3.1.1 Pyropia Red Rot disease

Pyropia spp. red rot is mainly caused by the oomycete Pythium porphyrae and is the most widely studied disease of Pyropia. Symptoms (shown in Figure 9) are characterised by small red patches on the blades in areas where the zoospores of P.porphyrae have germinated. Following germination, mycelium of P.porphyrae starts to colonise host cells intracellularly, killing them and progressively forming distinct patches of dead cells on the blades. Dead host cells turn from their natural reddish-brown colour to violet-red before they turn green and degenerate completely. The infection spreads quickly to other areas on the blade, and dead host tissue deteriorates forming numerous small holes. The holes merge into bigger holes, ultimately disintegrating the entire blade.

Figure 9 Red Rot macroscopic (a) and microscopic (b,c) symptoms observed on Pyropia blades. (a) Arrows correspond to areas infected by P.porphyrae. (b,c) Enlarged images of infected Pyropia cells showing intracellular colonisation by Pythium mycelium (Scale bars (b) 50 μm and (c) 10 μm).

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Three images of seaweed. From left to right: a picture of Pyropia frond showing brown marks associated with infection; the next two images are microscopic images showing infection at cellular level at increasing magnification.

Figure 9 Red Rot macroscopic (a) and microscopic (b,c) symptoms observed on Pyropia blades. (a) Arrows correspond to areas infected by P....

To date, Pythium porphyrae and Pythium chondricola have been reported as the main causing agents of the red rot of Pyropia. However, a recent paper identified a fungus of the genus Alternaria as another causal agent of the Red Rot disease.