252 THE ORCHID REVIEW. [Aucusr, 1914. 
those of last season only, are showing these minute spots (and it is to be 
noted that old leaves of imported plants have the same spots). I cannot 
admit that the insecticides used—XL All and Phytophyline—could be 
responsible for the damage, as the young leaves show nothing! 
As I have a house of 1500 plants of this Vanda I am very alarmed, and 
I ask you kindly to give me your advice about these spots. If it is of 
fungous nature what remedy do you suggest ?. In common with everybody 
having Vanda in cultivation, I have, in small degree, what is termed black 
spot at the apex ofthe leaves. Would the minute spots be of the same nature? 
Bois de Breux, Liége. _ CHARLES SLADDEN. 
The leaves sent have been submitted to microscopical examination, but, 
as in previous cases, no fungus mycelium can be detected. Weare inclined 
to think that the minute discoloured spots are only the earlier stages of 
what is know as Black Spot, and that both are due to the same cause, 
which may have been a chill caused by drip from the roof in cold weather. 
The immunity of the young leaves suggests that the cause has ceased for 
the time being, but these may also be affected when the cold season arrives, 
unless the cause can be prevented. 
Some years ago the Spot Disease of Orchids was worked out by Mr. G. 
Massee, of Kew, and a summary of his conclusions and experiments wat 
given in these pages (iv. pp. Ig-20). In two cases he succeeded In 
producing the disease artificially. It first appears in the form of minute 
pale spots on the upper surface of the leaf, sometimes few and scattered, at 
others more numerous. At first they are not conspicuous, and likely to 
escape observation unless specially looked for, but they soon assume a 
pale brown colour, and gradually increase in size, sometimes becoming 
confluent. The tissues are completely disorganised, and, owing to the 
collapse of diseased cells beneath the epidermis, the surface of the i 
becomes somewhat depressed. Mr. Massee concludes that the diseasé 
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 the temperature is exceptionally low and the roots copiously supplied 
with water. The effect of the chill produced by the drops of water 7 
the above conditions is to cause disorganisation of the cells of the = 
underlying the drops, and the disappearance of the chlorophyll, which : 
followed by the precipitation of tannin and other substances, os 
globular sphere in each cell, and eventually the complete disintegration © 
the cells. It is mainly caused by the three following conditions: (1) me 
high a temperature ; (2) too much water and not sufficient air in cr 
with the roots, and (3) watering or spraying with a falling instead 
rising temperature.—Ep. O.R. 
