DISEASES OF PLANTS. 



81 



leaf may be the means of infecting an entire orchard within 

 a very short time, or a single "rusted" leaf of wheat may be 

 the means of destroying the prospect of a crop. At this state 

 the value of spraying is obvious ; by applying a fungicide the 

 spores are destroyed, and the spread of the disease considerably 

 checked. The minute spores described above are known as 

 summer-spores, which, as already stated, germinate the moment 

 they are ripe, their use being to enable the fungus to extend 

 its area, and this method continues so long as the plant on 

 which the fungus is parasitic continues to grow actively. 

 During the autumn, when the host-plant, as the plant is 

 called on which a fungus is parasitic, is ceasing vigorous 

 growth, the mycelium of the fungus, which during the earlier 

 part of the year has given origin to summer-spores, now 

 begins to produce a totally different kind of spores, called winter- 

 spores or resting -spores. These winter-spores remain dormant 

 during the winter, and germinate the following spring, at the 

 time when the leaves of the host-plant are unfolding, their 

 use being to tide the fungus over that period of the year when 

 the host-plant is not in active growth. 



From the above account it will be seen how very important 

 it is to collect and burn all diseased leaves and twigs, and not 

 allow them to remain on the ground under the plants from 

 which they have fallen. It is true that such diseased leaves, 

 if allowed to lie on the ground, usually decay and disappear 

 during the winter ; but the resting-spores present in the tissues 

 of such diseased leaves do not perish, but remain in the soil 

 and germinate in due season, and in all probability a renewal 

 of the disease will follow. Here again is a case where pre- 

 ventive measures should be resorted to. Where a disease 

 has been known to exist during the previous season, the plants 

 should be sprayed at intervals of ten days during the period 

 of the expansion of the leaf-buds with a fungicide ; as, not- 

 withstanding every care in the removal of diseased parts, it 

 is probable that spores may be lurking in crevices of the bark, 

 &c, and the fungicide would destroy all such, and greatly 

 lessen the chance of a return of the disease. For spraying 

 purposes a solution, consisting of half an ounce of potassium 

 sulphide dissolved in a gallon of water, may be used ; this 

 will destroy most germinating spores, and, being clean, may 



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