1 82 FUNGOUS DISEASES OF PLANTS 



which may therefore be termed primary, secondary, etc. These 

 conidia may be propagated for some time in beerwort. On the 

 host germination may proceed normally, that is, by the direct pro- 

 duction of a true germ tube ; moreover, germination rather than 

 budding is occasionally observed in culture. The grayish cast 

 given to the surface of the leaf is due to the numerous asci, and a 

 mealiness may become evident later from the abundance of conidia. 



Infection. Since, as shown later, the disease may be in very 

 large part prevented by spraying prior to the opening of the 

 buds, it is evident that infection would seem to result, in general, 

 by spores or conidia which have been caught in the bud scales, 

 'or, at any rate, which were adherent to the buds at the time of 

 opening. It is therefore believed that the marked effect of con- 

 ditions upon the prevalence of the leaf curl is brought about in 

 this way : During cold, moist weather the young leaves within 

 the bursting buds would be in a state described as suffused with 

 water, and consequently attended by lowered vitality. The fungus 

 would therefore gain entrance more readily. If, however, at this 

 time the bud scales were well covered with a toxic fungicide, the 

 germ tubes of the fungus would in most cases fail to cause infec- 

 tion. Again, the effect of conditions does not end with infection, 

 and it is well known that the extent of the disease upon single 

 leaves or shoots is greater when the cold, moist weather is per- 

 sistent. It is, therefore, probable that the spread of the fungus 

 throughout an infected leaf or shoot is directly assisted by the 

 continuance of lessened vitality and water suffusion as growth 

 progresses. 



Control. Preventive measures for the leaf curl have been made 

 a subject of careful investigation throughout many years. It has 

 finally been clearly shown that a thorough application of a fungi- 

 cide, preferably Bordeaux mixture, during the late winter or early 

 spring just prior to the opening of the buds, may prevent from 

 90 to 95 per cent or more of the infections. I do not believe 

 that subsequent sprayings are of any importance except in a case 

 where the early spraying has been omitted ; and the fungus be- 

 ing abundant, it is desired to cover up all parts of the plant with 

 a spray so that the spores may be in large part killed as they are 

 produced, or as budding is attempted. 



