142 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



ON A METHOD FOR RENDERING CUCUMBER AND TOMATO 

 PLANTS IMMUNE AGAINST FUNGUS PARASITES. 



By George Massee, Y.M.H. 



The amount of forcing in the way of high temperature, excess of 

 moisture, and constant application of fertilisers considered necessary to 

 secure a fair margin of profit, when Cucumbers and Tomatos are grown 

 in a wholesale manner under glass, renders plants thus treated highly 

 susceptible to disease. 



There are two principal reasons for this susceptibility: (1) "soft" 

 foliage ; (2) the presence of fungi and eelworins in the soil. 



It is a well-known fact that, in the case of plants growing in the open 

 air, infection by fungus spores occurs almost entirely during the night, 

 when the vital activity of the foliage is somewhat checked, due to retarded 

 transpiration and respiration. Under these circumstances there is an 

 accumulation of those substances favouring infection present in the 

 leaves. This is what constitutes "soft" foliage. During the day the 

 vital activity of the leaves is unimpeded, and the various substances 

 formed are quickly conveyed away to other parts of the plant ; hence the 

 facility for inoculation by fungus spores dees not exist. This is the con- 

 dition expressed by the term " hard " foliage. 



In the case of plants growing under glass as described above, the con- 

 ditions are practically always highly favourable for the production of 

 <4 soft " foliage ; hence their extreme susceptibility to infection from fungus 

 spores. This condition of things explains the rapidity with which an 

 epidemic spreads when it once gains a foothold ; whereas plants of the 

 same kind grown out of doors, or even under glass, under more normal 

 conditions remain free from disease. 



Under the exceptional conditions of cultivation described above, not 

 only do the well-known parasites of the Tomato (Cladosporium tulvum, 

 Cooke, and Fusarium lycopersici, Sacc.) and of the Cucumber (Cercospora 

 melonis, Cooke) nourish luxuriantly, but certain other fungi, normally 

 occurring only on decaying vegetable substances in the open, now and 

 again assume a parasitic existence, when accidentally introduced into 

 hjuses where the conditions are so favourable to their development. 



The case of Dendryphium comosutn, Wallr., may be given as an 

 illustration. This minute fungus is not uncommon in Britain and other 

 countries. It grows on decaying plants, which it frequently covers with a 

 dense dull olive-coloured mould. 



During the present season a market-gardener, whose Cucumbers 

 Buffered severely from leaf-blotch (Cercospora melonis, Cooke) last year, 

 brought a number of diseased Cucumber plants to Kew for examination. 



The general aspect of the plants suggested a recurrence of the 

 epidemic of the previous season ; but microscopic examination and 

 repeated cultures and inoculations showed the blotches on the leaves to 



