Q4. (a) Describe the causal organisms, disease cycle and control measures of Early Blight of Potato and Blast of Rice.

Early Blight of Potato

Early blight is a common and often destructive fungal disease affecting potato (Solanum tuberosum) and other solanaceous crops like tomato and eggplant. It can significantly reduce yield, tuber size, and marketability.

Causal Organism

• The primary causal agent of Early Blight of Potato is the fungus Alternaria solani Sorauer.
• In some potato fields, a pathogen complex including A. solani and A. alternata may be observed.

Disease Cycle

The disease cycle of Alternaria solani is polycyclic, meaning multiple infection cycles can occur within a single growing season, leading to rapid disease progression under favorable conditions.

• Survival: The fungus overwinters as mycelium and conidia (spores) in infected potato plant debris (leaves, stems, tubers) in the soil and on alternative hosts (weeds). Conidia and mycelium can remain viable for over 17 months.
• Primary Infection:
  • In the spring, as temperatures rise and moisture becomes available, overwintered conidia germinate and produce new spores.
  • These primary inoculum are dispersed by wind, splashing rain, or irrigation water to healthy potato foliage, especially older, lower leaves. Spores can also be carried by equipment and workers.
  • Infection can occur through stomata, wounds, or direct penetration of the host epidermal cells.
• Symptom Development: Initial symptoms appear as small, dark brown to black, irregular or circular spots on the lower, older leaves. As these lesions enlarge, they develop characteristic dark concentric rings, giving them a "target spot" appearance. A yellow halo may surround these lesions. Elongated, brown to black lesions can also develop on stems and petioles. Tuber symptoms appear after harvest as dark, sunken lesions with a leathery to corky texture.
• Secondary Infection:
  • Once lesions form on the leaves, new conidia are produced on the infected tissue, especially during alternating wet and dry periods with optimal temperatures (around 20-25°C).
  • These secondary conidia are then dispersed by wind and rain, leading to new infections on healthy leaves within the same plant or neighboring plants.
  • Many cycles of spore production and lesion formation can occur, leading to an exponential increase in foliar infection.
• Tuber Infection: Tubers can become infected as they are lifted through infested soil at harvest, usually through wounds. Immature tubers and white/red-skinned varieties are more susceptible.

Control Measures

An integrated disease management (IDM) approach is most effective for controlling Early Blight of Potato, combining cultural, chemical, and genetic methods.

• Cultural Practices:
  • Crop Rotation: Rotate with non-host crops (e.g., small grains, corn, soybean) for at least 2-3 years to reduce inoculum levels in the soil.
  • Sanitation: Remove and destroy infected plant debris after harvest through burning or deep plowing to eliminate overwintering inoculum.
  • Disease-Free Seed: Use certified, pathogen-free seed tubers for planting.
  • Irrigation Management: Avoid overhead irrigation if possible, or time irrigation to allow leaves to dry before nightfall. Avoid irrigation during cool, cloudy weather.
  • Balanced Nutrition: Maintain proper plant nutrition and avoid nitrogen and phosphorus deficiencies, as stressed plants are more susceptible.
  • Weed Control: Eradicate solanaceous weed hosts (e.g., nightshade, horsenettle) that can harbor the pathogen.
  • Harvesting: Allow tubers to mature fully before harvest and handle carefully to avoid wounds. Dry tubers quickly if harvested under wet conditions.
• Chemical Control:
  • Fungicide Application: Apply foliar fungicides, such as those containing fluopyram (e.g., Luna Tranquility, Luna Pro) or strobilurins like azoxystrobin (though resistance can develop), chlorothalonil, mancozeb, or difenoconazole.
  • Timing: Timely application is crucial. Sprays typically begin after flowering or tuber initiation, or when physiological days (P-Days) accumulate, and are repeated at 7-15 day intervals depending on disease pressure and fungicide type.
  • Resistance Management: Rotate fungicides with different modes of action to prevent the development of resistant pathogen strains.
• Genetic Resistance:
  • Planting cultivars with field resistance or later maturity can help, as the disease favors older, senescing tissue. While no commercially produced potato cultivar is completely resistant, varieties like 'Kufri Sindhuri' show some degree of resistance.

Blast of Rice

Rice blast is considered the most devastating disease of cultivated rice (Oryza sativa L.) globally, capable of causing significant yield losses, sometimes leading to complete crop failure under severe conditions.

Causal Organism

• The disease is caused by the fungus Pyricularia oryzae Cavara (also referred to as Magnaporthe oryzae, its sexual stage).

Disease Cycle

Rice blast is a polycyclic disease that can affect all above-ground parts of the rice plant at any growth stage.

• Survival:
  • The primary inoculum overwinters as mycelium and conidia in infected rice stubble, crop residue, and infected seeds.
  • It can also survive on uncultivated grasses and some turf grasses, although these do not infect rice.
• Primary Infection:
  • When favorable conditions (high humidity, leaf wetness, optimal temperatures 25-30°C) occur, overwintered spores germinate.
  • Conidia are released and dispersed by wind, splashing rain, or irrigation water, infecting new rice plants.
  • Infection typically occurs when leaves are wet for prolonged periods (e.g., >10 hours).
• Symptom Development: The fungus produces lesions on various parts of the plant:
  • Leaf Blast: Small, dark, diamond-shaped or spindle-shaped lesions with gray or whitish centers and brown margins. In severe cases, leaves may blight and die, giving a "blasted" or "burnt" appearance.
  • Collar Blast: Affects the leaf collar, causing it to rot and leading to the collapse of the leaf blade.
  • Nodal Blast: Nodes on the culm become discolored (black) and weakened, often leading to the culm breaking.
  • Neck Blast (Rotten Neck): Infection of the panicle neck (just below the panicle) causes brown to grayish-brown discoloration and shriveling. If infection occurs before the milk stage, grains may not form (whitehead), leading to significant yield loss.
  • Panicle Blast: Lesions on panicle branches and spikelet pedicels, leading to partially filled or completely empty grains.
• Secondary Infection:
  • New conidia are produced on the lesions during moist conditions, forming spores that are readily dispersed by wind.
  • These spores cause subsequent infections, leading to rapid spread within and between fields.
  • Multiple disease cycles can occur in a season, making it highly destructive.

Control Measures

An integrated disease management (IDM) strategy is essential for effective control of Rice Blast, combining genetic resistance, cultural practices, and chemical control.

• Cultural Practices:
  • Resistant Varieties: Planting resistant or tolerant rice varieties is the most practical and economical method. However, the pathogen has high genetic diversity, quickly overcoming host resistance.
  • Seed Treatment: Use certified, disease-free seeds. Seed treatment with fungicides like carbendazim or tricyclazole, or biological agents like Pseudomonas fluorescens, can reduce primary inoculum.
  • Crop Rotation: Rotate rice with non-host crops to reduce inoculum buildup.
  • Field Sanitation: Burn or incorporate rice straw and stubble after harvest to destroy overwintering inoculum. Remove collateral weed hosts from bunds and channels.
  • Water Management: Maintain a continuous flood of 2-4 inches depth, especially in drill-seeded rice, as continuous flooding limits blast development by reducing nitrate availability and drought stress. Avoid extended drain periods.
  • Balanced Fertilization: Avoid excessive nitrogen application, which can lead to lush growth more susceptible to blast. Apply nitrogen in three split doses (basal, tillering, panicle initiation).
  • Planting Date and Density: Adjust planting dates to avoid periods favorable for disease development. Avoid dense planting, which creates humid microclimates conducive to the disease.
• Chemical Control:
  • Fungicide Application: Apply fungicides at critical growth stages, particularly during panicle initiation, booting, and heading. Systemic fungicides like tricyclazole, pyroquilon, and combination fungicides (e.g., trifloxystrobin + tebuconazole) are effective.
  • Timing: Prophylactic foliar sprays are important in high-risk areas. Early fungicide application during leaf blast stages might reduce inoculum, but head protection is often more economical.
  • Resistance Management: Alternate fungicides with different modes of action to prevent the development of fungicide resistance.
• Biological Control:
  • Application of bio-control agents like Pseudomonas fluorescens (seed treatment and foliar spray) can be effective as part of an integrated approach.

Comparative Summary of Early Blight of Potato and Blast of Rice

Feature	Early Blight of Potato	Blast of Rice
Causal Organism	Alternaria solani	Pyricularia oryzae (or Magnaporthe oryzae)
Host Crop	Potato, Tomato, Eggplant (Solanaceae)	Rice (Oryza sativa) and related grasses
Symptoms	Circular, dark-brown/black lesions with concentric rings (target spots) on leaves; sunken, dark lesions on tubers.	Spindle-shaped lesions on leaves; neck rot, nodal blackening, panicle blight leading to sterile or partially filled grains.
Primary Inoculum	Mycelium and conidia in infected crop debris, soil, alternate hosts.	Mycelium and conidia in infected crop debris, seeds, alternate grass hosts.
Dispersal	Wind, splashing rain, irrigation water, equipment.	Wind, splashing rain, irrigation water.
Favorable Conditions	Warm (20-25°C), humid periods, prolonged leaf wetness, plant stress, older leaves.	Warm (25-30°C), high humidity (>90%), prolonged leaf wetness (>10 hours), frequent rainfall, high nitrogen, dense planting.
Yield Loss Potential	5-50% (can be significant on susceptible cultivars).	10-30% annually, up to 100% in severe outbreaks.
Key Control Measures	Crop rotation, sanitation, disease-free seed, judicious irrigation, balanced nutrition, fungicides (chlorothalonil, fluopyram, strobilurins), resistant cultivars.	Resistant varieties, seed treatment, crop rotation, field sanitation, water management (continuous flood), balanced nitrogen, fungicides (tricyclazole, carbendazim, strobilurins), bio-control agents.