Fungal Diseases of Potatoes

Potato crops face numerous fungal diseases that impact yield and quality. This article explores key fungal diseases affecting potatoes, their symptoms, favorable conditions, and control measures. Understanding these diseases is crucial for effective management and maintaining healthy potato production.

Black Dot

Black dot, caused by Colletotrichum coccodes, affects potato tuber appearance, leading to downgrading in markets. While it typically does not impact plant growth, it can cause yield losses when combined with other factors like nematode infestations or wilt diseases.

A. Symptoms

Black dot is identified by small, black micro-sclerotia on tubers, stolons, roots, and stems. Dark-brown marks on tubers resemble silver scurf but are less defined. A hand lens reveals micro-sclerotia, distinguishing black dot from silver scurf.

B. Impact

The disease spoils tuber appearance, leading to cosmetic downgrading. Moisture loss in affected tubers reduces product quality. In severe cases, early die-off occurs, lowering yields, especially when combined with nematode or fungal wilt infestations.

C. Control Measures

1. Treat Tubers: In South Africa, treat tubers with registered fungicides days before planting to avoid eye damage.

2. Use Certified Seed: Plant disease-free, certified seed tubers to minimize infection risk.

3. Harvest Promptly: Harvest as soon as foliage dies to prevent disease spread in soil.

4. Store Properly: Keep tubers in dry, cool conditions and remove plant residues to reduce infection sources.

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Botrytis (Grey Mould)

Grey mould, caused by Botrytis cinerea, affects various crops, including potatoes. This widespread fungus spreads rapidly under favorable conditions, impacting leaves, stems, and tubers, leading to significant post-harvest losses.

A. Symptoms

Lesions develop on older leaf margins, appearing grey-green or transparent, often covered with grey spores. Infected tubers show dry rot symptoms with spore masses. Sclerotia form on dead stems or inside tubers, spreading the disease.

B. Favorable Conditions

Botrytis cinerea thrives at 18–24°C with relative humidity above 75%. Dense canopies promote high humidity, aiding disease development. Tuber infection occurs post-harvest, especially under wet conditions or through mechanical wounds.

C. Control Measures

1. Plant Healthy Seed: Use healthy seed potatoes to reduce infection risk.

2. Avoid Wet Harvests: Do not harvest in wet conditions to prevent disease spread.

3. Space Rows Wider: Wider row spacing reduces dense vegetation, limiting disease promotion.

4. Store Properly: Store potatoes under conditions that promote wound healing to minimize infection.

Brown Spot

Brown spot, caused by Alternaria alternata, is an emerging disease in South Africa, causing significant yield losses. It appears earlier than early blight and thrives in specific environmental conditions, making control challenging.

A. Symptoms

Symptoms appear as small, brown, water-soaked lesions on leaf undersides, enlarging over days. Unlike early blight, lesions lack concentric rings and remain smaller. The disease affects leaves from 50 days post-emergence or earlier.

B. Favorable Conditions

Warm, humid conditions and prolonged leaf wetness promote brown spot. Spores spread via air, water, or plant debris. The fungus infects through micro-injuries caused by insects, wind, hail, or chemical damage.

C. Control Measures

1. Plant Resistant Cultivars: Choose cultivars less susceptible to Alternaria species.

2. Remove Inoculum: Clear plant debris, weeds, and volunteer plants to reduce infection sources.

3. Use Crop Rotation: Include non-host crops like wheat or maize in rotation programs.

4. Apply Fungicides: Use fungicides at recommended rates, alternating active ingredients to prevent resistance.

Early Blight

Early blight, caused by Alternaria solani, is South Africa’s most common potato disease, causing up to 50% yield loss in severe cases. It affects leaves and tubers, spreading rapidly under specific conditions.

A. Symptoms

Dark-brown lesions with concentric rings form on leaves, followed by yellowing and die-off. Infected tubers develop single, sunken, dark-brown lesions during storage, remaining firm to a depth of 10–12 mm.

B. Favorable Conditions

Hot, moist conditions, rain, heavy dew, or irrigation promote early blight. The disease spreads rapidly during alternating wet and dry weather, particularly on light soils, increasing infection risk.

C. Control Measures

1. Plant Healthy Seed: Use certified, disease-free seed potatoes to reduce infection.
2. Apply Crop Rotation: Rotate crops to lower inoculum levels in the soil.
3. Use Fungicides: Apply registered fungicides to control disease spread.
4. Optimize Irrigation: Irrigate judiciously to maintain healthy plant growth and reduce disease risk.

Fusarium Dry Rot

Fusarium dry rot, caused by Fusarium species like F. solani and F. oxysporum, is a major post-harvest disease in South Africa. It infects tubers through injuries, leading to significant storage losses.

A. Symptoms

Symptoms appear 2–4 weeks post-infection, with brown to dark-brown sunken spots. Infected tubers may rot completely, showing wrinkled skin and hollows filled with fungal spores and mycelia.

B. Favorable Conditions

Dry rot is severe after hot, dry seasons and develops rapidly at 15–25°C with 90–98% relative humidity. Infection occurs through injuries during harvesting, sorting, or transport, even in cold storage.

C. Control Measures

1. Minimize Damage: Harvest mature tubers carefully to avoid mechanical damage.

2. Apply Fungicides: Treat tubers with fungicides within 4–6 hours post-harvest.

3. Store Properly: Store tubers at 13–16°C with high humidity for 10 days to promote wound healing.

4. Handle Carefully: Avoid unnecessary handling of cold-stored tubers to reduce infection risk.

Fusarium Wilt

Fusarium wilt, caused by Fusarium oxysporum and F. solani, can lead to 100% infection under optimal conditions, causing severe losses. It affects plant growth and tuber quality significantly.

A. Symptoms

Infected plants show lightening, wilting, yellowing, and curling of lower leaves mid-season. Stems exhibit corky decay, and tubers show sunken, wrinkled rot at the stem end or stolon attachment.

B. Favorable Conditions

Hot, dry weather and plant stress increase disease severity. High temperatures and low soil moisture promote infection, particularly when plants are under environmental stress.

C. Control Measures

1. Use Crop Rotation: Rotate with maize or wheat for three years to reduce inoculum.

2. Avoid Susceptible Cultivars: Do not plant susceptible varieties in affected areas.

3. Optimize Fertilization: Use high phosphate and potash, low nitrogen to suppress disease.

4. Plant Deeply: Plant well-sprouted tubers deep with narrow spacing to maintain low soil temperatures.

Gangrene

Gangrene, caused by Phoma exigua varieties, affects tubers during storage and handling. It delays emergence and reduces yield when infected seed potatoes are planted.

A. Symptoms

Tubers develop dark, sunken lesions or “thumbmarks” at injury sites, eyes, or lenticels. These lesions are characteristic and distinguish gangrene from other tuber diseases.

B. Favorable Conditions

Infection occurs through cuts, bruises, or lenticels during harvesting or handling. Cool, wet soils and storage temperatures of 2–10°C promote gangrene, while higher temperatures favor Fusarium dry rot.

C. Control Measures

1. Avoid Damage: Handle tubers carefully to prevent cuts and bruises.

2. Store Properly: Store at 13–16°C with less than 80% humidity for 10 days to promote healing.

3. Monitor Conditions: Avoid cool storage temperatures that favor gangrene development.

Late Blight

Late blight, caused by Phytophthora infestans, is a devastating disease that can destroy entire fields rapidly if uncontrolled. It affects leaves, stems, and occasionally tubers under specific conditions.

A. Symptoms

Small, light-green, water-soaked spots appear on older leaves, expanding into large, black lesions during cool, moist weather. Stems show brown lesions, while tuber infections are rare but occur in wet, cool soils.

B. Favorable Conditions

1. Temperature Range: Day temperatures of 15–24°C and night temperatures above 10°C.

2. High Humidity: Relative humidity above 90% with free moisture on plants.

3. Cloudy Weather: Cloudy conditions promote rapid disease spread.

C. Control Measures

1. Plant Resistant Cultivars: Choose cultivars with good late blight resistance.

2. Use Fungicides: Apply registered fungicides, avoiding phenylamides in resistant areas.

3. Monitor Fields: Inspect fields twice weekly and use disease-forecasting systems.

4. Avoid Wet Harvests: Do not harvest during wet weather to prevent tuber infection.

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Powdery Scab

Powdery scab, caused by Spongospora subterranea, affects tubers and roots in most potato-growing regions. The pathogen persists in soil for years, making control challenging.

A. Symptoms

Lesions are small, round, light-brown raised areas on tubers, enlarging to reveal powdery spore balls. Infected roots and stems develop necrotic lesions and milky white galls, resembling nematode damage.

B. Favorable Conditions

Cool (16–20°C), moist soils in early growth stages, followed by drying, favor infection. Heavy, poorly drained soils and over-irrigation enhance disease development, particularly before tuber formation.

C. Control Measures

1. Plant Disease-Free Seed: Use symptomless seed from disease-free areas.

2. Use Crop Rotation: Rotate crops for up to 10 years to reduce soil pathogen levels.

3. Ensure Drainage: Use cultivation practices to improve soil drainage.

4. Apply Zinc Products: Zinc-based products provide good control, though fungicides are ineffective.

Rhizoctonia (Black Scurf and Stem Canker)

Rhizoctonia, caused by Rhizoctonia solani, affects potato production globally, spoiling tuber appearance and damaging sprouts, particularly in cool, wet soils.

A. Symptoms

Black scurf appears as black sclerotia on tuber surfaces, resembling soil. Stem canker causes sunken, brown lesions on roots, stems, tubers, stolons, and sprouts.

B. Favorable Conditions

Cold, wet soils promote stem canker development. Infection can occur throughout the growing season, with cool, poorly drained soils increasing disease severity.

C. Control Measures

1. Plant Certified Seed: Use certified tubers treated with fungicides if black scurf is present.
2. Use Crop Rotation: Rotate with grasses or grains for 2–3 years to reduce infection.
3. Harvest Early: Harvest after skin set to minimize disease spread.
4. Avoid Cool Soils: Plant in well-drained soils to reduce infection risk.

Sclerotinia Rot

Sclerotinia rot, caused by Sclerotinia sclerotiorum, is generally minor in South Africa but can cause severe losses under favorable conditions, affecting leaves, stems, and petioles.

A. Symptoms

Water-soaked lesions with white mycelium appear on leaves, petioles, and stems. Infected stems contain large, black sclerotia. Small, mushroom-like apothecia may form, spreading spores via wind.

B. Favorable Conditions

Low temperatures (16–22°C) and high humidity (95–100%) promote sclerotia germination and infection. Mycelium can also develop directly from sclerotia, infecting plants rapidly.

C. Control Measures

1. Limit Irrigation: Avoid excessive night irrigation to reduce humidity.

2. Use Crop Rotation: Rotate with grain crops for four years or more.

3. Space Rows Widely: Wider row spacing reduces disease-promoting conditions.

4. Avoid Host Crops: Do not plant host crops in potato fields.

Sclerotinia Wilt

Sclerotinia wilt, caused by Sclerotium rolfsii, affects stems and tubers, particularly in warm, humid conditions. It is soil-borne and persists for long periods, complicating control efforts.

A. Symptoms

Infection occurs at or below the soil surface, causing yellowing and wilting. Thick, white fungal growth and light-brown sclerotia form at the stem base. Infected tubers show saffron-colored rot.

B. Favorable Conditions

High temperatures (28–30°C) and high humidity promote the disease. The soil-borne fungus spreads via mycelium fragments and sclerotia in plant remains or infested soil.

C. Control Measures

1. Plough Residues: Plough plant residues into the soil after harvest.

2. Avoid Contaminated Soils: Do not plant in soils with a history of infection.

3. Practice Sanitation: Maintain hygiene to prevent disease spread.

4. Monitor Conditions: Avoid planting during warm, humid periods to reduce infection risk.

Silver Scurf

Silver scurf, caused by Helminthosporium solani, affects tuber appearance, leading to market downgrading due to cosmetic issues and moisture loss, though it rarely impacts plant vigor.

A. Symptoms

Small, light-brown, circular spots enlarge into shiny, silvery lesions with darker margins due to spore production. Infected tubers lose moisture and weight, as the skin becomes permeable.

B. Favorable Conditions

Temperatures of 20–24°C and 90% relative humidity promote disease development. High humidity during storage exacerbates moisture loss and disease spread in affected tubers.

C. Control Measures

1. Plant Certified Seed: Use disease-free, high-generation seed in clean soil.

2. Treat Seed: Apply registered fungicides if silver scurf is suspected.

3. Harvest Early: Harvest promptly after canopy death to limit spread.

4. Store Properly: Keep tubers dry with good ventilation at low temperatures.

Verticillium Wilt

Verticillium wilt, caused by Verticillium albo-atrum and V. dahliae, is common in South Africa, particularly in fields with tomatoes or cotton. It causes significant losses under stress conditions.

A. Symptoms

Yellowing and browning of leaves start in lower plant parts, spreading upward. Wilted stems and tubers show brown vascular discoloration, a key symptom distinguishing it from other wilts.

B. Favorable Conditions

V. dahliae thrives at 22–27°C, while V. albo-atrum prefers 10–22°C. Stress conditions, such as drought or nutrient deficiency, increase disease severity in affected fields.

C. Control Measures

1. Plant Disease-Free Seed: Use certified, healthy seed potatoes.

2. Use Crop Rotation: Rotate with grasses or grains for 3–4 years to reduce inoculum.

3. Apply Soil Fumigation: Use treatments like chloropicrin to control soil inoculum.

4. Manage Irrigation: Maintain high soil moisture early to reduce infection risk.

Frequently Asked Questions

1. What causes black dot in potatoes?
Black dot is caused by the fungus Colletotrichum coccodes, which forms micro-sclerotia on tubers, stolons, and stems, affecting tuber appearance and quality.

2. How can grey mould be prevented in potato storage?
Store potatoes in dry, cool conditions with good ventilation, avoid wet harvests, and use healthy seed potatoes to minimize Botrytis cinerea infection.

3. What conditions favor brown spot development?
Warm, humid conditions, prolonged leaf wetness, and cyclic wet-dry periods promote brown spot, caused by Alternaria alternata, especially through micro-injuries.

4. How does early blight affect potato yield?
Early blight, caused by Alternaria solani, can reduce yields by up to 50% in severe cases, affecting leaves and tubers under hot, moist conditions.

5. What is the best way to control Fusarium dry rot?
Minimize mechanical damage, apply fungicides soon after harvest, and store tubers at 13–16°C with high humidity to promote wound healing.

6. How can late blight be managed effectively?
Use resistant cultivars, apply registered fungicides, monitor fields regularly, and avoid harvesting in wet conditions to control Phytophthora infestans.

7. Why is powdery scab difficult to control?
Spongospora subterranea persists in soil for up to 6 years, thrives in cool, moist conditions, and spreads via contaminated seed or manure, complicating control.

8. What are the key symptoms of Verticillium wilt?
Yellowing and browning of lower leaves, followed by vascular discoloration in stems and tubers, are primary symptoms of Verticillium wilt, caused by Verticillium species.

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