Diseases of Field & Horticultural Crops & Their Management-II
Unit-IV
Syllabus
Vegetable:
Cucurbits: downy mildew, powdery mildew, wilt;
Onion: purple blotch,;
Pea: downy mildew, powdery mildew, and rust.
Cucurbits
Downy Mildew:
Symptoms:
- Yellow angular lesions on the upper leaf surface.
- White to grayish-purple fuzzy growth on the lower leaf surface.
- Reduced plant growth and vigor.
- Premature defoliation leading to reduced yield and fruit quality.
Etiology:
- Caused by the oomycete pathogen Pseudoperonospora cubensis.
- Thrives in humid and wet conditions.
Disease Cycle:
- Infection: Pathogen infects leaves through stomata.
- Symptom Development: Lesions form on the upper leaf surface, and downy growth appears on the lower surface.
- Spore Production: Sporangia are produced on the lower surface and are easily dispersed by wind and rain.
- Dispersal: Sporangia are carried by air currents to other plants.
- Secondary Infection: Sporangia germinate and infect new leaves, continuing the cycle.
Management:
- Cultural Practices:
- Choose resistant cultivars when available.
- Rotate crops to reduce pathogen buildup.
- Avoid overhead irrigation which can create favorable conditions for the pathogen.
- Chemical Control:
- Fungicides can be applied preventively or curatively based on disease severity.
- Use fungicides with different modes of action to prevent resistance.
- Apply fungicides on a regular schedule, especially during periods of high humidity.
- Sanitation:
- Remove and destroy infected plant debris to reduce overwintering sources of the pathogen.
- Biological Control:
- Beneficial microorganisms and biopesticides can help suppress the pathogen's growth.
Powdery Mildew
Symptoms:
- White to gray powdery patches on the upper surface of leaves, stems, and fruits.
- Affected leaves may become distorted, curl, or die prematurely.
- Reduced plant growth and yield due to impaired photosynthesis.
- Fruits may develop with blemishes or deformities.
Etiology:
- Caused by different species of fungal pathogens, including Podosphaera xanthii and Erysiphe cichoracearum.
- Flourishes in warm and dry conditions.
Disease Cycle:
- Infection: Fungi produce spores (conidia) on the leaf surface, which are wind-dispersed.
- Germination: Conidia land on plant surfaces and germinate, forming hyphae.
- Mycelium Growth: Hyphae penetrate the plant's epidermal cells, extracting nutrients.
- Symptom Formation: As mycelium grows, the characteristic white powdery appearance develops.
- Conidia Production: Mycelium produces new conidia on the powdery growth.
- Secondary Infections: Conidia are carried by wind to other parts of the same plant or neighboring plants, initiating new infections.
Management:
- Cultural Practices:
- Choose resistant or tolerant cultivars when available.
- Provide adequate plant spacing to improve air circulation and reduce humidity.
- Avoid overhead irrigation to minimize leaf wetness.
- Chemical Control:
- Apply fungicides preventively before symptoms appear.
- Rotate fungicides with different modes of action to prevent resistance.
- Follow recommended application intervals based on disease pressure.
- Pruning: Remove and destroy heavily infected plant parts to reduce disease spread.
- Sulfur Dust: Sulfur-based products can provide effective control of powdery mildew.
- Biological Control:
- Some beneficial microorganisms can help suppress powdery mildew growth.
- Monitoring: Regularly scout plants for early symptoms to initiate control measures promptly.
As with any disease, integrated management strategies that combine different approaches offer the best chance of successfully managing Powdery Mildew of Cucurbits
Wilt
Symptoms:
- Wilting of leaves during the hottest part of the day, recovering at night.
- Yellowing and browning of leaves, starting from the margins.
- Stunted growth and reduced fruit production.
- Internal vascular discoloration in affected stems.
- Plants may die in advanced stages.
Etiology:
- Caused by soil-borne pathogens, particularly Fusarium oxysporum and Verticillium dahliae.
- These pathogens infect plant roots and block water uptake, leading to wilting.
Disease Cycle:
- Infection: Pathogens enter plant roots through wounds or natural openings.
- Colonization: Pathogens invade and colonize the vascular tissues, blocking water transport.
- Symptom Development: As the pathogen multiplies and clogs the vascular system, symptoms of wilting and yellowing appear.
- Spore Formation: Pathogens produce reproductive structures (spores) in the vascular tissues.
- Soil Contamination: Infected plant debris and spores can remain in soil, infecting subsequent crops.
Management:
- Resistant Varieties: Planting resistant cultivars is the most effective strategy.
- Sanitation: Remove and destroy infected plants to reduce pathogen inoculum.
- Crop Rotation: Rotate cucurbit crops with non-hosts to break the disease cycle.
- Soil Solarization: Covering soil with plastic in hot weather can reduce soilborne pathogens.
- Fumigation: Soil fumigation with appropriate chemicals can help control the pathogens.
- Seed Treatment: Treat cucumber, melon, and watermelon seeds with hot water to reduce pathogen presence.
- Avoid Stress: Maintain optimal irrigation and nutrition to reduce plant stress.
- Avoid Overhead Irrigation: Minimize wetting of foliage to prevent pathogen spread.
- Biological Control: Some beneficial microorganisms can help suppress pathogen growth.
- Chemical Control: Fungicides applied to soil can provide limited control.
Onions
Purple blotch
Symptoms:
- Purple to reddish-brown lesions with distinct yellow halos on leaves.
- Lesions expand and coalesce, causing leaves to wither and die.
- Severely affected plants have reduced bulb size and quality.
- Infections can also occur on bulbs, resulting in sunken lesions.
Etiology:
- Caused by the fungus Alternaria porri.
Disease Cycle:
- Infection: Fungal spores land on leaves and penetrate through natural openings or wounds.
- Lesion Development: Lesions form within a few days, starting as small purple spots.
- Spread: Spores are produced in the lesions and can be spread by wind, rain, or equipment.
- Overwintering: The fungus can survive on plant debris, bulbs, or in the soil.
- Secondary Infections: Infections can occur throughout the growing season under favorable conditions.
Management:
- Crop Rotation: Avoid planting onions in the same field for consecutive years.
- Sanitation: Remove and destroy infected plant debris.
- Resistant Varieties: Use onion cultivars resistant to Purple Blotch.
- Fungicides: Apply fungicides preventively or curatively, following recommended guidelines.
- Proper Spacing: Ensure good air circulation between plants to reduce humidity.
- Irrigation: Avoid overhead irrigation to minimize leaf wetness.
- Onion Bulb Treatments: Dip onion sets in fungicide before planting.
- Timing: Plant onions at the recommended time to avoid high disease pressure.
- Weed Management: Control weeds that may harbor the pathogen.
- Good Drainage: Ensure proper drainage to reduce conditions favoring disease development.
Pea
Downy Mildew
Symptoms:
- Yellowing and curling of leaves.
- Appearance of purplish-brown spots on the upper surface of leaves.
- Formation of downy growth on the lower leaf surface, giving a grayish appearance.
Etiology:
- Caused by the oomycete pathogen Peronospora viciae.
Disease Cycle:
- Infection: Pathogen survives in infected plant debris or as oospores in the soil.
- Spore Production: Sporangia are produced on infected plants and released during humid conditions.
- Spread: Sporangia are wind-dispersed and can infect nearby plants.
- Infection and Growth: Spores germinate in the presence of water and infect pea leaves.
- Secondary Infections: Sporulation continues under favorable conditions, leading to secondary infections.
Management:
- Resistant Varieties: Use pea varieties that are resistant to Downy Mildew.
- Crop Rotation: Avoid planting peas in the same area consecutively.
- Sanitation: Remove and destroy infected plant debris.
- Spacing: Adequate spacing between plants for good air circulation.
- Avoid Overhead Irrigation: Reduce leaf wetness to minimize disease development.
- Fungicides: Apply fungicides preventively or curatively as per recommendations.
- Planting Date: Plant peas early to avoid high disease pressure.
- Weed Management: Control weeds that can serve as alternative hosts.
- Avoid Excessive Nitrogen: Avoid excessive nitrogen fertilization, as it can promote disease development.
Powdery Mildew
Symptoms:
- Initial symptoms appear as white powdery spots on the upper leaf surface.
- As the disease progresses, the white spots coalesce to form a powdery white growth.
- Leaves may become distorted, curl, and exhibit necrosis.
- Pods and stems can also be affected, leading to reduced yield.
Etiology:
- Caused by the fungus Erysiphe polygoni.
Disease Cycle:
- Overwintering: The fungus can survive on crop residues, volunteer pea plants, or as cleistothecia (overwintering structures).
- Spring Infection: Conidia (asexual spores) are produced on overwintered structures and are wind-dispersed to infect young pea plants.
- Infection and Growth: Conidia germinate and penetrate plant tissues, forming a white powdery growth.
- Secondary Spread: Secondary conidia are produced and spread the infection to other parts of the plant.
- Cleistothecia Formation: In late season, cleistothecia form and serve as a source of primary inoculum for the next year.
Management:
- Resistant Varieties: Plant pea varieties with resistance to Powdery Mildew.
- Crop Rotation: Avoid planting peas in the same area consecutively.
- Sanitation: Remove and destroy infected plant residues.
- Spacing: Adequate spacing between plants for better air circulation.
- Fungicides: Apply fungicides preventively or curatively as per recommendations.
- Avoid Overhead Irrigation: Reduce leaf wetness to minimize disease development.
- Avoid High Nitrogen: Excessive nitrogen can promote disease development.
- Early Planting: Plant peas early to reduce exposure to disease pressure.
- Biological Control: Use beneficial microorganisms that can suppress Powdery Mildew.
- Pruning: Remove and destroy heavily infected plant parts.
Rust
Symptoms:
- The symptoms of rust on peas include the appearance of small, round to elongated pustules on the underside of leaves.
- These pustules are typically orange to reddish-brown in color.
- As the disease progresses, the pustules rupture and release rust-colored spores.
Etiology:
- Caused by fungal pathogens, such as Uromyces spp. and Uromyces pisi.
Disease Cycle:
- Spore Dispersal: Rust spores (urediniospores) are produced within pustules on infected plants.
- Wind Dispersal: Urediniospores are wind-dispersed to nearby pea plants.
- Infection: Spores land on pea leaves, germinate, and penetrate the leaf tissue.
- Disease Development: Once inside the leaf, the fungus grows and produces new pustules, completing the disease cycle.
Management:
- Resistant Varieties: Plant pea varieties with resistance to Rust.
- Sanitation: Remove and destroy infected plant debris.
- Crop Rotation: Avoid planting peas in the same location year after year.
- Fungicides: Apply fungicides as per recommendations to protect plants from rust infection.
- Timely Planting: Planting early or late may help reduce disease pressure.
- Spacing: Adequate spacing between plants for good air circulation.
- Remove Weeds: Eliminate alternate hosts, which can serve as a source of infection.
- Fertilization: Maintain balanced fertilization to prevent excessive nitrogen, which can promote disease.
- Monitor: Regularly scout the crop for early signs of rust and take action as needed.
Managing rust in peas requires a combination of cultural practices, fungicide applications, and the use of resistant varieties. The goal is to reduce the initial inoculum, prevent the spread of spores, and maintain plant health to minimize the impact of the disease on yield and quality.