60 QUEENSLAND AGRICULTURAL JOURNAL. [1 Jury, 1899. 
words, the virus becomes attenuated), unless it can periodically re-enter its host 
plant or plants. In addition to the ordinary potato-plant, hosts are afforded by 
both the tomato and egg plants, and possibly also by other members of the 
natural order (Solanacex) to which these, together with the potato, belong. 
The alternate mode of existence on the part of the germ, to which allusion 
has been made, can be passed for a considerable time in the soil itself. his 
accordingly may become infected, and therefore capable of communicating the 
disease to potato-plants or tomato-plants, &c., grown from clean seed or seed- 
tubers planted therein, even after many months have elapsed since its contamina- 
tion or infection has taken place. 
Infection of the soil may be brought about, in the first instance, by the 
decay or breaking up of any portion of a diseased plant (for instance, root, tuber, 
stem, or leaf), within or in contact with the soil; or in a secondary manner 
by the transference of earth that has been once thus infected to soil that is 
previously uncontaminated, such transference being capable of being accom- 
plished by implements, especially by the plough and moving water during 
drainage from a higher to a lower level, &e. 
The disease may further be conveyed from plant to plant as a result of 
accidental inoculation, such inoculation being capable of being brought about 
by insects whilst indulging in their leaf-eating habits, and visiting successively 
unhealthy and healthy plants, the germs of the malady becoming attached to 
their jaws during the former occupation. 
In addition to this, the germs of the malady may be temporarily 
associated with the potato tuber, especially when infected soil has become 
attached thereto, or when it has come in contact with diseased and rotten 
examples; and thus they may be conveyed to long distances to originate, when 
this is once used as seed, new occurrence of the malady. 
Such dissemination, moreover, may also be brought about by the employ-— 
ment of potatoes for seed purposes that harbour the disease in a latent or 
undeveloped condition. For it must be borne in mind that the rotting or 
decay of the tuber, by which the disease is usually recognised, is merely a 
secondary change; and that when commencement of the malady has been 
deferred until the tubers have been nearly ripe, they will, after being lifted and 
kept under conditions of comparative dryness, not undergo, in many cases, 
these destructive changes, although they are actually subject to its presence. 
Notwithstanding the numerous ways in which this disease can be thus 
propagated and become distributed, it usually happens that more than one 
season will elapse, subsequent to its first appearance, before the potato malady 
will become general throughout a plot of ground. It will at first manifest 
itself here and there in a sporadic manner, isolated plants being at first affected. 
These, however, if unattended to, will serve as new centres for dissemination, 
so that successive crops manifest it in greater and greater extent. 
The microbe, the cause of the disease, when isolated from the potato-plant, 
loses its vitality, and therefore its power to act in this capacity, when it has 
become thoroughly dried or has been submitted to a high temperature such as 
is frequently experienced during our Queensland summers.* _ It is also killed by 
contact with chemical substances, several of which are available for this purpose, 
even though their employment may not be admissible in practical agriculture. 
TREATMENT. 
Land in which this potato disease has once manifested its presence should 
be subjected to a process of quarantine ; an effort being made, in the first place, 
to deal with occurrences of it within its limits, to disinfect the soil therein, and to 
prevent the extension of the disease further afield. 
1. As soon as the disease is recognised by the drooping and wilting of the 
foliage, the affected plant should be lifted in its entirety and removed from the 
* The thermal death-point (ten minutes’ exposure) has not been determined exactly, but is 
peal about 52 degrees C. (93°6 degrees Fah.) It certainly is above 51 degrees C. (91°8 degrees 
ah.) and below 53 degrees C. (95°4 degrees Fah.).—E. F. Smith, op. cit., p. 18. 
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