Identifying and Controlling Diseases in Muga Silkworm Host Plants

Muga silk is one of India’s most prized natural textiles – its characteristic golden sheen is produced nowhere else in the world. The silkworm behind this treasure, Antheraea assamensis, is entirely dependent on the leaves of two primary host plants: Som (Persea bombycina) and Soalu (Litsea monopetala). These trees, endemic to the Brahmaputra Valley of Assam and northeastern India, are the foundation of every muga rearing cycle. When they fall sick, the silkworms go hungry – and silk production suffers. Two diseases in particular, Grey Blight and Red Rust, pose a recurring threat to these host plants and demand the attention of every muga farmer.

Why healthy host plant leaves matter so much 🔗

The muga silkworm is a multivoltine insect, completing five to six rearing cycles a year. Its larvae feed exclusively on host plant leaves, and the nutrition of the silkworm depends entirely on the quality of those leaves. Diseased or damaged foliage is either unpalatable for the larvae or physically unsuitable for feeding, directly reducing cocoon yield and silk quality. Research has shown that foliar diseases on Som alone can reduce total leaf yield by 13.8 to 41.6% annually. At that scale, disease management is not optional – it is central to sustaining the muga industry.

Grey blight: the most widespread fungal threat 🔗

Grey Blight is consistently ranked as the most damaging foliar disease affecting Som plants. It is caused by the fungus Pestalotiopsis disseminata (Thum) Stey, and Som (Persea bombycina) – the primary food plant of the muga silkworm – is its most economically significant host.

Symptoms to look for 🔗

The disease begins on tender young leaves, which are the most susceptible part of the plant. Minute brownish patches appear first, quickly turning grey. The disease progresses from the tender top leaves downward, causing affected leaves to dry out and take on a dark greyish colour. In advanced cases, the top of the plant can die back entirely as infected leaves wither and fall. The damage is not just aesthetic – leaves affected this way are unsuitable for silkworm feeding.

What drives the disease 🔗

Research conducted at the Central Muga Eri Research and Training Institute (CMER&TI) in Jorhat, Assam, found that a temperature of around 25°C combined with 70% relative humidity creates the optimal environment for spore germination of P. disseminata. This closely matches the warm, humid monsoon conditions that prevail across northeastern India for much of the rearing season. Young leaves – specifically the first four from the growing tip – are significantly more susceptible than older, tougher foliage. The fungus spreads via airborne spores, and studies on Som’s mycoflora show that P. disseminata is most prevalent in the aerosphere, making aerial spread the primary infection route.

Management of grey blight 🔗

The primary chemical control for Grey Blight is Bordeaux mixture applied at 1% concentration. This copper-based fungicide, when sprayed on affected Som plants, suppresses the spread of P. disseminata effectively. However, a critical precaution applies: sprayed leaves must not be fed to muga silkworms for at least one month from the date of spraying. Beyond chemical control, early removal and burning of infected leaves is strongly recommended. Catching the disease in its early stages prevents the fungal spores from spreading to healthy parts of the tree. Research has also explored biological approaches – application of vermicompost combined with plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) has been shown to slow disease progression in nursery conditions, suggesting promise for integrated disease management approaches.

Red rust: the algal disease affecting both som and soalu 🔗

Red Rust is distinct from Grey Blight in one fundamental way – it is not caused by a fungus. It is an algal disease, caused by Cephaleuros sp. (principally Cephaleuros parasiticus Karst.). Cephaleuros is a genus of parasitic thalloid green algae that grows between the cuticle and epidermis of infected plants, making it structurally very different from typical fungal pathogens. Its common name “red rust” reflects the vivid colouration of its lesions, which can look deceptively similar to fungal rust diseases.

Symptoms on Som and Soalu 🔗

On Som plants, Red Rust primarily targets older, mature leaves rather than tender growth. The disease appears as yellow-green, orange, or grey hairy pustules, mostly on the upper surface of the leaf. These pustules are circular or irregular in shape and are surrounded by chlorotic (yellowed) halos. As the infection intensifies, multiple pustules merge to cover a large area of the leaf blade. On Soalu, the disease behaves in the same way – it is restricted to older leaves and is particularly common during winter when leaves reach maturity. Heavily infected leaves yellow and drop prematurely, reducing the canopy available for silkworm feeding.

Conditions that favour the disease 🔗

Frequent rains and warm weather are the primary environmental drivers of Cephaleuros infections. Poor plant nutrition, poor soil drainage, and stagnant air around the canopy are predisposing factors that make host plants more vulnerable. The alga requires free water on the leaf surface to release its zoospores, so orchards with dense canopies that retain moisture are at greater risk. The disease is most severe during humid winters in northeastern India, which aligns with the Soalu-specific pattern of infection during the leaf maturation period.

Management of red rust 🔗

Control of Red Rust involves both cultural and chemical measures. On the cultural side, pruning and removal of infected leaves and plant parts is an effective first step in reducing disease spread. Timely intercultural operations and the application of potassic fertilizers at optimal doses also help contain the disease by improving plant vigour and resistance. For chemical control, Bordeaux mixture at 1% concentration serves as the standard protective spray, the same formulation used for Grey Blight. Research on related Cephaleuros species in tea gardens has confirmed that copper-based treatments like copper oxychloride consistently deliver over 68% disease reduction in field conditions, supporting the use of Bordeaux mixture as a reliable broad-spectrum option. As with Grey Blight management, sprayed leaves should not be used for silkworm feeding until the required waiting period has passed.

Integrated disease management: the bigger picture 🔗

Both Grey Blight and Red Rust underscore a common challenge in muga sericulture – the host plant and the silkworm are inseparable. A diseased tree does not just lose leaves; it breaks the entire rearing cycle. Effective management relies on combining field sanitation (regular removal and destruction of infected material), timely chemical intervention with Bordeaux mixture, and good agronomic practices such as improving canopy aeration, managing soil drainage, and maintaining adequate nutrition. Field sanitation is especially emphasized by specialists as a foundational measure that significantly reduces the overall incidence of both pests and diseases across Som and Soalu plantations.

The goal of all these measures is straightforward: keep the leaf canopy clean, healthy, and abundant so that silkworms have an uninterrupted supply of quality foliage through every rearing season. Given that muga silkworm rearing is deeply tied to the socioeconomic fabric of northeastern India, protecting the health of Som and Soalu is ultimately about protecting the livelihoods of thousands of farming families who depend on this golden silk for their income.

What do you think? If both Grey Blight and Red Rust can be managed with the same Bordeaux mixture spray, what practical challenges might farmers face in timing applications correctly without disrupting silkworm rearing? And given that foliar diseases can reduce Som leaf yield by up to 41%, do you think biological control methods like PGPR and vermicompost deserve more attention as long-term alternatives to chemical sprays in muga plantations?

References

1. https://sericulture.assam.gov.in/portlet-innerpage/food-plants-of-muga-silkworm
2. https://www.krishisewa.com/component/content/article.html?id=403:muga-host-disease
3. https://link.springer.com/article/10.1007/s10453-019-09588-w
4. https://www.sciencedirect.com/science/article/abs/pii/S0261219410001407
5. https://hbmahesh.weebly.com/uploads/3/4/2/2/3422804/non_mulberry_food_plant_pests_and_diseases_word.pdf
6. https://www.academia.edu/105562051/Management_of_grey_blight_disease_of_Som_plants_using_value_added_vermicompost_with_Glomus_constrictum_and_Bacillus_altitudinis
7. https://en.wikipedia.org/wiki/Cephaleuros
8. https://www.ctahr.hawaii.edu/oc/freepubs/pdf/pd-43.pdf
9. https://www.sciencedirect.com/science/article/pii/S1049964425002282
10. https://www.nature.com/articles/s41598-025-05987-x