PLANT DISEASES. 



743 



two or three seasons the trees so attacked are injured, or if young, as in 

 the case of nursery stock, killed outright. The form of the fungus grow- 

 ing on the Plum or Hawthorn leaves resembles little yellow horns, open 

 at the point, and occurring in clusters on a yellow spot. Much money and 

 time has been lost at various times in the endeavour to check the spread 

 of this disease on Hawthorns and Pears by spraying and other means, 

 acting on the assumption that the disease spreads directly from one tree 

 to another, which is not the case, as the spores from the Juniper alone 

 can infect the Hawthorn and Plum ; therefore, the only means of check- 

 ing the disease is to seek out the infected Juniper and remove the diseased 

 branches. Many other destructive fungi, especially those attacking 

 cereals, also grow on different plants daring different periods of their 

 existence. 



Most parasitic fungi produce at least two kinds of spores or reproduc- 

 tive bodies. First appear the summer spores, developed on the living host- 

 plant ; these are produced in immense numbers and in rapid succession, 

 and are always the direct cause of a sudden spread of fungus disease, 

 being dispersed by wind, rain, syringing, &c. ; hence, at the season when 

 plants are in the full vigour of growth, a constant watch must be kept, 

 and on the least indication of disease the affected plants should be 

 removed and isolated from those of their own kind. If a disease is 

 proved to be present, all neighbouring plants, especially if of the same 

 kind as those attacked, should be sprayed with potassium sulphide or 

 permanganate of potash solution. 



When the vigour of the host-plant begins to wane in the autumn, the 

 formation of summer-spores ceases, and winter-spores or resting-spores 

 are formed. The resting-spores may be produced in clusters presenting a 

 similar appearance to those of the summer-spores, as in the case of the 

 uredospores (=summer-spores) and the teleutospores (=winter-spores) 

 of rusts attacking cereals. Chrysanthemums, and many other plants, 

 or they may be quite different in form and structure and produced in 

 special conceptacles, as in the Nectria, causing Apple-tree canker. 



The use of winter-spores is to start the disease again the following 

 season ; and in the case of annuals, and also of perennials not ha%'ing 

 permanent mycelium of the fungus in their tissues, it is well to remember 

 that infection can only take place through contact with resting-spores 

 direct, or with secondary spores produced by the germinating resting- 

 spores that have passed the winter in the vicinity. Sclerotia are also 

 formed in the decaying tissues of many plants that have been attacked by 

 parasitic fungi, and these act in a manner similar to resting-spores by 

 producing spores after a period of rest. 



One of the greatest difficulties the gardener has to contend with is the 

 danger arising from the presence of resting-spores and sclerotia. Being 

 often firmly attached to the plant on which they are produced, they pass 

 unharmed through the process of the host being converted into manure, 

 and may even pass through the alimentary canal of an animal without 

 injury ; and, consequently, are in the end deposited on the land along 

 with the manure. As previously stated, it is impossible to burn all 

 infected plants, but in the case of garden produce there are instances 

 where it would be well to do so, rather than run the risk of a recurrence 



