Common Diseases in Tropical Tasar Silkworm Host Plants: Identification and Control

Tropical tasar silk – produced by the wild silkworm Antheraea mylitta Drury – is a significant cottage industry that supports millions of tribal and rural families across central and eastern India. The silkworm depends almost entirely on two primary host plants: Arjun (Terminalia arjuna) and Asan (Terminalia tomentosa). Healthy, disease-free foliage from these trees is non-negotiable for good cocoon yield and silk quality. Yet these host plants are routinely attacked by several foliar diseases that, if left unchecked, reduce leaf availability, lower leaf nutrition, and ultimately drive down silk production. Understanding what these diseases look like and how to manage them is a practical necessity for every tasar farmer.

Why host plant health matters so much in tasar sericulture 🔗

Tropical tasar silkworm is entirely outdoor-reared, which means the larvae feed directly on whatever foliage the host trees are producing at any given time. Unlike mulberry sericulture, where harvested leaves can be inspected before feeding, tasar larvae eat from living branches. A diseased tree is a diseased feeding ground. The Central Tasar Research & Training Institute (CTR&TI) at Ranchi, under India’s Central Silk Board, has documented three key foliar diseases specifically affecting T. arjuna and T. tomentosa: Black Nodal Girdling, Powdery Mildew, and Leaf Curling. Each has a distinct seasonal window, specific symptoms, and a defined set of control measures.

Black nodal girdling 🔗

Black Nodal Girdling is the most visually striking and structurally damaging of the three diseases. It is caused by a fungal pathogen and tends to appear from June onwards, building steadily through the monsoon months and peaking around October – precisely when the main tasar rearing season is underway. Research at CTR&TI stations in Ranchi, Dumka, Baripada, and Warangal has confirmed that incidence is strongly correlated with high relative humidity and rainfall, making wet years especially damaging.

Symptoms 🔗

The disease begins at the nectar glands of the leaf, which are small secretory structures located near the base of the leaf blade. From there, infection spreads downward to the petiole and into the internodal region of the shoot. As the fungus colonizes these tissues, it produces dense black sooty spores and mycelial growth that form a girdle-like ring around the node – hence the name “nodal girdling.” The girdled tissue effectively cuts off water and nutrient flow to the leaves above it, causing the entire leaf to wither and dry out. Once the infection girdles a node completely, that leaf becomes unsuitable for silkworm feeding and must be treated as a loss.

Management 🔗

Control of Black Nodal Girdling requires a combination of sanitation and timely chemical intervention. The recommended approach from the Central Silk Board includes:

• Cultural sanitation: Maintain proper field hygiene. Remove debris and avoid conditions that trap humidity around the plant base.
• Mechanical removal: Pluck and burn all infected leaves and affected shoot portions as soon as symptoms appear. Do not compost infected material – burning is essential to prevent spore dispersal.
• Fungicide spray: Apply Carbendazim @ 0.1% as a systemic fungicide, or alternatively Biltox @ 0.5% or Dithane M-45 @ 0.5%. Spraying should be timed before peak humidity months to provide protective coverage.

Powdery mildew 🔗

Powdery Mildew is a classic cool-season fungal disease and one of the most economically significant threats to tasar host plants. It is caused by an obligate fungal pathogen and is prevalent from October to March, with peak severity in November at most tasar-growing locations in India. Unlike Black Nodal Girdling, this disease shows a negative correlation with temperature – the cooler the weather, the more severe the infection – and is also negatively correlated with rainfall. This means dry, cool winter conditions are prime time for an outbreak.

The economic stakes are significant. Crop loss from powdery mildew has been reported at 25-30% in Arjun (T. arjuna) and 8-10% in Asan (T. tomentosa), and these figures likely underestimate the real impact since qualitative losses to leaf nutrition are harder to measure. T. arjuna is more susceptible, which is notable given that it is the preferred host in commercial tasar farming due to its faster growth and higher leaf yield.

Symptoms 🔗

The disease is easy to identify in its early stages. White, powdery patches appear on the ventral (lower) surface of leaves, which later develop distinct dot-like fruiting bodies within the powdery coating. As the disease advances, the corresponding upper (dorsal) surface of the leaf develops chlorotic (yellowing) patches, and the leaf lamina begins to curve or cup. In severe infections, affected leaves turn fully yellow and drop prematurely. This early leaf fall removes foliage that would otherwise have been available for silkworm feeding.

Management 🔗

Because powdery mildew thrives in cool, dry conditions – exactly when chemical sprays are least expected – prevention and early intervention matter most. Recommended measures include:

• Field sanitation: Carry out regular cultural operations. Remove and burn diseased leaves as soon as white patches appear.
• Chemical control: Spray Carbendazim @ 0.1% as a systemic option. Alternatively, Dinocap @ 0.2% (Karathane) or Sulfex @ 0.2% (wettable sulfur) can be used. Sulfur-based fungicides are particularly effective against powdery mildew pathogens, as UC IPM guidelines and field practice widely confirm.
• Timing: Begin spraying in October, before peak infection in November, and continue through the winter months if conditions remain cool and dry.

Leaf curling 🔗

Leaf Curling is a physiological disorder rather than an infection by a specific pathogen. It is believed to be caused by copper deficiency in the plant, though its precise etiology in tasar host plants is still considered tentative. It affects young, tender leaves – the very leaves that are most nutritious and most in demand for young silkworm larvae.

Symptoms 🔗

The first sign is withering of young and tender leaves shortly after they expand from buds. The leaves do not develop normally; instead, they begin to crinkle and fold inward along the midrib, producing a characteristic boat-shaped or inward-curled form. This curling is a structural deformity – the leaf blade fails to flatten out properly. As the condition worsens, affected leaves shrivel and die without ever becoming usable for rearing. Since the disease targets the youngest flush of growth, it can significantly reduce the amount of tender foliage available during early-instar silkworm feeding, when the larvae are most vulnerable and nutritional quality of the leaf matters most.

Management 🔗

Given the suspected link to copper deficiency, management focuses on correcting the nutritional status of the plant alongside standard sanitation practices:

• Pruning: Remove and destroy all affected shoots and leaves to prevent further development and to redirect plant energy toward healthy new growth.
• Burning infected material: As with other diseases, all removed plant parts should be burned – not discarded on the field – to minimize risk of further spread.
• Copper-based fungicide application: Spraying copper-based formulations can address both the suspected deficiency and any secondary fungal activity. Products such as Blitox (copper oxychloride) are commonly used in this context in sericulture plantations.

Common principles across all three diseases 🔗

While Black Nodal Girdling, Powdery Mildew, and Leaf Curling each have distinct causes and seasonal patterns, the management principles recommended by India’s Central Tasar Research & Training Institute share a common logic: act early, act mechanically first, and use fungicides as a follow-up rather than a standalone solution. Pruning infected parts and burning them prevents pathogen spread far more reliably than chemical sprays applied after an infection has already taken hold. Field hygiene – keeping the plantation clean of fallen infected debris – reduces the inoculum load season after season. Chemical fungicides such as Carbendazim, Dinocap, Sulfex, and Blitox serve as a protective shield when applied preventively, but lose much of their value when used reactively on heavily infected trees.

It is also worth noting that T. arjuna monoculture plantations are inherently more vulnerable to disease outbreaks than mixed or diversified planting systems. Maintaining plantation diversity, ensuring adequate spacing for air circulation, and monitoring disease calendars issued by CTR&TI can all reduce the frequency and severity of outbreaks across seasons.

Seasonal disease calendar: a practical guide 🔗

Understanding when each disease strikes is as important as knowing what it looks like. The following seasonal pattern, documented across major tasar-growing states by the Central Silk Board, helps farmers plan their interventions proactively:

• June-October: Black Nodal Girdling risk is highest. Inspect nodes weekly during high-humidity months. Begin Carbendazim sprays in June as a preventive measure.
• October-March: Powdery Mildew season. Start spraying Dinocap or Sulfex from October. Remove and burn affected leaves as soon as white patches appear.
• Spring flush (new growth period): Watch for Leaf Curling on tender foliage. Prune affected shoots promptly and apply copper-based sprays if symptoms emerge.

The ability to read a plantation’s disease status at a glance – and to respond with the right tool at the right time – is what separates a productive tasar season from a failed one. For the tribal and rural communities whose livelihoods depend on these host plants and the silkworms they support, that knowledge translates directly into income security.

What do you think? Given that Black Nodal Girdling peaks exactly during the main rearing season and powdery mildew hits during the winter months when preventive sprays are least expected, how feasible is it for small-scale tribal tasar farmers to maintain consistent disease monitoring calendars without institutional support? And with T. arjuna being more susceptible to powdery mildew than T. tomentosa, should commercial tasar plantations reconsider relying so heavily on T. arjuna as the primary host despite its faster growth rate?

References

1. https://link.springer.com/chapter/10.1007/978-981-10-3304-9_10
2. https://www.sciencedirect.com/article/abs/pii/S0580951721000106
3. http://silks.csb.gov.in/jhansi/diseases-and-pests-of-food-plants/
4. https://www.cabidigitallibrary.org/doi/10.1186/s43170-023-00204-z
5. https://ipm.ucanr.edu/m/pn7406-3.html
6. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/antheraea-mylitta
7. https://link.springer.com/article/10.1186/s43170-023-00204-z
8. https://sericulture.assam.gov.in/portlet-innerpage/food-plants-of-tasar-silkworm