REMEDY FOR GRAPE-BLIGHT. 229 



round black bodies on the surface of the leaf: those of the 

 Perononfora are in the interior of the leaf. 



A microscopic examination, then, shows us that it is not 

 correct to speak of grape mildew, or blight, as a distinct dis- 

 ease. We have just seen that there are at least two different 

 fungi which produce a blight ; and the two differ decidedly 

 in their habit and growth, — so much so, that the means taken 

 to prevent the growth of one will not apply to the other. 

 Let us consider this practical point more at length. We will 

 suppose that the grape-raiser recognizes that his plants are 

 attacked by the first form of blight described, — Unoinula 

 spiralis. As a microscopic examination shows that the fun- 

 gus is on the surface, and not in the interior, of the leaves, it 

 is plain that the object should be to check the growth of the 

 mycelium on the leaf. The injury that the Uncinula does to 

 the grape is, that it covers the leaves, which in a certain 

 sense may be said to be the respiratory organs of a plant, so 

 that the necessary supply of light and air is shut off. The 

 growth of the fungus may be checked by the use of sulphur 

 strewn over the plants. We must also consider how the 

 disease is propagated from one plant to another. The conid- 

 ial spores already described are light, and easily blown from 

 one plant to another. Wherever they fall, if the weather is 

 only moist enough, they begin to send out threads, which 

 form the mycelium of a new Uncinula. The threads only 

 grow to any length, as far as we yet know, when the spores 

 have fallen on or near grape-vines. From this we can infer 

 that the Uncinula does not live entirely upon material found 

 in the air, or accidentally on the surface of the grape-leaves, 

 but that it also requires some peculiar substance produced 

 only by the grape-plant. A great many of the species of 

 fungi, however, which are botanically closely related to the 

 Uncinula., are not limited to the plants of a single genus, but 

 grow indifferently on plants which are l )t nearly related 

 botanically. 



Another question also arises, — How does the fungus sur- 

 vive the cold of winter? The conidial spores which can 

 spread the disease rapidly are killed by the cold. The round 

 black bodies which contain the winter sj^ores are much 

 tougher. The winter spores are not ripe until late in the 

 autumn, and fall to the ground with the leaves on which 



