Fusarium is a large genus of fungi – many of which are plant pathogens.

We talk about fusarium like it is one singular thing, but in actuality there are over 300 different species of the fungus. A few species in particular are the biggest pain for cut flower growers – fusarium oxysporum, proliferatum and solani, just to name a few. Fusarium oxysporum, the culprit of most fusarium wilts, can be further broken down into over 100 formae speciales which are host-specific, each one affecting a different plant.

As you are already imagining, this is a complex issue with no single cure!

Fusarium wilt occurs when the soil-borne fungi enters through the root tips or lateral roots, and travels up the vascular system where it starts creating reproductive spores. Eventually, the spores and mycelia build up to the point where the vascular system becomes clogged, and the flow of water and nutrients from the roots to the leaves slows down and ultimately stops. The plants show this infection typically by wilting in the heat of the day and recovering at night, then wilting entirely. Sometimes the leaves turn yellow and the death of the plant is gradual, but more often than not, the plants look totally fine one day, and are completely wilted and dead within a couple days.

If left alone, the fungus will invade all plant tissue, sporulate, and continue to infect the soil and neighboring plants. It’s for this reason you need to pull and dispose of any infected plant – do not compost or the spores will infect the compost and you will move the fungus from bed to bed. Once the soil is contaminated, the spores will exist in the soil for many years.

Unfortunately, there is no cure for fusarium oxysporum – but there are thankfully some methods to manage the fungus. The easiest solution is to plant resistant varieties, improve drainage in stagnant soils, and clean up infected plant material instead of letting it break down in the field.

If you grow in a greenhouse or a hoophouse, you can use industrial steamers to ‘cook’ the soil, rendering it lifeless – however this isn’t a solution that works for the majority.

There are many cut flower varieties that seem to go unaffected in soils known to contain fusarium spores – but China Asters and Lisianthus do not fall under this category, and I have a feeling you love those varieties as much as we do here at Farmer Bailey.

Not all hope is lost however, as we do have one weapon in our battle against fusarium, and its name is Trichoderma (RootShield).

You have probably seen us recommend “RootShield” or have seen it mentioned in a plethora of flower farming resources. Many (most?) people don’t know what it is, or how it works, so buckle up for a science lesson. RootShield is a biofungicide – a formulation of living organisms used to control the activity of plant-pathogenic fungi and bacteria. Clear as mud, right? Let me break it down for you.

RootShield contains the dormant spores of the effective, and well proven, active ingredient Trichoderma harzianum strain T-22. When mixed with water and applied to the growing medium, the spores germinate within 24 hours and the emerging mycelium immediately start coiling around the plant roots and any plant pathogen cells that may be present. These mycelia actively seek out plant roots and colonize the root hairs, branching sites, the root surface, and any callus tissue (the scar tissue that forms on the end of cuttings during propagation). Hint hint, these are also the sites in which fusarium typically enters the plant’s vascular system.

After colonization (which happens quickly – when applied at the typical application rates, over 100 billion spores are applied to the growing medium in one application) RootShield has two predominant modes of action. The first is called competitive exclusion; this is when the Trichoderma gets to the part of the plant that pathogens use to enter the plant’s system first, preventing diseases (Pythium, Fusarium, Rhizoctonia, etc) from getting into the roots. It creates a living barrier or shield around the roots, protecting the plant from the soil-borne invaders.

The second mode of action is mycoparasitism, whereby Rootshield encounters a pathogen (disease) and secretes enzymes which dissolve the pathogen’s cell wall, killing it.

Here is the catch – RootShield is not considered an eradicant or rescue treatment, which means if you are seeing active signs of root disease like fusarium in your field, you are too late for an initial treatment.

It must be applied preventatively, when the plants are young, and the roots not yet affected by disease.

We recommend treating Farmer Bailey plugs by dunking the entire tray – foliage, roots and all – into the solution prior to planting in the field. In addition – and this is something that is overlooked by many growers – one RootShield application is only effective for 10 to 12 weeks. Due to microbial competition, the Trichoderma concentration slowly declines to levels below which root disease prevention is considered ideal. After 10-12 weeks from the initial treatment of plugs, a second, half-strength treatment should be applied as a soil drench to extend the root protection an additional 10-12 weeks.

All of the information in this article was sourced from the website of Bioworks, manufacturer of Rootshield and other horticultural products for disease and insect control.

BioWorks has published additional resources to help with application methods, application rates and compatibility with other products such as pesticides or fertilizers. RootShield is not cheap, but it’s an investment into the health of your plants, and I suggest you read all the literature they offer to make sure you are using the product correctly and getting the most bang for your buck.

EXTRA CREDIT:

• Utilizing Dips guide by BioWorks [MANUFACTURER INSTRUCTIONS]
• Growing Mustard as a Biofumigant for Fusarium [BLOG]
• Lisianthus Forever webinar [VIDEO]