THE SPREAD OF PLANT DISEASES. 125- 



tant one. The subject is new even to scientific men, and most 

 cultivators know nothing about it. I^'evertheless, the indirect 

 damage done by these animals as carriers of disease germs is 

 often immenseh^ greater than the immediate and visible injuries 

 they induce by feeding. Again I shall refer only to a few cases, 

 but these I consider to be well-established ones. 



Wagner's Exjjeriment with Snails. — Last year a German by 

 the name of G. Wagner published an interesting paper on the 

 spread of fungi by snails. He experimented with downy and 

 powdery mildews, ascomycetous fungi, and rust fungi. Snails of 

 various sorts were transferred from diseased to healthy plants, 

 and the disease appeared on the latter ; they were fed various 

 parasitic fungi, and a subsequent examination of the excreta 

 showed that the spores of these fungi passed through the animals 

 uninjured, and in condition to germinate. Finally, the dung of 

 these snails was dissolved in water and painted on the leaves or 

 stems of host plants and the disease followed. 



In this way, three downy mildews, two powdery mildews; one 

 Nectria, and one rust fungus were transferred to healthy plants 

 of various sorts. Mr. Wagner thinks that fungi are very com- 

 monly distributed by snails. 



GalbraitJv's Experiments. — Mr. Galbraith, an Englishman living 

 in the Seychelles islands, has recently obtained equally interesting 

 results with snails, which he finds to be largely responsible for 

 the spread of a vanilla disease prevalent in those islands. 



The Bacterial Broxon Rot of Cabbage. — This disease is preva- 

 lent in many parts of the United States, and is tolerably well 

 known to market gardeners. It blackens the veins of the leaves 

 and the woody ring in the stem, causes the leaves to fall off, and 

 prevents the formation of heads, or spoils those which have al- 

 ready formed. It is due to a micro-organism, and is disseminated 

 in various ways. I have myself the present winter obtained strik- 

 ing infections by means of the common greenhouse slug. These ani- 

 mals were fed on diseased cabbage leaves and then transferred for a 

 few hours to healthy cabbage plants, the disease appearing a week 

 or two later. Insects are probably also carriers of this disease.' 



I will next invite your attention to the danger of spread of 

 parasitic diseases by way of the manure pile. 



1 Since this was written I have obtained conclusive evidence, usingthe" Southern cabbage 

 worm " (larvae of Plusia brassicce), and have been able to confirm these green-house ex' 

 perlmenta by many observations in the field. 



