1. Early Blight
Symptoms
Early blight primarily affects older leaves first, progressing upward. Initial symptoms appear as small, dark brown to black circular or angular spots with concentric rings, giving a characteristic "target board" or "bull's eye" appearance. These lesions are typically surrounded by a yellow halo. As the disease progresses, affected leaves turn yellow, wither, and drop prematurely. The pathogen can also infect stems, causing dark, sunken lesions, and fruits, producing leathery, dark brown spots usually at the stem end with concentric markings.
Etiology
Early blight is caused by the fungus Alternaria solani, a necrotrophic pathogen belonging to the class Dothideomycetes. The pathogen produces characteristic multicellular, beaked conidia with both transverse and longitudinal septa. It thrives in warm temperatures (24-29°C) with high humidity and can survive on infected plant debris and in soil.
Disease Cycle
The fungus overwinters in infected plant debris, soil, and on volunteer tomato plants. In spring, conidia are produced and dispersed by wind, rain splash, and irrigation water to healthy plants. The pathogen enters through stomata or wounds and establishes infection. Under favorable conditions of warm temperature and leaf wetness, the fungus sporulates, producing numerous conidia that initiate secondary infections. The disease cycle continues throughout the growing season, with multiple infection cycles occurring.
Management
- Cultural practices: Remove and destroy infected plant debris; practice crop rotation with non-solanaceous crops for 2-3 years; avoid overhead irrigation; ensure proper plant spacing for air circulation; mulch to prevent soil splash onto lower leaves
- Resistant varieties: Use tolerant cultivars when available
- Chemical control: Apply fungicides containing chlorothalonil, mancozeb, azoxystrobin, or copper-based products preventively or at first disease appearance; alternate fungicide groups to prevent resistance
- Biological control: Apply Bacillus subtilis or Trichoderma species as biocontrol agents
2. Late Blight
Symptoms
Late blight causes irregular, water-soaked, gray-green to brown lesions on leaves, typically starting at leaf tips and margins. Under humid conditions, a white, downy fungal growth appears on the lower leaf surface around lesions. Infected leaves quickly turn brown, dry up, and die. Stems develop dark brown, elongated lesions. Fruits show irregularly shaped, greasy-appearing, brown to purplish lesions that remain firm initially but become susceptible to secondary bacterial infections. The disease can devastate entire crops within days under favorable conditions.
Etiology
Late blight is caused by Phytophthora infestans, an oomycete (water mold) pathogen belonging to the class Oomycetes. Despite being fungus-like, it is more closely related to algae. The pathogen reproduces through asexual sporangia that can directly germinate or release motile zoospores. Sexual reproduction produces thick-walled oospores that can survive harsh conditions. The pathogen requires cool temperatures (10-25°C) and high relative humidity (>90%) or free moisture for infection.
Disease Cycle
The pathogen overwinters as mycelium in infected tubers, plant debris, or as oospores in soil. Sporangia are produced on infected tissue and dispersed by wind over long distances. Under cool, wet conditions, sporangia germinate directly or release zoospores that swim in water films to infection sites. The pathogen penetrates leaf tissue, colonizes it, and produces new sporangia within 3-7 days. This short cycle allows rapid disease spread during favorable weather. Secondary infections continue throughout the season, creating polycyclic epidemics.
Management
- Cultural practices: Use disease-free transplants and seed; destroy cull piles and volunteer plants; avoid overhead irrigation, especially in evening; provide adequate plant spacing; remove infected plants immediately
- Resistant varieties: Plant resistant or tolerant cultivars carrying resistance genes
- Chemical control: Apply protective fungicides (chlorothalonil, mancozeb) before disease appears; use systemic fungicides (metalaxyl, dimethomorph, cymoxanil) during active infection; spray at 5-7 day intervals during favorable weather
- Forecasting: Use disease forecasting systems to time fungicide applications
3. Tomato Leaf Curl
Symptoms
Tomato leaf curl disease causes severe upward or downward curling and rolling of leaves, giving them a cup-like appearance. Young leaves show severe stunting, crumpling, and chlorosis (yellowing). Infected plants exhibit reduced plant size, shortened internodes, and a bushy appearance due to excessive branching. Flowers often drop, and fruit set is severely reduced or completely absent. Fruits that do develop are small, misshapen, and of poor quality. Symptoms are more severe in younger plants infected early.
Etiology
Tomato leaf curl disease is caused by a complex of begomoviruses (family Geminiviridae), including Tomato Leaf Curl Virus (ToLCV), with various geographical strains. These are single-stranded DNA viruses with circular genomes. The viruses are exclusively transmitted by the whitefly vector Benisia tabaci (biotype B) in a persistent, circulative manner. The virus requires 15-20 hours acquisition feeding and 8-12 hours latent period in the whitefly before transmission.
Disease Cycle
The virus overwinters in perennial weeds, infected tomato plants, and alternative hosts. Viruliferous whiteflies acquire the virus by feeding on infected plants for several hours. The virus circulates through the whitefly body and reaches the salivary glands. Infected whiteflies remain viruliferous for life and transmit the virus while feeding on healthy plants. The virus multiplies in plant cells and spreads systemically. New whitefly generations acquire the virus from infected plants, continuing the cycle. The disease spreads rapidly with high whitefly populations and susceptible varieties.
Management
- Vector control: Use yellow sticky traps to monitor and trap whiteflies; apply systemic insecticides (imidacloprid, thiamethoxam) as soil drench or foliar spray; use neem-based products
- Cultural practices: Use virus-free transplants; establish barrier crops around tomato fields; remove and destroy infected plants and weeds that serve as alternate hosts; use reflective mulches to repel whiteflies; avoid overlapping crops
- Resistant varieties: Grow resistant or tolerant cultivars where available
- Physical barriers: Use insect-proof nets in nurseries and field borders to exclude whiteflies
4. Tomato Mosaic
Symptoms
Tomato mosaic disease produces characteristic light and dark green mottling or mosaic patterns on leaves. Leaves may show malformation, including narrowing (shoestring effect), curling, and distortion. Young leaves often exhibit a fern-like appearance with reduced leaf blade. Plants may be stunted with shortened internodes. Fruits develop irregular ripening with yellow blotches, internal browning, and necrotic spots. Infected plants show reduced yield and fruit quality. Symptoms intensity varies with virus strain, plant age at infection, and environmental conditions.
Etiology
Tomato mosaic disease is primarily caused by Tomato Mosaic Virus (ToMV) and Tobacco Mosaic Virus (TMV), both belonging to the genus Tobamovirus, family Virgaviridae. These are single-stranded RNA viruses with rod-shaped particles. The viruses are extremely stable and can survive for extended periods (years) in dried plant tissue, soil, and on contaminated tools and surfaces. They are highly infectious and spread mechanically through plant sap.
Disease Cycle
The virus overwinters in infected plant debris, soil, seeds (seed coat contamination), and on contaminated tools and greenhouse structures. Transmission occurs primarily through mechanical means during cultural operations like transplanting, pruning, and tying plants. The virus enters through wounds and spreads systemically through the plant. Workers' hands, clothing, and tools contaminated with plant sap from infected plants serve as major transmission routes. The virus can also spread through seed and contaminated soil. Once established in a field, the virus spreads rapidly through routine handling and maintenance operations.
Management
- Sanitation: Wash hands frequently with soap; disinfect tools, stakes, and equipment with 10% bleach solution or trisodium phosphate; handle plants carefully to minimize sap transmission; workers should avoid tobacco use as tobacco can be a virus source
- Cultural practices: Use certified disease-free seed and transplants; remove and destroy infected plants immediately; control weeds that may serve as alternative hosts; avoid working with plants when wet; practice crop rotation
- Resistant varieties: Plant varieties with Tm-1, Tm-2, or Tm-22 resistance genes for effective protection
- Preventive measures: Treat seed with trisodium phosphate; use protective clothing when handling plants; establish new plantings away from infected areas
