Fusarium Wilt

Fusarium wilt, a prevalent fungal disease affecting the vascular system of plants, manifests through symptoms of plant wilting or death. It is regarded as one of the most severe plant diseases that can potentially devastate growers and farmers economically.

The culprit behind Fusarium wilt is the pathogenic fungus called Fusarium oxysporum. This fungus is further classified into different forma specialis categories based on the specific host crop it infects.

Disease Cycle and Development

Fusarium oxysporum, widely distributed across the globe, holds the distinction of being the most prevalent species within the Fusarium genus. It lacks a known sexual stage but generates three types of asexual spores: microconidia, macroconidia, and chlamydospores. Microconidia, produced abundantly, take on oval, elliptical, or kidney-shaped forms and emerge from aerial mycelia. Macroconidia, consisting of three to five cells with gradually pointed or curved edges, are present on sporodochia situated on the surface of diseased plants (although sporodochia may be scarce or absent in culture conditions). Chlamydospores, usually formed individually or in pairs but occasionally found in clusters or short chains, are spherical spores with thick walls. They develop within or at the terminal ends of older mycelium or within macroconidia. Unlike the other spores, chlamydospores can endure in the soil for an extended period.

Fusarium oxysporum, a prevalent soil pathogen and saprophyte, sustains itself by consuming deceased and decomposing organic material. It survives in the form of mycelium and various spore types within soil debris, with chlamydospores being the most commonly found. This pathogen disseminates through two primary methods: short-range dispersion occurs through water splashes and planting equipment, while long-range transmission happens via infected transplants and seeds. F. oxysporum infects healthy plants by either utilizing mycelium or by the germination of spores that penetrate the tips of plant roots, root wounds, or lateral roots. The mycelium progresses intracellularly through the root cortex and enters the xylem. Once inside the xylem, the mycelium remains exclusively within the xylem vessels and produces microconidia (asexual spores). These microconidia have the ability to enter the plant’s sap stream and are transported upward.

Upon the cessation of sap flow, the microconidia initiate germination. Over time, the spores and mycelium begin to obstruct the vascular vessels, hindering the plant’s ability to absorb and transport nutrients. Consequently, the plant experiences excessive transpiration beyond its transport capacity, leading to the closure of stomata, wilting of leaves, and eventual demise. Following the plant’s death, the fungus invades all tissues, undergoes sporulation, and persists in infecting nearby plants.

Environment Where It Thrives Best

As mentioned earlier, Fusarium oxysporum is a prevalent soil saprophyte capable of infecting a broad range of plant species worldwide. It exhibits the ability to survive in various soil types, including arctic, tropical, desert, cultivated, and non-cultivated soils. While F. oxysporum can be found in diverse locations and environments, the development of the disease is particularly favored by elevated temperatures and warm, moist soils. Optimal growth on artificial media occurs within the temperature range of 25-30 °C, while the ideal soil temperature for root infection is 30 °C. However, infection through seeds can occur even at lower temperatures, as low as 14 °C.

Host Crops and Symptoms

Fusarium oxysporum, the fungal pathogen, has a wide range of hosts, affecting plants of various ages. Tomato, tobacco, legumes, cucurbits, sweet potatoes, and bananas are among the most susceptible plants, although it can also infect other herbaceous plants. Fusarium oxysporum typically induces symptoms such as wilting, chlorosis, necrosis, premature leaf drop, browning of the vascular system, stunting, and damping-off. Vascular wilt is the most significant symptom observed.

The onset of Fusarium wilt is characterized by vein clearing in younger leaves and drooping of older lower leaves. This is followed by plant stunting, yellowing of lower leaves, defoliation, marginal necrosis, and ultimately, plant death. Symptoms on older plants become more distinct during the blossoming and fruit maturation stages.

Fusarium oxysporum is categorized into specialized divisions known as formae speciales (singular forma specialis, abbreviated f.sp.). There are over 100 formae speciales divisions, each with one or two distinct races. Each forma specialis is specific to a particular host plant and exhibits different symptoms:

  • oxysporum f.sp. batatas affects sweet potatoes, resulting in leaf chlorosis, stunting, and leaf drop. It is transmitted through soil and vascular wounds in plant material.
  • oxysporum f.sp. betae affects sugar beets, while F. oxysporum f.sp. Phaseoli affects dry beans. Severely affected plants experience yellowing, wilting, and premature death, leading to reduced yields or complete crop loss.
  • oxysporum f. sp. cubense causes Panama disease in bananas, prevalent in Africa, Asia, Central and South America. It attacks bananas of all ages and spreads primarily through the soil, leading to wilting and yellowing of leaves.
  • oxysporum f. sp. lycopersici causes vascular wilt in tomatoes. The disease initially manifests as yellowing and drooping on one side of the plant, progressing to leaf wilting, plant stunting, browning of the vascular system, leaf death, and reduced fruit production.
  • oxysporum f. sp. Melonis affects melons, muskmelons, and cantaloupes. It causes damping-off in seedlings and induces symptoms such as chlorosis, stunting, and wilting in mature plants. Necrotic streaks may appear on the stems.

Treatment and Control Measures

To combat Fusarium wilt, various treatment and control measures can be implemented. Some recommended options include:

  • PYRAMID 50G/20L as a foliar spray and 100GM/20LTR as a drench.
  • CHARIOT 20MLS/20L as a drench.
  • CHARIOT 20MLS/20MLS as a spray.
  • RANSOM 15G/20MLS as a spray and drench.

Fungicide drenches should commence early after germination and be repeated every 3 weeks to 1 month.

Add your comment