Phenology of farmed seaweed Kappaphycus alvarezii infestation by the parasitic epiphyte Polysiphonia sp. in Madagascar

epiphytic
filamentous algae (EFA) disease has appeared in many
regions of Madagascar. This infestation has dramatic consequences
for local farmers as it alters drastically farmed algal
growth and has caused farming activity to collapse in many
places. The present study characterizes the structure and ultrastructure
of the stages observed in the life cycle of Polysiphonia
sp. and gives the results of a monitoring of 18 months made in
three Kappaphycus alvarezii farming sites in the southwest of
Madagascar. Transmission electron microscopy (TEM) was
used to analyze the ultrastructure of the cortex in infested K.
alvarezii. Five stages have been observed in the life cycle of
Polysiphonia sp.: the infesting stage that is a small dark spot
observed at the surface of K. alvarezii, the male gametophyte,
the female gametophyte, the tetrasporocysts, and the undifferentiated
stage where individuals show normal thalli without sexual
differentiation. EFA infestation was never recorded in
Sarodrano, but often in the two other monitored villages
(Lambohara, Tampolove). Prevalence of infestation varied from
40 to 100 % and the rates of infestation from 42 to 78
epiphytes cm−2. Prevalence of infestation showed significant
seasonal variation and a between-sites variation; the rates of
infestation were not significantly different between sites and
did not vary with the period. The ways of infestation between
K. alvarezii individuals in an infested field and from infested to
healthy fields are discussed at the light of the present results.