Diseases of the Poppy Plant.
PAPAVER
somniferum.
the Economic
Products of India.
5 7
him, from whatever locality obtained, agreed in showing the presence of
Peronospora arborescens , a fungus closely allied to that which occurs in
potato disease. " The fungi," he says, " which belong to this genus, besides
the two species mentioned above, include various others affecting other
host-plants, are true entophytes, their mycelium or vegetative apparatus
being developed within the tissues of the plants which play the part of hosts,
whilst their reproductive apparatus consists partly of filaments bearing
the so-called spores or conidia which emerge from the substance of the
leaves and project on the surface as a whitish mouldy layer, and partly of
larger reproductive bodies termed Oogonia which are developed within the
substance of the affected tissues. These consist of a sac containing a mass
of living protoplasm which subsequently forms the Oospore, and is ulti-
mately invested with a thickened membrane, frequently marked with pro-
jecting tubercles, ridges or reticulations. The former class of reproductive
bodies, the conidia, are produced in extreme abundance and serve for
the rapid diffus on of the parasite from plant to plant, whilst the latter,
wrapped up in the tissues of the host and additionally protected by the
thickened character of their envelopes, are supposed to be capable of
retaining their vitality under conditions destructive to that of the myce
- lium and conidia, and to be thus endowed with a capacity for germination
after prolonged periods of rest. This process has, however, never been actu-
ally observed to occur in them, but is only inferred to exist from what has been
observed in regard to the corresponding bodies in the allied genus Cystopus .
It is by means of such O ospores that the species of Peronospora affecting
various plants at home are supposed to survive the winter and to be ena-
bled to appear year after year." Dr. Cunningham next alludes to a method
of survival which pertains in perennial host-plants. "For example,
Oogonia and their contained Oospores have never been detected on the
Peronospora infestans, as that parasite exists in Europe on the potato
plant, and yet it survives the rigours of the winter and appears year after
year because portions of its mycelium affecting- the tissues of the tubers
remain protected and dormant, wrapped up in the substance of the latter
and ready to grow and spread outwards with the young shoots arising
from the tubers when they are planted." In the case of the potato
disease Dr. Cunningham says that so far as the evidence goes it would
seem as if certain breeds of the potato had a hereditary predisposition to
attack or exemption, co-existent with perfect health in the host. This well
accepted view,regarding the potato disease and some other parasitic diseases,
makes it very probable that Mr. Scott's observation is correct, namely,
that certain forms of the poppy plant are less liable to the attacks of
the disease than others. In support of the idea it may be added that
Dr. Cunningham noted that the disease presented different appearances
according to the nature of the leaves in which it was found. In his micros-
copical investigations also he failed to detect any haustoria or penetrating
portions of the mycelium. The filaments were seen to be closely adapted
to their containing intercellular spaces and to exhibit projections and dilata-
tions corresponding with the irregularities of the intercellular spaces of the
leaf-tissue within which they freely anastomosed, but were never seen to pene-
trate the cells of the host. The filaments were also observed to be devoid of
any septa. " The fertile filaments are erect, strung, and persistent, many
remaining recognisable adherent to dried fragments of leaves attached to
dead plants, or obtained from the neighbouring soil. When quite fresh,
they are perfectly colourless and full of finely molecular protoplasm ; but
on keeping, the latter shrinks into granular masses of bright yellow colour
and remains without further change for many months in dried specimens."
" The conidia appear very rapidly to lose the power of germinating. In
C ULTIVATION
Diseases.
Perenospora.