936 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Garden plants do, however, often receive treatment which renders 

 them more liable to attacks of fungi than the wild forms. Broad areas of 

 one sort of plant, overcrowding and overfeeding, are instances of this ; 

 and again, weakly plants are sometimes produced from poor cuttings 

 (through trying to get up too rapidly a stock of some new variety, e.g. of 

 a Chrysanthemum). With greenhouse plants, too, " susceptibility " to 

 disease may be induced by soft unripened growth, if insufficient ventilation 

 is given; or the constitution of the plants may be weakened in some other 

 way. Forcing, needless to say, is apt to do this ; and to those who know 

 the countless myriads of spores that are produced by fungi the wonder is 

 that plants which are constantly forced, like Tomatos, Cucumbers, &c, are 

 not more subject to epidemic diseases than as a matter of fact they are. The 

 latter plant especially must be very resistent to fungi, as the atmosphere 

 that is required for forcing it is ideal for the growth of fungus parasites, 

 and yet the number of forms which attack it is comparatively small. But, 

 apart from the special conditions, cultivated plants should not be more 

 liable to diseases than wild species. 



Before giving a list of those wild plants which afford suitable hosts 

 for these fungus parasites, we will refer to two widespread diseases, viz. the 

 Club Root of the Cabbage tribe, and the disease of fruit-trees caused by 

 Eutypclla, as affording an illustration of the twofold danger which wild 

 plants may bring about. 



The Club Root of Cabbage, Turnip, "Wallflower, &c, affords an 

 excellent example of how a garden parasite can make use of common 

 weeds which may have been left to thrive. The disease is caused by a 

 microscopic fungus (Plasmodiopthora brassicce) which gains an entrance 

 into the young roots of the crops, and when inside grows and flourishes 

 to such an extent that it completely distorts the roots, and causes the 

 characteristic swellings so familiar to gardeners. The remedy usually 

 given is to lime the ground and not grow Crucifers for at least five 

 years. But it must be remembered that the Club Root fungus can live 

 in the ground for several years without any host at all, and though it does 

 not flourish or increase under these conditions, it manages to get along 

 somehow or other, and is ready to seize every opportunity of infecting the 

 roots of Shepherd's Purse, Mustard, or such common cruciferous weeds on 

 which the gardener may have had compassion. If these weeds are 

 available, it is enabled again to increase, and it obtains, as it were, a new 

 lease of life. Shepherd's Purse is one of the commonest weeds, and is no 

 doubt a welcome friend to Plasmodiophora ; but most, if not all, cruci- 

 ferous weeds form a harbour for this pest. 



A very important point in connection with this disease may be 

 emphasised here, though not strictly within the bounds of our subject. 

 Great care should be exercised in keeping the seed-beds free from the 

 fungus. The plants are mostly attacked when quite young, though the 

 swellings may not be visible until after planting out ; but if the seed-beds 

 are kept clean and well limed, the danger of an attack by "club" is 

 greatly diminished. (G. Massee, " A Text-book of Plant Diseases," 

 p. 334.) 



The new disease of fruit trees caused by Eutypella is an instance of 

 a fungus common on wild plants turning its attention to cultivated forms, 



