276 THE ORCHID REVIEW. 
SPOT IN ORCHIDS. 
A LECTURE on the Diseases of Plants, given at the Drill Hall, on March 
17th last, by Mr. George Massee, F.L.S., is printed in the last part of the 
Journal of the Royal Horticultural Society (Vol. XXI., Part 1), and contains 
the following account of the Orchid Leaf Rust (Gloeosporium cinctum, 
Berkl.) :-— 
“This is the most generally distributed of fungus parasites with which 
the cultivator of Orchids has to contend. It does not as a rule kill the 
host-plant, but the leaves are disfigured by its presence, and in addition to 
robbing the plant of a certain amount of material elaborated for its own use, 
proves injurious in rupturing the epidermis in many places, and thus 
facilitating the escape of water from the leaf, and, asa consequence of which, 
the balance of absorption and transpiration is upset. This fungus may be 
considered as a very simple type of parasite, inasmuch as, so far as is 
known, only summer spores are formed, but as the leaves of Orchids are 
always present, the spores find a suitable place for germination throughout 
the year; secondly, the fungus completes its development on one and the 
same host-plant. . 
‘Inoculation takes place by the spores being conveyed by some means 
on to the surface of a damp Orchid leaf, germination takes place at once, 
and within a few hours the germ-tube, or first-formed mycelium, produced 
by the germinating spore, pierces the epidermis of the leaf, and enters the 
tissues, where it spreads in the interior of the leaf-cells, finally rupturing the 
epider-nis to form its spores on the surface of the leaf. The fruit appears 
in small patches, a quarter to half an inch across: these patches are 
at first pale green, then yellowish or almost white, and in the case of fleshy 
leaves, sunk a little below the general surface of the leaf,as if they had 
been pressed down by the tip of a finger. When examined with a pocket- 
lens, each fruit, of which there are several on each pale patch, is seen to be 
surrounded by a blackish ring, hence the specific name of the fungus. 
“The potassium sulphide solution mentioned . . . destroys the 
germinating spores of this fungus, and as the parasite has been observed on 
several different kinds of Orchids, spraying all the plants present in a house 
where the pest has been observed is necessary.” —l.c., pp. 83, 84. 
The solution alluded to consists of half an ounce of potassium sulphide 
dissolved in a gallon of water, which, when sprayed on to the plants, will 
destroy most germinating spores, and, being clean, may be used wholesale 
in greenhouses where Bordeaux mixture could not be used on account of the 
whitewash effect produced on everything with which it comes in contact. 
Potassium sulphide dissolves most readily in hot water, which should be 
allowed to cool before spraying. 
