THE OCCURRENCE OF PLANT DISEASES IN ARID CLIMATES AND 

 THEIR AGRICULTURAL SIGNIFICANCE 



Professor I. Reichert 

 (Rehovot, Israel) 



INTRODUCTION 



One of the great advances in plant pathology in the last thirty years is without 

 doubt the recognition of the responsibility of environmental factors in the occurrence 

 and distribution of plant diseases. The significance of these factors and especially 

 of climate, has been appreciated for a long time by plant growers, who take them into 

 consideration whenever plant introductions are made. Unfortunately, however, the 

 phytopathological side is still much neglected. The fact that plant diseases can be 

 introduced along with plants has not always been given the consideration that it de- 

 serves. 



In another place (Reichert, 1950), I described phytopathogeographical methods 

 that enable plant growers to be more certain in predicting the reappearance of plant 

 diseases that may have been introduced with their hosts. 



The application of these methods makes possible the exact planning of new im- 

 portations and may obviate diseases otherwise incurred by introducing plants from 

 foreign places. 



It will be shown that the exothermic climate may be utilized within the frame of 

 phytopathogeographical planning for plant disease- control. The application of the 

 principles to be described will diminish to a great extent the affliction of crops by 

 diseases. It is known, that only a limited number of pathogens are able to establish 

 themselves and achieve their cycle of development in a xerothermic climate. This key 

 fact must therefore be utilized whenever introductions are made and the timing of sea- 

 sonal cultivations planned. The most important point that must be kept in mind in res- 

 pect to xerothermic climates is that a great part of the summer (the chief growing sea- 

 son for many crops) is either rainless or nearly so, and that only a relatively few 

 pathogens can surmount this obstacle and accomplish their cycle of development. This 

 fact has great agricultural significance. 



Below, some examples taken from observations on plant diseases made in Israel 

 and other parts of the Middle East are quoted. The maladies mentioned are hampered 

 in their development in this area which is characterized by rainless summers, and a 

 winter precipitation (from November — December to March — April). In Israel this 

 diminishes in quantity from North to South and from West to East, thus, Safad in the 

 mountains of northern Galilee has an average of 744mm. of rain per year, whereas 

 Beersheba in the South (Negev) has 193mm. and Haifa on the coast has 6l2mm. There 

 is approximately 400mm. average yearly rain in Ain Harod in the great interior valley 

 of Je2yeel in the eastern part of the country average July temperatures range from 22.4 C 

 at Safad, 24.5°C at Beersheba, 25.5°C at Haifa, to 27.7°C at Ain Harod. Relative hu- 

 midities during July average 48% at Safad, 65% at Beersheba, 74%at Haifa and 56% at 

 Ain Harod. (Anon., 1938; Ashbel, 1949). 



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