Fusarium – Identification Characters and Plant Disease control

Fusarium – Identification Characters and

Plant Disease control

Fusarium – An Introduction

Fusarium is a large genus of filamentous fungi widely distributed in soil and in association with plants. Most species are harmless saprobes, and are relatively abundant members of the soil microbial community. Some species produce mycotoxins in cereal crops that can affect human and animal health if they enter the food chain. The main toxins produced by these Fusarium species are fumonisins and trichothecenes.

The name of Fusarium comes from Latin fusus, meaning a spindle.

Identifying characteristics

• Saprophytic Fungi.
• Grows in plant and animal tissues, and soils.
• Lab cultures often show a cottony-pink mycelium.
• Macronidia shape is main basis for identification.
• Causes many plant diseases, mainly root rots and vascular wilts.
• Spores dispersed by air movement and rain splash.
• Over thirty unique species within the genus have been isolated.

Fusarium cells

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The picture on the left shows typical Fusarium cells

Taxonomic description

The genus Fusarium consists of species that are highly variable due to their genetic structure and also because environmental changes can easily cause changes in their morphology. Many Fusarium species require specific conditions to form their optimal morphologies and also tend to mutate rapidly, causing further difficulties in identification.

The specific shape of the Fusarium's slimy, banana-shaped, septate macronidia is the main identifying characteristic. Some species also form distinctly different sequences of micronidia in their aerial mycelium. Also, some species form chlamydospores in varying patterns.

Fusarium is a genus of the hyphomycetes, formally classified as a genus of the deuteromycetes. There are thirty species of Fusarium that are most commonly recognized, but many additional species have been isolated.

However, due to the varying conditions under which these were cultured and the mutational possibilities of these species, not all scientists recognize them as unique. It cannot be stressed enough as to the myriad of subtle and qualitative differences among Fusarium species.

The above picture shows micronidial chains in the aerial mycelium of various species of Fusarium.

Isolation and ecology

Isolation of Fusarium species can be achieved from samples of soil, running water, insects, and seeds and roots from most plants. Because of the many difficulties in identifying the various species, an evolving set of isolation principles is gaining favor, including the following:

 1) nutrient poor media such as carnation or banana leaf agar must be used to culture the microscopic characteristics of Fusarium for accurate identification to be possible,

2) exposure to fluorescent light and/or UV light is necessary for optimal macronidia growth, and 3) the potato dextrose and potato sucrose agars commonly used in the past to culture Fusarium species should no longer be used as the high sugar levels in these media tend to promote mutation in many species, therefore making accurate identification an almost impossible task.

As stated earlier, Fusarium is found to be widely distributed in nature in various environments. There are several toxic species that can cause disease in both plant and animals, including humans. Infection in animals by a Fusarium rarely occurs, and most often only does so when a break in the skin allows for the organism to enter the body.  

The most common diseases associated with Fusarium are common in plants, mainly root rots and vascular wilts of field crops such as potatoes, tomatoes, and small grains such as wheat, oats, barley, and rye. 

Chlamydospores in clumps of Fusarium

The above picture shows chlamydospores in clumps of Fusarium.

B. subtilis – Control  over Fusarium

B. subtilis bacteria produce antibiotics, including some called iturins, which help the bacteria compete with other microorganisms either by killing them or reducing their growth rate.

When applied directly to seeds, B. subtilis bacteria colonize the developing root system, competing with various disease organisms that attack root systems. According to the manufacturers, B. subtilis also inhibits plant pathogen spore germination and interferes with the attachment of the pathogen to the plant.

When soil or seed-applied, it is claimed that B. subtilis feeds off plant root exudates, depriving disease pathogens of a food source.

B. subtilis is also reported to induce systemic acquired resistance (SAR) against bacterial pathogens. SAR is when a plant’s own defense mechanisms are induced by prior treatment with either a biological or chemical agent. The concept of SAR has been studied for many years and is an exciting prospect for disease management.

Effective disease management in onions, or any crop, is usually the result of an integrated approach that includes cultural practices like crop rotation, sanitation and variety selection. On organic farms, biological fungicides such as Bacillus subtiliscan be used as an additional tool. Studies show this material to be relatively effective in helping to suppress onion diseases