Identifying and Controlling Soil-Borne Diseases in Mulberry

Mulberry (Morus spp.) is the exclusive food source for the silkworm Bombyx mori, making its health directly tied to the success of the entire silk industry. Among the many threats mulberry faces, soil-borne diseases are particularly destructive because they attack the plant’s root system – silently, underground – long before visible damage appears above the soil surface. Two diseases stand out for the severity of their economic impact: Root Knot, caused by the nematode Meloidogyne incognita, and Root Rot, caused primarily by the fungus Fusarium solani along with several other soil-borne pathogens. Understanding how to identify and control these diseases is essential for anyone involved in mulberry cultivation and sericulture.

Why soil-borne diseases are a serious concern in mulberry 🔗

Unlike foliar diseases that are visible on leaves, soil-borne pathogens live and persist in the soil, often going undetected until the plant is severely compromised. Soil-borne diseases can cause tissue discoloration, wilting of foliage, root decay, and sudden plant death – all of which directly reduce leaf yield and quality. In mulberry, a decline in leaf quality translates immediately to poor silkworm growth and reduced cocoon production, which hits farmers economically at every stage of the value chain. Research from India and China has documented that root-knot nematode infestation alone can reduce mulberry leaf yield by anywhere from 20 to 45%, with losses exceeding 75% in severely affected fields.

Root knot disease: the nematode threat 🔗

Root Knot disease in mulberry is caused by Meloidogyne incognita, a microscopic plant-parasitic nematode that is considered one of the most economically damaging plant pathogens in tropical and subtropical regions worldwide. It is widely distributed across all major silkworm-rearing countries, including India, Japan, Brazil, and China, and is especially problematic in sandy loamy soils under irrigated conditions.

How the nematode infects mulberry 🔗

M. incognita begins its life cycle when second-stage juveniles (J2) hatch from eggs in the soil and migrate toward plant root tips. Once inside the root, the juveniles move through cortical tissues toward the vascular zone and establish permanent feeding sites called giant cells. The plant responds by forming abnormal swellings around these feeding sites, which are the characteristic galls or knots visible on infected roots. The females then mature and lay eggs into a gelatinous mass on the root surface, restarting the cycle. The entire life cycle takes as little as 37 days at 21°C, meaning populations can build rapidly within a single growing season.

Symptoms of root knot in mulberry 🔗

Above-ground symptoms are easy to miss initially. Severely affected plants show stunted growth, reduced leaf moisture, and yellowing of leaf margins. As the disease progresses, leaves become sparse and the plant wilts even when soil moisture is adequate. The definitive diagnosis, however, comes from examining the roots. Root knot causes the formation of spherical galls of varying sizes – young galls are small and yellowish-white, while older galls turn pale brown and may merge, distorting the root system significantly. This physical damage to roots blocks nutrient and water uptake, which explains the above-ground wilting and stunted growth. The disease spreads primarily through contaminated soil, infected planting material, farm implements, and run-off irrigation water.

The economic consequences extend beyond yield. Studies have shown that nematode infestation reduces both the quantity and nutritive quality of mulberry leaves, which directly impairs silkworm growth and cocoon yield – representing compounded losses for silk farmers.

Root rot disease: the fungal complex 🔗

Root Rot in mulberry is not caused by a single pathogen but by a complex of soil-borne fungi. The primary causal agents identified in Indian sericulture regions include Fusarium solani, Fusarium oxysporum, Macrophomina phaseolina (charcoal root rot), Lasiodiplodia theobromae (black root rot), Rhizoctonia solani, and Sclerotium rolfsii. Research from India identifies M. phaseolina as the most prevalent pathogen in the South Indian sericulture belt, capable of causing up to 35% leaf yield loss, while Fusarium solani and dry rot forms are consistently reported across all major mulberry-growing zones.

Symptoms of root rot in mulberry 🔗

Infected plants display dried and wilted foliage with a brown coloration, soft and decayed root systems, and squashy bark that releases gummy exudates with an unpleasant smell. In the case of Lasiodiplodia and Macrophomina, the mycelium spreads beneath the bark and blackening of the bark progresses rapidly. Fusarium solani specifically causes what is known as dry root rot – the root cortex browns and shrivels, and the vascular tissue becomes discolored, cutting off water and nutrient flow to the shoots. Above-ground symptoms of Fusarium infection include wilting, stunted growth, and chlorosis, often appearing first on one side of the plant before spreading. If the infected plant is not removed promptly, the pathogens spread to neighboring plants through the soil, potentially causing the abandonment of entire mulberry gardens.

An added complication is that root rot pathogens and root-knot nematodes frequently occur together. Nematode feeding wounds on roots create ideal entry points for fungal pathogens, meaning that a plant already weakened by M. incognita is far more vulnerable to root rot infections – and vice versa.

Management of soil-borne diseases in mulberry 🔗

Effective management of Root Knot and Root Rot requires a multi-pronged strategy that combines cultural practices, chemical treatments, and biological approaches. No single method alone provides adequate control, particularly because many of these pathogens persist in the soil for years as dormant survival structures.

Cultural control methods 🔗

Cultural practices form the foundation of soil-borne disease management and work best as preventive measures. Key practices include:

Deep plowing: Turning the soil to a depth of 30 cm or more exposes dormant pathogens and nematode eggs to sunlight and desiccation, significantly reducing their viability. Deep tillage and soil exposure are recognized as effective sanitation measures that disrupt pathogen survival structures in the upper soil layers.

Soil solarization: Covering moist soil with transparent polyethylene sheets during hot summer months raises soil temperature to levels that kill many fungal pathogens and nematode juveniles. This is particularly useful before establishing a new mulberry garden.

Removal and destruction of infected plants: Diseased plants should be uprooted immediately, and their roots burned. The pit soil should be exposed to sunlight for several days before replanting. Leaving infected plant debris in the field allows pathogens to multiply and spread.

Use of disease-free planting material: New saplings should be sourced from pathogen-free nurseries. When planting, roots can be pre-treated by dipping them in approved fungicide solutions to reduce the risk of introducing pathogens into clean soil.

Crop rotation and antagonistic plants: In fields with a history of nematode problems, introducing antagonistic crops such as Tagetes (marigold) species helps reduce nematode populations. Certain Tagetes species have well-documented nematicidal properties when incorporated into soil.

Chemical control measures 🔗

For established infestations, chemical treatments provide faster action, though they must be used carefully to avoid harming silkworms or the beneficial soil microbiome.

For Root Rot: The Central Sericultural Research and Training Institute (CSRTI), Mysore, developed a target-specific formulation called Navinya (80% herbal and 20% chemical components) for root rot control. The application method involves pruning infected shoots 15-30 cm above the ground, making a shallow ring in the soil around the stump, and drenching the pruned stump with Navinya solution (10 g per liter of water). Surrounding plants should also be treated to prevent spread. New saplings can be protected by root-dipping them in 0.2% Navinya solution before planting.

Fungicide treatment at planting: Applying 10 g of Indofil M-45 (Mancozeb) per planting pit and soaking new saplings in a 0.1% Indofil M-45 solution for 30 minutes before planting provides protection during the critical establishment phase. Follow-up applications of 500 g per plant in the root zone at intervals of four months help maintain protection.

For nematode management, periodic soil sampling is recommended to monitor population levels before they reach economically damaging thresholds, allowing for timely intervention.

Biological control and bioformulations 🔗

Given that mulberry leaves are fed directly to silkworms, minimizing chemical residues is a strong practical reason to prioritize biological approaches. Bioformulations are increasingly central to soil-borne disease management in mulberry.

Nursery Guard is a bioformulation used during nursery establishment to protect young mulberry saplings from soil-borne pathogens before and during transplanting. Applied as a soil drench or root dip, it helps protect the vulnerable early root system from initial pathogen attack.

Bionema is a biological nematicide used specifically against root-knot nematodes. It typically contains entomopathogenic fungi or nematophagous organisms that parasitize nematode eggs and juveniles in the soil, reducing nematode populations without chemical toxicity to the plant or to silkworms.

Research has demonstrated that biocontrol agents including Trichoderma harzianum, Pseudomonas fluorescens, and actinobacteria from the mulberry rhizosphere show strong antagonistic activity against root rot pathogens including Fusarium solani, Macrophomina phaseolina, Rhizoctonia solani, and others. These agents work through antibiosis, competition for nutrients, and induction of systemic resistance in the plant.

Studies on botanical-based soil amendments, including leaf powder from Passiflora foetida, have also demonstrated nematicidal activity against M. incognita in mulberry, showing reductions of up to 80% in root galls when used as soil incorporation treatments. Such botanical options are particularly valuable in organic or low-input sericulture systems.

Integrated disease management (IDM) approach 🔗

The most effective and sustainable results come from combining all three pillars – cultural, chemical, and biological – into an integrated disease management (IDM) strategy. In practice, this means:

Preparing fields with deep plowing and soil solarization before planting, using certified disease-free saplings treated with fungicide or bioformulation root dips, applying bioformulations like Nursery Guard and Bionema mixed with farmyard manure to the planting zone, monitoring the garden regularly for early symptoms, and using targeted chemical treatments only when disease pressure exceeds acceptable thresholds. Combining biological agents with organic amendments like farmyard manure creates synergistic effects – the organic matter feeds beneficial microorganisms and helps them establish stable populations that provide ongoing protection.

The advantage of IDM over single-method approaches is long-term sustainability. Chemical-only programs can build pathogen resistance over time, disrupt beneficial soil biology, and leave residues that affect silkworm health. A well-managed IDM program reduces input costs, maintains soil health, and provides more durable protection across seasons.

Early detection is the key 🔗

Both Root Knot and Root Rot are far easier to manage when caught early. Wilting during cooler parts of the day, unexplained stunting in patches of the garden, and yellowing of lower leaves are the first signs that something is wrong underground. Whenever these symptoms appear in mulberry, pulling up a sample plant and examining the roots directly – looking for galls, dark discoloration, or decayed tissue – provides a rapid on-site diagnosis. Confirming the pathogen type, whether nematode, Fusarium, or another fungus, helps in choosing the right management approach quickly. Delaying action typically results in the disease complex spreading to neighboring plants and compounding the damage to leaf yield and silkworm output.

What do you think? Given that root-knot nematodes and root rot fungi often co-infect mulberry simultaneously, would a bioformulation targeting both pathogens at once be more practical for small-scale silk farmers than managing each disease separately? And how feasible is it for farmers in high-pressure nematode zones to rely primarily on biological controls rather than chemical nematicides?

References

1. https://soilhealth.ucdavis.edu/soil-challenges/soil-borne-diseases
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC7265890/
3. https://en.wikipedia.org/wiki/Meloidogyne_incognita
4. https://silks.csb.gov.in/gaya/wp-content/themes/Common_District/gaya/dpm-frame.html
5. https://arccjournals.com/journal/agricultural-science-digest/D-5229
6. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0200099
7. https://ejbpc.springeropen.com/articles/10.1186/s41938-022-00532-8
8. https://www.koppert.com/plant-diseases/fusarium-spp/
9. https://www.mdpi.com/2077-0472/10/1/16
10. https://www.mdpi.com/2311-7524/7/2/33