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Scientific Memoirs by
fungal elements being in many places swarming with mites such as those
which are figured in Plate II, Figs. 2-5. It might have remained an open
question whether these were primary or secondary elements in the disease—
whether their presence was determined by or determinative of that of the
mycelium—had it not been that in a previous sample of blighted tubers obtained
from Jullunder in November 1892, they had occurred alone and apart from any
fungal elements as a cause of extensive destruction. In this case the tubers
presented conspicuous brownish or black scars superficially in sites correspond
-
ing with some of the eyes. The texture of the tissues at such points was dry,
friable, and permeated by the mycelium of various common moulds superficially,
but on section there was no evidence of the invasion of the deeper tissues by
any fungal elements. The substance of the tubers was permeated by a com
-
plicated system of fissures and cavities bounded by smooth, moist walls, swarm
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ing with acari and their ova. Where fresh surfaces of intact tissue were
exposed by section, the mites speedily invaded these and proceeded to excavate
cavities in them. In cases where the general surface of the epidermis was left
intact and the tubers were exposed to a humid atmosphere, they continued to
destroy more and more tissue until a mere shell remained persistent, but where
the interior of the tubers was exposed by section or rupture of the rind, the
tissues rapidly dried up, and with this the progress of the disease was abruptly
arrested, the dead portions shrinking and separating from the intact ones and
becoming converted into masses of débris full of dead mites and ova.
Here there could be no question that the acari were the primary and
efficient cause of the disease. No fungi save purely saprophytic ones were
present, and these were confined solely to those portions of the tubers where the
tissues had already been completely destroyed and where acari were no longer
present, whilst from the distribution of the systems of cavities and the superficial
appearances of the tubers there could be no question that the original sites of
invasion had been the soft tissues of the eyes. The acari are therefore certainly
capable of acting as an efficient and independent cause of the destruction of
tubers, and are in addition a source of danger to the latter by rendering them
liable to the attacks of destructive fungal elements which, without their aid as
factors of rupture of the continuity of the rind, would be unable to attain access
to the interior of the tissues. In this case the only fungi which had attained
access were pure saprophytes, and therefore of practically no importance in rela
-
tion to the progress of the disease, but in the specimens from Poona this was no
longer the case, because the mycelium was here possessed of parasitic properties
and came into play as a secondary destructive agent. The mites alone are
capable of effecting the complete destruction of tubers, but the progress of
destruction is much more rapid when they are associated with parasitic fungal
elements whose access to the tissues of the interior they have facilitated.