Mas see. — The ‘ Spot ’ Disease of Orchids . 427 
conditions as to temperature, showed that some other unde- 
termined factor exercised an influence. After repeated experi- 
ments this proved to be the relative amount of moisture 
present in the plant. After a pseudo-bulb with its ac- 
companying leaf had been removed from a plant and allowed 
to remain for three days in a dry place, it was found 
impossible to produce spot by the method mentioned above, 
whereas with a similar specimen removed from the same 
plant, and having the pseudo-bulb placed in water at once, 
fully developed c spot ’ could be produced in four days. 
Similar results were obtained when experiments were made 
with entire plants ; those copiously supplied with water at 
the root, and grown in a high temperature, ‘ spotting ’ readily ; 
whereas plants in a resting condition, scantily supplied with 
water and kept in a low temperature, usually resist all 
attempts to produce £ spot ’ artificially. 
It may be mentioned that, other conditions being equal, 
‘ spot ’ can be produced with the greatest certainty, and in 
the shortest amount of time, when the experiment is con- 
ducted in an atmosphere saturated with moisture. This 
agrees with the experience of gardeners, who state that 
‘ spot * is most prevalent in foggy weather. 
Experiments show that ‘ brunissure,’ or browning of vine 
leaves, when the plants are grown in the open air, can be 
caused by the following combination of meteoric conditions. 
A copious deposition of dew and rapid fall of temperature, 
following heavy rain. Similar conditions produce the disease 
in the leaves of tomatoes, which has been described by 
Abbey 1 as due to an organism named by him Plcismodio - 
phora tomati . 
Summary. 
The orchid disease known as ‘ spot ’ is of non-parasitic 
origin ; the initial cause being the presence of minute 
drops of water on the surface of the leaves at a time when 
1 The ‘drooping’ disease in Tomatoes. Journ. Hort., Ser. 3, Vol. xxx, p. 360 
(April 25, 1895). 
