bourhood of Damascus and in lower tgypt remain untouched by the standard maladies 

 that are usually met with in humid regions. The only serious disease remaining here 

 is powdery mildew, and this can be easily controlled. Citrus and other southern trees 

 escape the bulk of plant plagues if grown in xerothermic regions. This is especially 

 true of the leaf diseases usually concomitant with them in warm, damp climates. Thus 

 grapefruit groves in the interior valleys of Israel, as well as the large citrus planta- 

 tions in the vast western desert of Egypt are free from all leaf diseases. The same 

 may be said of cereals, particularly maize, and of many vegetables and industrial 

 plants, all of which, under these conditions, are quite free of hygrophilic diseases. 



The above data show that, a distinction must be drawn between two types of xero- 

 thermic region. The first may be designated as interior areas, far removed from the 

 sea, which are characteristically free of certain diseases. The second type comprises 

 the area bordering the sea. It is naturally subjected to higher humidity and to humid 

 breezes, which modify the immunity found in the areas of the first category. For ex- 

 ample, the downy mildew of vine {Plasmopara viticola) which is prevalent, in a mild 

 form in the coastal plains of the Mediterranean, disappears in steppes and deserts far 

 from the coast (Reichert, 1927). 



Another important factor in the controlling effect of arid climate on plant disease 

 must be recognised. This, is the difference between summer and winter periods. The 

 winter climate in a xerothermic region is cooler and more humid than that of summer, 

 and therefore may enhance the development of certain diseases that are suppressed in 

 the summer. Thus, Sclerotinia sclerotiorum attacks plants during the winter and not 

 during the summer because it has a low minimal temperature — 1°C and a low optimal 

 temperature — between 17 and 21°C. Decisive factors in the success of infection are 

 the amount of precipitation, the relative air humidity, dew, and last but not least, the 

 altitude of the locality. 



As a good example, the distribution of the vine disease, Plasmopara viticola may 

 again be mentioned. This is limited to the coastal plains where sufficient humidity 

 and dew are present, whereas it disappears in the inland valleys where they are ab- 

 sent, as in the high mountains of Galilee, where freely blowing winds quickly dry up 

 any dew that is formed and diminish humidity of the air. The great importance of hu- 

 midity in arid countries in the propagation of the Plasmopara pathogen was demon- 

 strated during this study of the comparative distribution of the disease (Reichert, 

 1927). 



Another important ecological point that must be emphasized is the duration of 

 night and the subsequent dew formation. In northern countries, parasitic fungi avail 

 themselves of the necessary dampness for infection both in the night and in the day- 

 time. In the dry regions however, these conditions prevail only during the night. It 

 has been demonstrated experimentally by a study of the ecological conditions on 

 downy mildew of cucumber {Pseudoperonospora cubensis) the extent to which dew for- 

 mation is essential for the development of diseases in dry regions. Vvhen dew was 

 prevented from forming on plants by means of canvas covers, no infection took place 

 (Goldsworthy & Smith, 1931). 



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