440 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



this way; and that other mechanical injuries — such as that caused by a 

 bruise — tend to destroy, by affecting the vitality of the leaf -cells, the 

 immunity to disease which a plant may have previously possessed. * 



On many of the examples of mildewed leaves of E. japonicus which 

 have been sent to me minute orange-coloured larvae were to be observed 

 among the powdery masses of conidia. These proved to be the larvae of 

 a dipterous insect belonging to a genus (probably Mycodiplosis) of the 

 order Cecidomyidce. These larvae, which are very common on a great 

 number of mildews (see 9), feed exclusively on the conidia. Notwith- 

 standing their voracious appetite — for they appear to feed continuously 

 on the conidia during their larval condition — these larvae seem to be of 

 little or no effect in stopping the rapid growth of the fungus when the 

 external conditions are favourable. Their presence probably serves, 

 rather, to increase the disease, since a great number of ripe conidia are 

 usually found adhering to the bodies of the larvae, and these conidia are 

 sown over the surface of the leaf as the larva* crawl about. The prob- 

 ability of such larvae serving as agents in spreading a fungus disease has 

 been pointed out by Lindroth (5) in the case of similar Cecidomyidce 

 feeding on spores of the Uredinece. 



With regard to the question of the best means of combating the 

 present disease, it is obvious that, as the fungus is so conspicuous, it is 

 possible to collect and burn in the winter months the leaves which bear 

 the white patches of hibernating mycelium, before conidia are produced 

 on these which would infect the new leaves in the spring. Since the 

 fungus is wholly confined to the external surface of its host-plant, the 

 destruction of all such leaves would lead to the complete extirpation of 

 the disease. Where, however, such means are not practicable, fungicides 

 must be used. Although in the case of the present disease no trial of 

 the efficacy of any special fungicide has up to the present been made, it is 

 safe to predict, from the successful results obtained in dealing with other 

 mildews belonging to the same family, that sulphur will be found the 

 most valuable agent in destroying the fungus. The sulphur should be 

 applied either in the form of the dry powder known as " flowers of 

 sulphur," or as potassium sulphide (" liver of sulphur "). The " flowers of 

 sulphur " should be well dusted over the affected leaves, and the applica- 

 tion should be made on a still day during sunshine. The potassium 

 sulphide (1 oz. to 2 gallons of water) is to be used as a spray, and the 

 application should be repeated about every ten days. Should heavy rains 

 occur, the fungicide should be at once applied again immediately fine 

 weather sets in. Trial should be made, too, of the fungicide lately 

 recommended by Halsted and Kelsey (10) as being specially efficacious 

 against the powdery mildews. The fungicide is a kerosene emulsion, 

 made as follows : — Kerosene, 2 pints ; soap (any good hard kind), 1 oz. ; 

 water, 8 gallons. The soap is first dissolved in about a gallon of boiling 

 water, and after removing from the fire the kerosene is added. Ten 

 minutes' vigorous mixing will produce a creamy mass that will not 

 separate. After adding the proper amount of water the emulsion is ready 

 for use. The plants should be sprayed once in ten days, using a nozzle 



* In this connection it may be pointed out that in many districts the leaves of 

 E. japonicux are much subject to injury from the attacks of a caterpillar. 



