📈 Diseases of Citrus — Canker, Greening, Gummosis

Citrus is one of the most commercially important fruit crops worldwide. India is the third-largest citrus producer, with major species including sweet orange, mandarin, lime, and lemon. Canker, greening, and gummosis are the three most destructive diseases limiting citrus production.

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Citrus Canker

Causal Organism: Xanthomonas axonopodis pv. citri (Bacterium)

Symptoms

• Raised, corky, crater-like lesions on leaves, stems, and fruits
• Lesions are surrounded by a characteristic yellow halo (water-soaked margin)
• Severely affected fruits are unmarketable due to disfiguring canker spots
• Leaf fall and twig dieback occur in advanced infections

Disease Cycle

The bacterium survives in old canker lesions. Rain splash and wind-driven rain are the primary modes of spread. Leaf miners (Phyllocnistis citrella) create wounds that facilitate bacterial entry. Warm temperatures (25-35 degrees C) and high rainfall favour epidemics.

Management

• Use canker-free nursery stock and resistant rootstocks
• Spray Streptomycin sulphate (500 ppm) + Copper oxychloride (0.3%) during flushes
• Control citrus leaf miner with insecticides to reduce wound-mediated infection
• Eradicate severely infected trees in new plantations

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Citrus Greening (Huanglongbing)

Causal Organism: Candidatus Liberibacter asiaticus (Bacterium, phloem-limited)

Symptoms

• Asymmetric blotchy mottling on leaves — one side of the leaf is yellow while the other remains green
• Fruits remain small, lopsided, and develop a bitter taste
• Seeds are aborted, and the columella is stained orange-brown
• Gradual decline of the tree over several years

Vector

The disease is transmitted by the Asian citrus psyllid (Diaphorina citri).

Parameter	Detail
Pathogen type	Phloem-limited bacterium
Vector	Diaphorina citri
Latent period	6 months to several years
Diagnostic method	PCR-based detection

Management

• Use pathogen-tested budwood from certified nurseries
• Control psyllid populations with systemic insecticides (Imidacloprid, Thiamethoxam)
• Remove and destroy infected trees to reduce inoculum source
• No curative treatment is available once the tree is infected

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Gummosis

Causal Organism: Phytophthora nicotianae var. parasitica and P. citrophthora

Symptoms

• Dark, water-soaked bark lesions at the base of the trunk near the soil line
• Profuse gum exudation from the affected bark
• Bark splits and peels away, exposing dead cambium tissue
• Crown and root rot in waterlogged soils lead to tree decline

Management

• Budding on resistant rootstocks such as Trifoliate orange or Rangpur lime
• Apply Metalaxyl-Mancozeb paste on trunk lesions
• Improve drainage and avoid deep planting
• Soil drenching with Fosetyl-Al or Metalaxyl around the tree base

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Summary Cheat Sheet

Disease	Pathogen	Hallmark Symptom	Core Management
Citrus canker	Xanthomonas axonopodis pv. citri	Corky lesions with yellow halo	Copper + Streptomycin, clean nursery stock
Greening (HLB)	Candidatus Liberibacter asiaticus	Asymmetric leaf mottling and lopsided fruit	Psyllid control + rogue infected trees
Gummosis	Phytophthora spp.	Gum exudation and basal bark lesions	Drainage + resistant rootstock + fungicidal drench

References

2 sources • [1] [2]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

CABI Crop Protection Compendium

Website