PLANT DISEASES CAUSED BY FUNGI. 



125 



than the summer form, appearing to the naked 

 eye as very minute black points, half buried in 

 the white felty mycelium. 



The use of winter fruit to the fungus is very 

 different to that of the summer fruit. The 



Fig. 161 



Mildew. 



1. Winter form of fruit on Rose hip (natural size). 2. A single fruit with its 

 appendages (magnified 200). 3. Spores produced in the winter fruit 

 (magnified 350i. 4. A single spore germinating (magnified 350). {Journal 

 Royal Horticultural Society.) 



spores of winter fruit will not germinate at 

 once when ripe, but only after a period of rest. 

 The winter fruit of the Eose Mildew may remain 

 on the shoots all winter, or may fall to the 

 ground, and the part of the Eose on which it 

 grew may completely decay, but the winter 

 fruit does not decay, but remains lying on the 

 ground, and the following spring, just when 

 the young Eose leaves are expanding, the spores 

 of the winter fruit of the fungus are set free 

 and blown about by wind, and those that 

 happen to alight on the damp leaves of Eose- 

 trees germinate, enter the tissues of the leaf, 

 and soon give origin to the summer form of 

 the fungus. 



The use of winter fruit is to continue the 

 fungus from one generation to another and, by 

 its power of remaining for some time before it 

 can germinate, to tide the fungus over that I 

 period of the year when the plant on which it ! 

 is parasitic is also in a resting condition. 



The life-history of fungous parasites, as ex- 

 plained above, must be clearly understood before 

 the prevention of a disease can be carried out 

 on scientific principles, and with any hope of 

 success. The course to be followed resolves 

 itself into two distinct parts. 



1. Preventing the Spread of a Disease. 

 — Parasitic fungi, as a rule, do not attack in- 



discriminately every living plant that their 

 spores happen to alight on, but are confined to 

 a single kind, or at most to plants that are 

 botanically closely related. 



An epidemic or extensive wave of disease can 

 only occur where large numbers of the same 

 kind of plant are growing in close proximity, 

 as a field of corn or Potatoes, or a house crowded 

 with Tomatoes, Chrysanthemums, &c. Of course 

 it is impossible to alter the manner of growing 

 field-crops, but it is doubtful whether it is wise 

 to crowd plants of one kind in houses. At all 

 events it must ever be borne in mind that crowd - 

 ing plants of the same kind favours the spread 

 of disease. 



If a disease shows itself, the affected plants 

 should be removed at once — an operation too 

 frequently neglected — and the remainder of the 

 plants sprayed without delay. Spraying will 

 not cure a disease, but if properly and promptly 

 done, it will prevent the spread of a disease by 

 destroying all spores that may have been de- 

 posited on the leaves of healthy plants; and 

 furthermore, as the fungicide or liquid used for 

 spraying adheres for some time to the surface 

 of the leaves, it destroys all spores that alight 

 on the leaves before they germinate and enter 

 the tissues. Spraying therefore is purely a 

 preventive measure, but none the less valuable 



Fig. 162.— Apple-tree Canker (JS'ectria ditissima). 

 Branches of an Apple tree showing the hark destroyed by the fungus. The 

 little white points in the cracks on the diseased parts are the fruits of 

 the fungus, which are of a bright-red colour (natural size). {Gard. Chron-) 



on that account, for " prevention is better than 

 cure", and also much easier to accomplish. 

 2. Preventing a Repetition of a 



