Fusarium proliferatum in cannabis – symptoms, spread inside the stem and significance in propagation

Fusarium proliferatum is a sac fungus with a very broad host range that was first described on cannabis in 2020 as the cause of crown and stem rot and of pith necrosis. What stands out is less the external damage, which resembles that of other root diseases, than the route the fungus takes inside the plant. For operations that keep their own mother plants, it is therefore above all a propagation problem.

Symptoms

Affected plants show leaf yellowing, leaf necrosis and leaf drop, together with rot at the crown and an internal brown to black discolouration of the pith tissue. On rooted cuttings the disease appears as damping-off: the young plants topple over and die. The pith necrosis cannot be seen from the outside; it only shows in a longitudinal or cross section.

In the detection trials, fifteen cannabis strains were affected, both mother plants and plants in the vegetative and the flowering stage. Resistance in particular strains is therefore not in sight.

Spread inside the plant

As the tissue ages, the stem of a cannabis plant becomes hollow inside because the pith parenchyma breaks down. This channel has been described as a route along which mycelium, spores and bacterial cells can move upwards through the plant. In the case of Fusarium proliferatum, the pathogen could still be detected one hundred to one hundred and fifty centimetres from the crown.

Under the electron microscope, the pith tissue of inoculated cuttings showed collapsed parenchyma cells and cell death within five days, and mycelium grew from the epidermal cells into the pith. The speed at which symptoms developed suggests that, besides the fungus itself, its metabolic products are also involved in the damage.

In practical terms: a pathogen that has once settled in the pith spreads where no spray solution can reach.

Points of entry

The recovery of fungi from stem tissue is highest at the crown and decreases further up. Fresh cut surfaces are the second point of entry: after pinching, Cladosporium and Penicillium species were isolated from the wounds, and on plants from indoor rooms also Fusarium oxysporum. Untreated coco substrate also regularly carries fungal spores, Fusarium among them.

Several Fusarium species have been described on cannabis, including Fusarium oxysporum and Fusarium solani, which cause similar symptoms. They cannot be told apart by the damage pattern, only by culture and sequence analysis. In a comparison, Fusarium proliferatum colonised the stem of cannabis cuttings more extensively than Fusarium oxysporum.

What follows from this

Because the pathogen moves internally and can be passed on via cuttings, the effective approach lies in the hygiene of mother plants and cutting tools, not in treating the visible plants. A stem cut open is the most informative diagnostic object: brown or blackened pith tissue is a finding, while clean white or empty pith is not.

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