8 DISEASES OF CULTIVATED PLANTS 



The development of an epidemic of fungus disease upon 

 weather conditions may not be universal, but it undoubtedly 

 applies to other than potato disease. Who ever saw or heard 

 of an epidemic of cereal rust during a hot, dry season, or an 

 outbreak of peach 'leaf-curl,' excepting following a sudden 

 snap of cold, dull, damp weather, preceded by conditions 

 favourable to the growth of the peach-tree. 



Traditional beliefs die hard ; the ideas of farmers and 

 gardeners concerning the cause of mildew, blight, etc., are 

 very much the same to-day as they were centuries ago. It is 

 the common practice of scientists to smile, or even to sneer, 

 at such antiquated notions, nevertheless there is generally a 

 substratum of truth in these old traditions, the outcome of 

 centuries of observation ; in fact the arguments advanced by 

 such people are often quite correct as far as they go, and only 

 fail where they could not possibly be expected to succeed, 

 that is in giving a scientific interpretation Of the facts 

 observed. 



Atmospheric conditions have undoubtedly much to do 

 with determining whether a given plant can be infected or 

 not; or even, when infection has taken place, whether the 

 parasite can manifest itself to any injurious extent. The 

 practical man believes that blight, mildew, etc., are caused by 

 cold east winds in the spring. This is quite correct as far as 

 it goes, and all that the scientific man knows in addition is 

 the fact that mildew and blight are due to a fungus, the 

 growth and development of which is favoured by the weather 

 conditions indicated above. It is common knowledge that 

 when favourable conditions for plant growth are continuous, 

 fungus diseases are absent or very much in abeyance : whereas 

 a genial period in the spring, followed by a cold spell of east 

 wind or frost is as certainly followed by blight, or a rapid 

 development of fungus growth. 



The man who grows potatoes knows too well that a period 

 of cloudy, damp, warm weather will be followed by potato 

 disease. The fungus was already present in the potatoes, but 

 so long as conditions favoured the growth of the potato plant, 

 the fungus could tiot make headway. The atmospheric con- 

 ditions indicated prove unfavourable for the continuous 

 healthy growth of the- potato plant, but, on the other hand, 

 favour the growth of the fungus, and an epidemic is the 

 result, more or less severe, depending entirely on atmospheric 

 conditions. 



