14 DISEASES OF CULTIVATED PLANTS 



are also favourable for infection. It will be noticed that 

 the symptoms described above agree in the main with the 

 gardener's conception of 'soft' foliage, a condition which 

 is well known to favour the development of disease. Now 

 plants grown under glass, more especially when exposed to 

 an abnormally high temperature and excess of moisture, 

 have comparatively speaking always 'soft' foliage, and con- 

 sequently are more susceptible to fungus disease than plants 

 of the same kind grown under more natural conditions. 



HOW FUNGUS DISEASES ARE DISSEMINATED 



It is very important to remember that fungus diseases may be 

 disseminated in different ways, (i) By means of spores or 

 specially produced reproductive bodies. (2) By what is termed 

 a vegetative method, that is by a portion of the mycelium of 

 the fungus present in the seed or some other portion of a plant, 

 as rootstock, tuber, bulb, etc. The mycelium remains in a 

 passive condition until the seed or tuber commences growth, 

 when the mycelium grows up along with the new plant. 

 This method of infection is most dangerous and most difficult 

 to prevent, as the seed, or whatever portion of the plant 

 harbours the mycelium, is infected every year, the fungus 

 simply passing from the old to the new generation without 

 ever leaving the plant. (3) Sclerotia or concentrated masses 

 of mycelium, replete with food, are also often formed in the 

 substance of bulbs, tubers, etc. 



In dealing with infection by means of spores, it is again 

 necessary to treat the subject from two standpoints, (i) The 

 spread of disease from one locality or district to another in 

 the same country. 



Wind is undoubtedly an important factor in distributing 

 spores within a limited area, as are also insects and various 

 kinds of mites and other minute creatures that creep or fly 

 about from one plant to another. Birds, rabbits, and hares 

 also carry spores from one place to another. Snails and slugs 

 are responsible to a considerable extent for the diffusion of 

 spores, and consequent spread of disease. Many kinds of 

 fungi are eaten by slugs, and so far as is at present known the 

 spores of some kinds of fungi germinate only after having 

 passed through the alimentary tract of a slug. Slugs are 

 eaten by toads, and here again we have an additional agent 

 in effecting the dispersion of spores. If a slug is placed on 



