1920.] Cockayne. - Yellow-leaf Disease in Phormium tenax. 191 
also by means of marked plants to gain some fairly accurate knowledge as 
to the progress of the disease during a period of fourteen months. 
In an earlier report I put forth the view that yellow-leaf was not due 
either to a fungoid or a bacterial disease, but that it was “ caused by the 
roots of the flax being exposed in winter to an excess of stagnant water 
and in summer to too great dryness.” I stated, moreover, “ even if it is 
eventually proved that yellow-leaf is caused by a fungus or a bacterium, 
a faulty environment for the flax-plant may probably be an important 
factor in inducing the disease.” Since these statements were made the 
facts dealt with under the next head make it abundantly clear that they 
must be considerablv modified. 
2. Further Facts regarding the Environment of Affected Plants 
and the Causes of the Yellow-leaf. 
During the spring and early summer of 1918-19 yellow-leaf so rapidly 
increased in the Manawatu flax areas that there could no longer be any 
doubt as to the affection being an infectious disease of some kind or other, 
which most probably would be either fungoid or bacterial. This state¬ 
ment is strongly supported by the fact that throughout the South Island, 
with hardly an exception, there is more or less yellow-leaf. Nor are the 
diseased plants confined to milling-areas or even swamps. On the con¬ 
trary, they occur amongst plants of dry stations at all altitudes up to the 
altitudinal limit of the plant ; indeed, there is possibly no situation where 
the flax-plant can live secure. 
Notwithstanding what is said in the last paragraph, flax-plants of a 
wet station are apparently far more liable to become affected than are 
those of drier positions. Also, I do not know whether plants growing with 
their roots in pure running water are ever attacked. 
The flax area of Mr. Brown at Waikanae is an instructive case. This 
area had entirely escaped the trouble until about the month of September, 
1918, when it broke out in the wettest part of the area and spread rapidly. 
In December, 1918, I paid a visit to this swamp. At that time but few 
plants in that splendid flax where the soil is comparatively dry near the 
highroad were affected—so few, indeed, that hardly any yellow-leaf was 
apparent when walking along the road. In the wet hollow near the sand¬ 
hills the position of affairs was strikingly different. There, in places, was 
a veritable epidemic, perhaps one-third of the plants being diseased. In 
company with Mr. Brown I also visited that area of magnificent flax 
(Hadfield’s) between Waikanae and Paraparaumu, where we noted only 
one diseased plant, and that very slightly affected. 
As a general rule I have seen very little badly diseased flax when the 
plants grew naturally under dry conditions. Disease there may be, it is 
true, but generally it is negligible. 
As for the cause of the disease, a fungus has been isolated by Mr. 
Waters in the laboratory of the Biologist to the Department of Agriculture, 
Weraroa, and pure cultures prepared for inoculation experiments. Such 
have to be conducted with the greatest care ; and, of necessity, considerable 
time must elapse before definite conclusions can be reached. Also, the 
fungus dealt with may not be the one causing the disease ; nor must it be 
forgotten that when the actual organism causing the disease is discovered, 
if it be fungoid or bacterial, science has yet no remedy for such a trouble. 
The plants to be cured are not in the orchards, gardens, or even fields 
where spraying is practicable, but they occur in dense masses mixed with 
dying leaves, tall grass it may be, or other weeds of flax areas. Nor is it 
foliage amenable to spraying which is attacked, but the roots far-spreading 
beneath the soil. 
