A) Tikka Disease of Groundnut (Early and Late Leaf Spots)

Tikka disease is one of the most destructive diseases affecting groundnut crops worldwide, especially in India, where it causes significant yield losses. The disease manifests as two distinct types of leaf spots: Early Leaf Spot and Late Leaf Spot, caused by different fungal species.

Causal Organisms

1. Early Leaf Spot:
   • Pathogen: Cercospora arachidicola
   • Sexual Stage: Mycosphaerella arachidis
2. Late Leaf Spot:
   • Pathogen: Phaeoisariopsis personata (Synonym: Cercosporidium personatum)
   • Sexual Stage: Mycosphaerella berkeleyii

Economic Importance

• Tikka disease is the most devastating foliar disease of groundnut, prevalent in almost all groundnut-growing areas.
• It is particularly severe when the plants are infected early in the season.
• The disease can cause pod yield losses of up to 50%.
• Late leaf spots are more commonly observed than early leaf spots in many regions, including parts of India.

Symptoms

1. Early Leaf Spot (Cercospora arachidicola)

• Appearance Time: Usually appears within 35 Days After Sowing (DAS).
• Location: Primarily affects leaflets but can also occur on rachis, petioles, stipules, and stems as elongated, elliptical spots with defined borders.
• Description of Spots:
  • Spots are irregularly circular (1-10 mm in diameter).
  • Reddish-brown or dark brown on the upper leaf surface, surrounded by a bright yellow halo.
  • On the lower surface, spots are light brown to tan.
  • Coalescence of spots leads to drying and premature defoliation of leaves.
• Distinct Features:
  • Appears earlier (before 35 DAS) than late leaf spot, hence called Early Leaf Spot.
  • Lesions may also form on petioles, stems, and pegs.

2. Late Leaf Spot (Phaeoisariopsis personata)

• Appearance Time: Typically appears after 35 DAS.
• Location: Affects leaflets, petioles, stems, and pegs.
• Description of Spots:
  • Spots are circular with a bright yellow halo around mature spots.
  • Usually darker than early leaf spots, appearing deep black in color.
  • No yellow halo is visible on the lower leaf surface.
  • Clusters of conidiophores bearing conidia are arranged in a concentric pattern.
• Distinct Features:
  • Causes severe defoliation when infection is intense.
  • Often starts on the lower leaves and progresses upwards.

Pathogen Details

1. Cercospora arachidicola (Sexual Stage: Mycosphaerella arachidis)

• Mycelium: Both intercellular and intracellular.
• Sporulation: Abundant on the upper leaf surface.
• Conidiophores:
  • Olivaceous brown or yellowish-brown, short, 1-2 septate, unbranched, and geniculate.
  • Arise in clusters.
• Conidia:
  • Sub-hyaline or pale yellow, obclavate, often curved.
  • 3-12 septate, measuring 35-110 x 2.5 – 5.4 µm.
  • Rounded to distinctly truncate base with a sub-acute tip.
• Sexual Stage (Mycosphaerella arachidis):
  • Produces asci in pseudothecia that are globose or broadly ovate with a papillate ostiole.
  • Asci are cylindrical to clavate, containing 8 ascospores.
  • Ascospores are hyaline, slightly curved, and two-celled, with the apical cell larger than the lower cell.

2. Phaeoisariopsis personata (Sexual Stage: Mycosphaerella berkeleyii)

• Mycelium: Both intercellular and intracellular.
• Conidiophores: Long, continuous, 1-2 septate, geniculate, arising in clusters on the lower leaf surface. Olive brown in color.
• Conidia:
  • Cylindrical or obclavate, short, measuring 18-60 x 6-10 µm.
  • Hyaline to olive brown, usually straight or slightly curved with 1-9 septa (mostly 3-4 septate).
• Sexual Stage (Mycosphaerella berkeleyii):
  • Produces asci in pseudothecia which are globose or broadly ovate with a papillate ostiole.
  • Asci are cylindrical to ovate, containing 8 ascospores.
  • Ascospores are 2-celled, constricted at the septum, and hyaline.

Disease Cycle

• Survival:
  • Fungi survive as conidia, dormant mycelium, and pseudothecia in infected plant debris.
  • They also persist on volunteer groundnut plants, contaminated pods, and seeds.
• Primary Infection: By ascospores or conidia liberated from infected plant debris.
• Secondary Spread: Through wind-blown conidia. Rain splash also aids the spread of conidia.

Favourable Conditions

• Prolonged high relative humidity for 3 days.
• Low temperatures (25-30°C) with dew on leaf surfaces.
• Heavy application of nitrogen and phosphorus fertilizers.
• Magnesium deficiency in the soil.

Management Practices

i) Cultural Practices:

• • Remove and destroy infected plant debris to reduce inoculum load.
  • Eradicate volunteer groundnut plants that may harbor pathogens.
  • Crop rotation with millets or other non-host crops.

ii) Chemical Control:

• • Seed Treatment: With Captan or Thiram at 4g/kg or Carbendazim @ 0.2%.
  • Foliar Sprays:
    • Carbendazim @ 0.1%.
    • Mancozeb @ 0.2%.
    • Chlorothalonil @ 0.2%.
    • Repeat spraying after 15 days if necessary.

iii) Resistant Varieties:

• • Vemana (resistant to both early and late leaf spots).
  • Naveen, Tirupathi-3 (resistant to early leaf spot only).

B) Rust of Groundnut

Causal Organism: Puccinia arachidis

Economic Importance

• Rust of groundnut is one of the most destructive diseases affecting groundnut production, especially in India.
• Historical Background: First observed in Punjab, India, and was officially recorded for the first time at Tirupati in 1971.
• Impact on Yield:
  • Yield loss can be as high as 38% when the infection occurs early in the crop growth stage.
  • Severe infestations lead to the formation of small and shriveled seeds, reducing both quantity and quality of the yield.
• Geographical Spread:
  • Prevalent in tropical and subtropical groundnut-growing regions with high humidity and moderate temperatures.

Symptoms

• The disease attacks all aerial parts of the plant, including leaves, petioles, and stems.
• Usually appears when plants are about 6 weeks old.

Leaf Symptoms

• Initial Signs: Small, minute pale yellow to light brown pustules (uredosori) appear on the lower surface of the leaves. The pustules enlarge and can reach a size of about 1 mm in diameter.
• Development of Pustules: As the pustules mature, the epidermis ruptures, releasing a powdery mass of uredospores. Corresponding to the uredosori on the lower surface, small, necrotic brown spots appear on the upper surface of leaves.
• Late-Season Symptoms: In advanced stages, brown teliosori appear as dark pustules amidst the necrotic patches. Severe infections cause premature drying and defoliation of lower leaves.

Stem and Petiole Symptoms

• Rust pustules may also appear on petioles and stems.
• The infection spreads upwards, affecting new growth.

Effect on Pods and Seeds Infected plants produce small and shriveled seeds due to reduced photosynthetic activity.

Pathogen Characteristics

Classification

• Kingdom: Fungi
• Phylum: Basidiomycota
• Class: Pucciniomycetes
• Order: Pucciniales
• Family: Pucciniaceae
• Genus: Puccinia
• Species: Puccinia arachidis

Reproductive Stages

The pathogen exhibits two major reproductive stages on groundnut:

1. Uredial Stage (Asexual Reproduction)
   • Uredospores are the main source of secondary infection.
   • Characteristics:
     • Pedicellate, unicellular, yellow, oval or round, and echinulate (spiny surface).
     • Produced abundantly on groundnut.
2. Telial Stage (Sexual Reproduction)
   • Teliospores are dark brown, bicellular, and are involved in the overwintering of the pathogen.
   • Production is limited compared to uredospores.

Absent Stages

• Pycnial and Aecial stages are not observed.
• There is no evidence of an alternate host, unlike many other Puccinia species.

Disease Cycle

• Primary Source of Inoculum: The fungus survives as uredospores on volunteer groundnut plants. It can also persist in infected plant debris left in the soil.
• Secondary Spread: Uredospores are the primary means of secondary spread, dispersed mainly by wind. Contaminated seeds and pods also contribute to long-distance dissemination.
• Modes of Transmission:
  • Wind-borne uredospores are the most common means of spread.
  • Rain splash and farming implements can also disseminate the spores locally.
• Collateral Hosts:
  • The pathogen can survive on other species within the genus, such as Arachis marginata and Arachis prostrata.

Favourable Conditions

• High relative humidity (> 85%).
• Heavy rainfall and frequent leaf wetness.
• Moderate temperatures ranging from 20°C to 25°C.
• Dense canopy which retains moisture, favoring spore germination and infection.

Management Strategies

i) Cultural Practices

• Avoid Monoculture: Practice crop rotation to break the disease cycle.
• Sanitation and Hygiene: Remove volunteer groundnut plants and collateral hosts to eliminate primary inoculum.
• Plant Spacing and Pruning: Ensure adequate spacing between plants to reduce humidity and improve air circulation.

ii) Chemical Control: Fungicidal Sprays: Chlorothalonil or Tridemorph at 0.2% concentration is effective. Spraying should begin at the onset of symptoms and continue at 10-15 day intervals during favorable conditions.

iii) Biological Control: Biocontrol Agents: Research is ongoing on the use of antagonistic fungi and bacterial species to inhibit P. arachidis growth.

iv) Resistant Varieties: Breeding for Resistance: Utilization of Arachis glabarata in breeding programs to develop resistant varieties. Integrated Disease Management (IDM): Combine cultural, chemical, and biological control methods with the use of resistant cultivars for effective management.

Recent Advances in Research

• Molecular Studies: Research on host-pathogen interaction and gene expression for rust resistance.
• Genomic Tools: Use of molecular markers for marker-assisted selection (MAS) in breeding programs.
• Nanotechnology: Exploration of nano-fungicides for improved delivery and efficacy.