1. Anthracnose of Beans
Symptoms
Anthracnose, caused by Colletotrichum lindemuthianum, affects all above-ground parts of bean plants. On leaves, small brown spots with dark borders appear, often along the veins. These lesions may coalesce, causing extensive blighting. Stems and petioles develop elongated, sunken, dark brown to black cankers that can girdle the plant, leading to wilting and death of shoots above the infection point.
The most characteristic symptoms appear on pods, where circular to oval, sunken lesions develop. These spots have dark brown to black borders and tan to pink centers containing salmon-colored spore masses during humid conditions. Seeds from infected pods show brown to black discoloration and may carry the pathogen, serving as the primary source of inoculum for subsequent crops.
Etiology
The causal organism, Colletotrichum lindemuthianum, is a fungal pathogen belonging to the class Ascomycetes. It produces asexual spores called conidia in acervuli, which appear as pink or salmon-colored masses on infected tissue. The fungus thrives in cool to moderate temperatures (13-26°C) with high humidity or frequent rainfall. Multiple physiological races of the pathogen exist, making resistance breeding challenging.
Disease Cycle
The pathogen survives between seasons primarily on infected seed and plant debris. Seed-borne inoculum is the most important source of primary infection. When infected seeds are planted, the fungus grows systemically through the seedling, producing lesions on cotyledons, stems, and leaves. Secondary spread occurs through rain splash and wind-dispersed conidia from these initial infections.
Under favorable environmental conditions—cool temperatures combined with high moisture from rain, dew, or overhead irrigation—the pathogen produces abundant spores. These conidia are disseminated to healthy plants, initiating new infection cycles. The disease intensifies during prolonged wet periods, with multiple infection cycles occurring within a single growing season.
Management
Cultural Practices
Plant only certified, disease-free seed from reputable sources. Practice crop rotation with non-host crops for at least 2-3 years to reduce inoculum in soil and plant debris. Avoid working in fields when plants are wet to prevent mechanical transmission of spores. Deep plowing to bury infected crop residues helps reduce inoculum survival. Use drip irrigation instead of overhead sprinklers to minimize leaf wetness duration.
Resistant Varieties
Select bean varieties with resistance to the prevalent races of anthracnose in your region. However, recognize that resistance may break down due to the emergence of new pathogen races, necessitating continuous monitoring and variety evaluation.
Chemical Control
Seed treatment with fungicides such as thiram or captan protects against seed-borne infection. Apply foliar fungicides preventively when environmental conditions favor disease development. Effective fungicides include copper-based products, chlorothalonil, mancozeb, and azoxystrobin. Begin applications before symptoms appear and continue at 7-14 day intervals during wet weather.
2. Bacterial Blight of Beans
Symptoms
Bacterial blight, caused by Xanthomonas axonopodis pv. phaseoli (common blight) and Pseudomonas syringae pv. phaseolicola (halo blight), produces distinctive symptoms on bean plants. Common blight begins as small, water-soaked spots on leaves that enlarge into irregular brown necrotic areas surrounded by a narrow yellow halo. The lesions often appear greasy or shiny when fresh and may coalesce, causing extensive blighting of foliage.
Halo blight is characterized by small water-soaked lesions surrounded by a broad, bright yellow-green halo, particularly visible on the underside of leaves. On pods, both diseases cause water-soaked, reddish-brown spots that may exude bacterial ooze under moist conditions. Severely infected pods become distorted, and seeds may develop discoloration ranging from yellow to brown, serving as sources of inoculum.
Etiology
Xanthomonas axonopodis pv. phaseoli and Pseudomonas syringae pv. phaseolicola are gram-negative, rod-shaped bacteria. Xanthomonas produces a yellow pigment and favors warm, humid conditions (28-32°C), while Pseudomonas thrives in cooler temperatures (16-24°C). Both bacteria require free moisture for infection and spread, entering plants through natural openings (stomata, hydathodes) or wounds.
Disease Cycle
Bacterial blight pathogens survive primarily on or in infected seeds and on crop debris. Seed-borne bacteria are the most critical source of primary infection, introducing the pathogen to new fields. When infected seeds germinate, bacteria multiply and spread to emerging plant parts through water films. Secondary spread occurs through rain splash, wind-driven rain, irrigation water, and mechanical contact during cultivation or harvesting.
The bacteria multiply rapidly in the intercellular spaces of plant tissue under warm, humid conditions with free moisture. Storms, hail, and insect feeding create wounds that facilitate bacterial entry. Once established, the bacteria produce multiple generations within a season, with disease severity increasing progressively under favorable environmental conditions.
Management
Cultural Practices
Use certified, pathogen-free seed exclusively, as seed transmission is the primary means of introducing bacterial blight. Implement a minimum 2-3 year crop rotation away from legumes to allow reduction of bacterial populations in soil and debris. Avoid field operations when foliage is wet, as bacterial transmission is greatly facilitated by water films and mechanical injury to wet plants.
Remove and destroy volunteer bean plants and crop residues that harbor bacterial populations. Practice field sanitation by cleaning equipment between fields to prevent mechanical transmission. Use drip or furrow irrigation rather than overhead sprinklers to minimize leaf wetness and bacterial splash dispersal.
Resistant Varieties
Plant bean varieties with genetic resistance to common blight and halo blight. Resistance levels vary among cultivars, and complete immunity is rare, but resistant varieties significantly reduce disease severity and economic losses.
Chemical and Biological Control
Copper-based bactericides (copper hydroxide, copper sulfate) provide some suppressive effect when applied preventively before infection occurs and during conditions favoring disease development. However, bacterial diseases are difficult to control with chemicals once established. Some bacterial strains have developed copper tolerance, reducing efficacy.
Biological control agents containing beneficial bacteria (Bacillus species) may provide modest protection when applied preventively. Combine chemical or biological treatments with rigorous cultural practices for optimal disease management. Applications should begin early in crop development and continue at 7-10 day intervals during wet weather.
