1. Wilt Disease of Guava
Symptoms
Wilt disease manifests progressively in guava plants with characteristic symptoms. Initial signs include yellowing of leaves on one or more branches, beginning from the margins and progressing inward. Affected leaves eventually turn brown, wither, and drop prematurely while remaining attached to the stem. The wilting typically starts from the top branches and gradually moves downward. Vascular browning is evident when affected branches are cut longitudinally, showing dark brown to black discoloration of xylem tissues. In advanced stages, entire branches or the whole plant may wilt and die. The roots of infected plants show darkening and rotting symptoms.
Etiology
Guava wilt is primarily caused by Fusarium oxysporum f. sp. psidii and Fusarium solani. These are soil-borne fungal pathogens that invade the vascular system of the plant. The pathogen enters through the root system, particularly through wounds or natural openings. Additional causal organisms include Macrophomina phaseolina and various species of Verticillium. The disease is more severe in plants under stress conditions, including waterlogging, drought, nematode infestation, and nutrient deficiency.
Disease Cycle
The pathogen survives in soil as chlamydospores or mycelial fragments in plant debris for several years. When favorable conditions arise, these spores germinate and infect young feeder roots through wounds created by nematodes, agricultural implements, or natural root damage. After penetration, the fungus grows through the cortex and enters the vascular system (xylem vessels). The mycelium and conidia produced within the vessels are carried upward in the transpiration stream, colonizing the entire vascular system. The fungus produces toxins and physically blocks water transport, leading to wilting symptoms. Sporulation occurs on dead plant material, and spores are disseminated through irrigation water, contaminated tools, and soil movement, initiating new infection cycles.
Management
Cultural practices: Remove and destroy affected plants including root systems to reduce inoculum. Avoid waterlogging by ensuring proper drainage. Follow crop rotation with non-host crops for 2-3 years. Use disease-free planting material from certified nurseries. Apply well-decomposed organic matter to improve soil health and suppress pathogen activity.
Physical methods: Solarize nursery beds and field soil by covering with transparent polythene sheets during hot summer months for 4-6 weeks. This reduces soil-borne inoculum significantly.
Biological control: Apply Trichoderma harzianum or T. viride at 5-10 g/plant mixed with organic manure around the root zone. Treat seeds and seedlings with these bioagents before planting. Use Pseudomonas fluorescens as soil drench at 10 g/liter of water.
Chemical control: Drench soil around the root zone with carbendazim (0.1%) or thiophanate-methyl (0.1%) at 2-liter solution per plant. Apply fungicides at 15-day intervals during disease occurrence. Treat planting pits with formaldehyde (2%) or copper oxychloride before planting.
2. Anthracnose of Guava
Symptoms
Anthracnose affects leaves, twigs, flowers, and fruits of guava. On leaves, small circular brown to dark brown spots with gray centers appear, often surrounded by yellow halos. Severe infection causes premature leaf drop. On young twigs, dark brown elongated lesions develop, leading to die-back. Flowers turn brown and drop prematurely. Fruit symptoms are most economically significant, with small, circular, dark brown to black sunken lesions developing on mature or ripening fruits. These spots enlarge rapidly under humid conditions and may coalesce to cover large fruit areas. In advanced stages, salmon-pink to orange spore masses appear on lesions. Infected fruits rot completely, become mummified, and remain attached to trees. Post-harvest losses are substantial as latent infections manifest during storage and transit.
Etiology
Anthracnose is caused by Colletotrichum gloeosporioides (teleomorph: Glomerella cingulata). The fungus produces both conidia (asexual spores) and ascospores (sexual spores). Conidia are hyaline, single-celled, and oval to cylindrical, produced in acervuli on infected tissue. The pathogen thrives in warm, humid conditions with temperatures between 25-30°C and high relative humidity above 80%. Rainy weather, overhead irrigation, and dense canopies favor disease development. The fungus can infect fruits at any stage but often remains quiescent in immature fruits, becoming active as fruits ripen.
Disease Cycle
The pathogen overwinters as mycelium in infected twigs, mummified fruits, and plant debris on the ground or in trees. During favorable weather, especially monsoon season, acervuli develop on overwintered structures producing numerous conidia. These spores are disseminated by rain splash, wind-driven rain, insects, and contaminated tools. Conidia germinate on fruit or leaf surfaces in the presence of free moisture, producing appressoria that penetrate directly through the cuticle or enter through wounds and natural openings. In immature fruits, the fungus often establishes quiescent infections, remaining dormant until fruit ripening when increased sugar content and decreased antifungal compounds activate pathogen growth. The fungus colonizes tissues, producing acervuli with new spores that continue the disease cycle. Multiple infection cycles occur during a single growing season under favorable conditions.
Management
Cultural practices: Prune diseased twigs and remove mummified fruits, fallen leaves, and debris to reduce inoculum sources. Ensure proper spacing between plants for good air circulation and light penetration. Avoid overhead irrigation and irrigate early in the day to minimize leaf wetness duration. Harvest fruits at proper maturity and handle carefully to prevent wounds.
Physical methods: Bag individual fruits with paper or polythene bags 30-40 days after fruit set to protect from infection. Hot water treatment of harvested fruits at 50-52°C for 5 minutes reduces post-harvest infections without affecting fruit quality.
Biological control: Apply Trichoderma viride (0.5%) as foliar spray. Use Pseudomonas fluorescens (0.5%) at flowering and fruit development stages. These bioagents compete with the pathogen and induce systemic resistance.
Chemical control: Apply protective fungicides like mancozeb (0.25%) or copper oxychloride (0.3%) at 15-day intervals during rainy season starting from fruit set. Systemic fungicides such as carbendazim (0.1%), thiophanate-methyl (0.1%), or azoxystrobin (0.1%) provide better control. Alternate between contact and systemic fungicides to prevent resistance development. Pre-harvest sprays 15 days before harvest are critical. Post-harvest dip treatment with carbendazim (0.05%) for 5 minutes reduces storage losses.
Integrated approach: Combine cultural, biological, and chemical methods for sustainable disease management. Select resistant varieties when available. Maintain plant vigor through balanced nutrition with emphasis on potassium and micronutrients to enhance disease resistance.
