542 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



; 



the fungus in pure culture, and carried out infection experiments 

 reproducing the disease in the lettuce by its means. 



The colourless mycelium of the fungus is somewhat restricted in 

 its growth, being present almost solely in the tissues of the spots which 

 it has killed. The spores occur clustered closely together in small; 

 but numerous groups on the dead spots, being at first covered by I 

 the epidermis, but later breaking through and exposed. They are 

 produced at the apex of very short, erect hypha?, and are about 11 

 to 20 ju long, and 3 to 4 ju broad, colourless and two-celled, much 

 broader at the apex than the base. The septum is not always easy 

 to see, but with proper illumination it becomes evident. 



In 1896 a very similar disease attacked lettuce in a greenhouse m 

 Troy, Ohio. In this case Selby"^ describes the leaves as showing! 

 numerous perforations from 1 mm. to 2 mm. in diameter with irregular 

 borders. The spots were at first pale in colour, but soon fell out. 

 It was attributed to a new species of fungus by Ellis and Everhart, 

 who called it Marsonia perforans. In this case the spots were 

 1 to 2 mm. in diameter, whereas in the case of the Italian disease, 

 they were described as having a diameter of 3 to 5 mm. In our speci- 

 mens they were intermediate between these measurements, but the 

 figure given in the American publication shows them somewhat 

 larger than the dimensions quoted in the description. The spores are 

 described as 11 — 15 x 2.5 — 3 fx exceptionally reaching 20 jj. long in 

 M. perforayis, and 15-20x3 — 4 fx in M. Panattoniana. In our speci- 

 men they were 11 — 18 x 3 fx. Except in the size of the spots and the 

 spores no difference appears to exist between the fungi or in the effect 

 they produce on the lettuce, and M. perforans is probably specifically! 

 identical with M. Panattoniana. j 



The disease does not appear to have spread to any serious exteni 

 in America, although it has been noticed in some other districts beside^| 

 Troy. In the single case recorded in Ohio one-fifth of the crop was:! 

 destroyed, and the German outbreak f is referred to as somewhaj 

 serious. It seems, therefore, so far as our present evidence goes, tha| 

 where the disease appears it may, if the conditions be suitable, assunu] 

 an epidemic form, and that it is likely to be more prevalent in crops: 

 under glass than elsewhere, for probably there the most suitable conj 

 ditions are found for its development. It would be well that growerf 

 of lettuce under glass, whether in frames, houses, or under cloches 

 should be on the watch for it. i 



The remarks made by the grower at Troy, Ohio, are instructive..: 

 He regarded watering the lettuce as the quickest mode of spreadin< 

 the disease. It first occurred along a strip on the greenhouse benches 

 and when about one-fifth of the crop had been attacked the growe 

 used sulphur and oil on the steam pipes and watered carefully, keeping 

 the house well ventilated. After these measures were adopted ui 



* A. D. Selby, " Investigations of Plant Diseases in Forcing-house am 



Garden. I. Diseases of Lettuce." U.S.A. Exp. Stn., Ohio, BnU. 73 (1897) 

 t Appel and Laibach, I.e. 

 X See Selby, I.e. 



