CHAPTER VII. 

 ECONOMIC IMPOETANCE OF DISEASES OF PLANTS. 



§ 13. The economic importance of any plant-disease depends on 

 its distribution, its intensity, and the value of the plants attacked. 

 Of most consequence are those epidemic diseases of fungoid origin, 

 which cause rapid death of their host, and spread with great 

 rapidity over wide areas. Such, through repeated attacks, 

 may render the cultivation of certain plants impossible 

 in a locality. Almost equal damage may result from those 

 parasites, which, although they do not kill their host, yet 

 destroy or prevent the development of that part for which we 

 grow the plant. Amongst these are species which inhabit 

 flowers or fruits, the wood-destroying fungi of forest-trees, 

 and forms inimical to the foliage, roots, or tubers of plants 

 of economic value. 



As examples of parasitic fungi which bring about rapid death 

 of their host, are the originators of many diseases of young 

 plants. Phytophthora omnivora may during a few days of damp 

 weather completely kill out not only healthy beds of seedling 

 beech or conifers in the nursery, but even the young plants 

 by which a forest is being naturally regenerated. Pestalozzia 

 Hartigii, a few years ago in the beech-forests in some districts 

 of Bavaria, exterminated three-fourths of the naturally-sown 

 plants from one to four years old. Herpotrichia nigra is 

 capable of completely destroying the young spruce plantations, 

 so important for the afforestration of bare slopes in mountainous 

 districts, and it may attack with such violence nurseries 

 established at great cost and labour that they have to be 



