1. Alternaria Leaf Spot
Symptoms
Alternaria leaf spot manifests initially as small, circular to irregular dark brown to black spots on the older lower leaves. These spots are typically surrounded by a yellow halo and may display characteristic concentric rings, giving them a target-like appearance. As the disease progresses, the lesions enlarge and coalesce, leading to extensive blighting of the foliage. Severely infected leaves become chlorotic, wither, and eventually drop from the plant. The pathogen can also infect stems, producing elongated dark lesions, and seed pods, causing premature ripening and seed discoloration. In seedlings, damping-off and collar rot may occur under favorable conditions.
Etiology
The disease is caused by necrotrophic fungi of the genus Alternaria, predominantly A. brassicae and A. brassicicola. These pathogens thrive in warm, humid conditions with temperatures ranging from 20-28°C and high relative humidity above 80%. The fungi produce dark, multicellular conidia with characteristic beaks and septa. Infection is favored by extended periods of leaf wetness, poor air circulation, and dense plant canopies. Host stress factors such as nutrient deficiency, particularly nitrogen imbalance, and water stress predispose plants to severe infection. The pathogen has a wide host range within the Brassicaceae family, affecting cabbage, cauliflower, broccoli, mustard, radish, and other cruciferous crops.
Disease Cycle
Alternaria species survive between growing seasons as mycelium and conidia in infected crop debris, volunteer plants, and contaminated seeds. The pathogen can remain viable in soil and plant residues for extended periods. Under favorable environmental conditions, conidia are produced on infected tissues and disseminated by wind, rain splash, and mechanical means including farm equipment and workers. Primary infection typically occurs on older, senescing leaves where natural openings or wounds facilitate pathogen entry. The fungus can also penetrate directly through the cuticle. Following infection, the pathogen establishes itself in host tissue, and within 3-5 days, new conidial production begins, completing the asexual cycle. Secondary spread occurs rapidly through successive cycles of sporulation and infection, particularly during periods of high humidity and moderate temperatures. Seed transmission is an important means of introducing the pathogen to new areas and can result in systemic infection of seedlings.
Management
Cultural Practices: Implement a minimum 2-3 year crop rotation with non-cruciferous crops to reduce inoculum buildup. Remove and destroy infected plant debris immediately after harvest. Ensure adequate plant spacing to promote air circulation and reduce leaf wetness duration. Avoid overhead irrigation; instead, use drip or furrow irrigation to minimize foliar wetness. Balance fertilization programs, particularly nitrogen, to prevent excessive vegetative growth that increases disease susceptibility. Practice field sanitation by controlling volunteer crucifer plants and alternate weed hosts.
Resistant Varieties: Select and cultivate varieties with reported tolerance or resistance to Alternaria leaf spot where available, though complete resistance is limited in most commercial cultivars.
Chemical Control: Apply protective and systemic fungicides at the first appearance of symptoms or preventively during favorable weather conditions. Effective fungicides include mancozeb, chlorothalonil, azoxystrobin, difenoconazole, and tebuconazole. Alternate fungicides with different modes of action to prevent resistance development. Follow recommended application intervals, typically 7-14 days depending on disease pressure and weather conditions.
Seed Treatment: Use certified disease-free seeds or treat seeds with appropriate fungicides such as thiram or captan before planting to eliminate seed-borne inoculum and prevent seedling infection.
2. Black Rot
Symptoms
Black rot is characterized by distinctive V-shaped yellow to brown lesions that develop from the leaf margins, with the apex of the "V" pointing toward the midrib. The veins within infected areas turn black, creating a striking network of darkened vascular tissue that distinguishes this disease from other leaf spots. As infection advances, the lesions expand and may encompass entire leaves, causing them to become necrotic and drop prematurely. In severe cases, the pathogen invades the vascular system systemically, moving through the petioles into stems and heads. Infected cabbage heads show internal blackening of veins, while cauliflower curds may develop brown to black discoloration and exhibit a foul odor. On young seedlings, the bacterium causes damping-off and systemic wilting. The disease significantly reduces crop quality and marketability, often rendering produce unsaleable.
Etiology
Black rot is caused by the gram-negative bacterium Xanthomonas campestris pv. campestris, a highly specialized pathogen of cruciferous plants. The bacterium is rod-shaped, motile with a polar flagellum, and produces characteristic yellow pigmented colonies on culture media. Optimal conditions for disease development include warm temperatures between 25-30°C and high humidity or free moisture on leaf surfaces. The pathogen requires wounds or natural openings such as hydathodes and stomata for infection. Heavy rainfall, overhead irrigation, and dew formation facilitate bacterial spread and infection. The disease is particularly severe in fields with poor drainage, dense plant populations, and when mechanical damage from cultural operations provides entry points. X. campestris pv. campestris exhibits high host specificity within the Brassicaceae family and can infect all commonly cultivated cruciferous vegetables.
Disease Cycle
The bacterium overwinters in infected crop residues, volunteer crucifer plants, and contaminated seeds, maintaining viability for several months to years. Seed transmission is a critical mechanism for long-distance dissemination and field introduction. Under moist conditions, bacteria exude from infected tissues and are dispersed by splashing rain, irrigation water, wind-driven rain, insects, and contaminated tools and equipment. Primary infection commonly occurs through hydathodes at leaf margins when guttation fluid is present, or through wounds caused by insects, mechanical injury, or cultural practices. Once inside the plant, bacteria multiply rapidly in the intercellular spaces and colonize the vascular system. Bacterial ooze containing millions of cells is released from infected tissues, serving as inoculum for secondary infections. The disease spreads rapidly within and between plants during warm, wet weather. The bacterium can complete its cycle from infection to symptom expression and new bacterial exudation within 7-14 days under optimal conditions, leading to explosive disease development in susceptible crops.
Management
Cultural Practices: Practice extended crop rotation of at least 2-3 years with non-cruciferous crops to reduce soilborne inoculum. Eliminate volunteer crucifer plants and control cruciferous weeds that serve as reservoir hosts. Implement proper field sanitation by removing and destroying all infected plant debris after harvest; avoid composting diseased material unless properly heated. Ensure good field drainage and avoid waterlogged conditions. Reduce plant stress through balanced fertility management and consistent moisture availability. Minimize physical damage to plants during cultivation, weeding, and harvesting operations, as wounds facilitate bacterial entry.
Water Management: Avoid overhead irrigation systems that promote leaf wetness and bacterial dispersal. Use drip irrigation or furrow irrigation to keep foliage dry. When overhead irrigation is unavoidable, irrigate early in the day to allow rapid foliage drying.
Resistant Varieties: Plant resistant or tolerant cultivars when available. While no cultivars possess complete immunity, several exhibit varying degrees of resistance that can significantly reduce disease severity.
Seed Treatment and Certification: Use only certified disease-free seeds from reputable sources. Hot water seed treatment (50°C for 20-30 minutes) can eliminate seed-borne bacteria but must be performed carefully to avoid reducing seed germination. Chemical seed treatments with antibiotics or bactericides may also be employed where permitted.
Chemical Control: Bactericide applications have limited efficacy once symptoms appear but can provide some protection when applied preventively. Copper-based bactericides such as copper hydroxide or copper sulfate can be applied before infection or at the first sign of disease. Fixed copper formulations combined with mancozeb provide both antibacterial and antifungal protection. Applications should be repeated at 5-7 day intervals during conditions favorable for disease development. Bacterial strains resistant to copper have been reported in some regions, reducing effectiveness. Biological control agents containing Bacillus subtilis or Pseudomonas fluorescens may provide supplementary control when integrated with other management practices.
Integrated Management: The most effective black rot management relies on integration of multiple strategies including use of clean seed, resistant varieties, cultural practices that minimize leaf wetness and physical damage, crop rotation, and judicious use of bactericides. No single control method is sufficiently effective when used alone.
