early Symptoms: Diseased silkworms don’t show obvious symptoms during the feeding period. This makes early diagnosis very difficult.
- Late-Stage Manifestations: As the silkworms mature, the disease manifests with the following symptoms:
- Most silkworms die in a contracted (scorpion-like) position.
- Some silkworms form a thin, weak cocoon.
- Others may die during the silking process or lie flat.
- Carcass Hardening: The dead silkworm’s body rapidly becomes rigid, turns an earthy-yellow color, and has a brittle texture.
- Fungal Growth: Irregularly shaped lesions of varying sizes (up to 1 square millimeter) are visible on the body surface.
- Post-Mortem Development: When incubated at 26°C and 90% relative humidity, the body darkens to a brownish-red after a week. Papillary protrusions then develop which elongate into root-like earthy yellow hyphal bundles.
3. Characteristic Hyphal Bundles
The formation of hyphal bundles is a defining characteristic of grassy muscardine.
| Characteristic | Description |
|---|---|
| Number | Ranges from a few (4-5) to hundreds per silkworm. |
| Appearance | When few in number, hyphae bundles are thick and long (1-1.5 mm in diameter and 6-10 cm long). When many in number, they are thin and short (0.5-1 mm in diameter and 1-3 cm long). |
| Overall effect | The hardened silkworm carcass is covered in these hyphal bundles, giving it a grass-like appearance. |
4. Susceptibility and Diagnosis
Young silkworms are more susceptible to grassy muscardine, and due to the lack of obvious symptoms in the early stages, diagnosis is hard. It can only be confirmed through corpse culture by incubating dead silkworms and observing the development of hyphae.
5. Management and Control
The control methods for grassy muscardine are similar to those used for other fungal silkworm diseases, such as those caused by Beauveria bassiana (a different type of fungus):
- Hygiene: Maintain a clean rearing environment to minimize fungal spores.
- Environmental Control: Ensure proper ventilation and temperature to reduce humidity which promotes fungal growth.
- Biological Control: Use specific biological control agents.
- Quarantine: Isolate and dispose of infected silkworms and bedding materials.
By understanding the lifecycle and symptoms of Hirsutella patouillard, sericulturists can better manage grassy muscardine and minimize its impact on silk production.
