Give an account on mycoplasmal diseases and physiological disorders of Potato.

Mycoplasmal Diseases of Potato

Mycoplasmal diseases in potatoes are caused by phytoplasmas, which are wall-less bacteria, obligate intracellular parasites, and are primarily transmitted by insect vectors, mainly leafhoppers. These diseases can lead to severe yield losses and affect tuber quality.

Key Mycoplasmal Diseases and their Characteristics:

• Potato Purple Top Roll (PPTR):
  • Symptoms: Characterized by upward rolling and purple or pink discoloration of the basal part of the top leaves. Other symptoms include stunting, chlorosis, proliferation of axillary shoots, aerial tuber formation, and swelling of nodes. In some varieties, only rolling occurs without discoloration.
  • Causal Organism: Phytoplasmas belonging to the "Aster Yellows" group.
  • Transmission: Primarily transmitted by leafhoppers (e.g., Orosius albicinctus) and infected tubers serve as a significant source of inoculum.
  • Impact: Can cause 15-20% loss in potato yield in affected regions.
• Potato Witches' Broom:
  • Symptoms: Extreme stunting of the plant, formation of numerous slender, filamentous stems with unusually small, simple leaves. The affected plants produce very small, numerous tubers or no tubers at all, leading to significant or total crop loss. The internodes become shorter, and tuber formation is poor.
  • Causal Organism: Phytoplasmas.
  • Transmission: Transmitted through leafhoppers and infected tubers. Grafting can also transmit the disease.
  • Impact: Severely reduces yield and quality, often resulting in complete economic loss for the affected plant.
• Marginal Flavescence of Potato:
  • Symptoms: Main symptom is chlorosis (yellowing) on the margins of the upper young leaves. In severe cases, the affected plants may develop a pinkish coloration on leaflet bases and show rolling. Hairy root symptoms on tubers have also been observed.
  • Prevalence: More common in hills and plateau areas where cool weather persists during the crop season.
  • Impact: Can lead to substantial crop losses, ranging from 40-75%.
  • Transmission: Transmitted by leafhoppers and grafting.
• Little Leaf of Potato:
  • Symptoms: Characterized by a significant reduction in the size of the leaves, giving the plant a stunted appearance. There is often a failure of proper tuber development.
  • Causal Organism: Phytoplasmas.
  • Impact: Affects tuber yield and quality adversely.

Management of Mycoplasmal Diseases:

• Use of disease-free seed tubers: Essential to prevent primary infection.
• Vector control: Managing leafhopper populations through insecticides can reduce disease spread.
• Rouging: Removing and destroying infected plants promptly to prevent further spread.
• Hot water treatment: For marginal flavescence, hot water treatment of tubers at 50°C for 10-15 minutes has been shown to eliminate the pathogen without adverse effects on tubers.
• Resistant varieties: Developing and using varieties with resistance to phytoplasmas.

Physiological Disorders of Potato

Physiological disorders are non-pathogenic abnormalities in potato tubers or plants caused by unfavorable environmental conditions, nutritional imbalances, or improper cultural practices, rather than by pathogens or pests. These disorders can significantly reduce marketability and quality.

Common Physiological Disorders and their Causes/Symptoms:

Disorder	Symptoms	Causes	Management Strategies
Hollow Heart	A star-shaped or irregular cavity in the center of the tuber, which may or may not be discolored. Often preceded by brown center.	Rapid growth following a period of stress (e.g., fluctuating soil moisture, sudden nutrient availability, wide plant spacing), high nitrogen levels, or insufficient calcium.	Maintain uniform growth conditions, avoid over-irrigation, ensure optimal plant population, balanced fertilization (especially calcium). Select less-prone cultivars.
Black Heart	Internal blackening of the tuber flesh, often in the center. The affected tissue turns dark gray to black.	Oxygen deficiency in tubers, especially during storage or transportation under high temperatures (e.g., above 32°C) or poor ventilation. Can also occur in fields with waterlogged soil.	Ensure adequate ventilation during storage and transit. Maintain appropriate storage temperatures (avoiding extremes). Avoid leaving harvested tubers in direct sun on the field.
Internal Brown Spot (IBS) / Brown Fleck	Irregular brown spots or flecks scattered throughout the tuber flesh, often dry and corky. No external symptoms initially.	Linked to high soil temperatures and severe calcium deficiency during the growing season. Moisture fluctuations also contribute.	Maintain consistent soil moisture, ensure adequate and balanced nutrition, particularly calcium. Avoid high soil temperatures.
Green Tuber / Greening	Development of green pigmentation (chlorophyll) on the surface of tubers, often accompanied by accumulation of toxic solanine alkaloids, making them bitter and potentially harmful.	Exposure of tubers to sunlight or artificial light in the field (due to shallow planting, soil cracks) or during storage, transport, and display.	Proper hilling to cover tubers, adequate planting depth, minimize exposure to light after harvest and during storage. Choose varieties less prone to greening.
Growth Cracks	External cracks or splits on the tuber surface, varying in size and depth.	Sudden changes in growth rate, often due to fluctuating soil moisture (e.g., heavy rain after a dry spell), inconsistent temperatures, or excessive nitrogen application.	Maintain uniform soil moisture through irrigation, avoid sudden environmental shifts, and provide balanced nutrition.
Black Spot / Pressure Bruise	Discoloration (dark patches) beneath the potato skin, caused by mechanical impact or pressure. No visible external injury.	Mechanical bruising during harvest, handling, or storage. Low pulp temperature, low humidity, and loss of moisture from tubers during storage increase susceptibility.	Careful handling of tubers at all stages. Warm tubers to 10°C before grading. Maintain appropriate storage humidity and temperature to prevent shrinkage.
Sprouting Disorders (e.g., Internal Sprouting, Secondary Tubers)	Sprouts growing into the tuber or formation of bead-like miniature tubers directly from the seed potato in storage or immediately after planting.	Physiological aging of tubers, high storage temperatures (above 16°C), low concentrations of sprout inhibitors, or planting old seed into cold, dry soil.	Proper storage conditions (temperature and humidity) to control physiological age. Use healthy, appropriately aged seed. Apply effective sprout inhibitors when necessary.
Cold Damage / Freezing Injury	Internal gray, reddish, or black discoloration, often in vascular tissue. Frozen tissues become soft and watery.	Exposure of tubers to temperatures at or below freezing point, either in the field (e.g., shallow tubers during frost) or in storage/transit.	Harvest before severe frost. Provide frost protection during storage and transport. Do not dig if the ground is frozen.

Integrated Management Approach for Potato Health:

An integrated approach combining cultural, biological, and chemical methods is crucial for managing both mycoplasmal diseases and physiological disorders. This includes:

• Certified Seed: Using disease-free, certified seed tubers adapted to local agro-climatic conditions.
• Optimal Agronomic Practices:
  • Balanced fertilization, especially micronutrients and calcium.
  • Timely and adequate irrigation to maintain consistent soil moisture.
  • Proper planting depth and hilling to protect tubers from light.
  • Appropriate plant spacing.
• Pest Management: Control of insect vectors like leafhoppers through systemic insecticides or biological control methods to curb phytoplasma spread.
• Harvesting and Storage:
  • Careful handling during harvest to prevent mechanical damage.
  • Prompt removal of harvested tubers from the field, especially during extreme temperatures.
  • Maintaining optimal temperature, humidity, and ventilation in storage facilities.
• Variety Selection: Choosing potato varieties known for their resistance to specific diseases and disorders, and those well-suited to the local climate. The ICAR-Central Potato Research Institute (CPRI) has developed over 50 improved varieties suited to Indian agro-climatic conditions.