OBSERVATIONS 



Cereals 



Wheat. As the first example of a disease derived from a cool climate that sel- 

 finds adequate conditions for development in arid climates, we may mention Tilletia 

 tritici. It does not often occur in Israel, is especially rare in the interior valleys and 

 is to be found only when rain and ensuing cold weather follow closely upon the sowing 

 of the grain. The optimum temperature for seed germination is 25°C while that for ger- 

 mination of the spores is 10- 16°C. Since rain generally does not ^pear until some 

 time after sowing, the seedling usually has enough time in which to evade attack. Such 

 a case was observed in Palestine in 1924 (Reichert, 1928 a). The same conditions and 

 results are known in other xerothermic countries, such as southern India, southern Rus- 

 sia, the Spokane Valley in Washington, and others (Heald, 1933; Walter, 1950). 



A second example is Claviceps purpurea, which is favoured by 'abnormally wet 

 seasons with reduced amount of sunshine' (Heald, 1933). Since arid regions are al- 

 ways dry during the blossoming period of wheat, no infection can take place here. Avi- 

 zohar (1947) was able artificially to produce this disease in Israel only by increasing 

 the humidity around the plant by artificial means. Similar environmental conditions 

 prevail in the distribution of the disease in other countries. (Heald, 1933). 



Maize. One of the healthiest summer crops grown in Israel is maize, the only 

 diseases affecting the plant here being Puccinia sorghi and ilstitago zeae. They oc- 

 cur only near the sea or under irrigation ; conditions that raise the humidity to such an 

 extent that the pathogen can successfully attack the crop. This is particularly true in 

 the case of the rust. It is interesting to note that, although all the maize varieties 

 were originally derived from localities in the U.S. and were sown here without seed 

 treatment, they did not reveal any of the serious diseases recorded in other maize - 

 growing countries, e.g. Diplodia zeae, Sclerospora macrospora, and Physoderma zeae- 

 maydis. These three diseases require abundant and frequent rains throughout the grow- 

 ing period of the corn crop, conditions not to be found in arid regions. They are there- 

 fore also excluded from the arid south-western part of the U.S. (Heald, 1933; Walker, 

 1950). 



Wheat and maize in our country have not been affected by bacterial diseases like 

 Pseudomonas translucens var. undulosa and P. desofvens, that cause great damage in 

 other countries. 



Vegetables 



A good example of how an arid climate can preclude or limit the occurrence of an 

 important vegetable disease is to be found in the case of 'black rot' of cruciferous 

 plants (Pseudomonas campestris). It is widespread in coastal regions of Israel (Rei- 

 chert, 1939 b), but its presence in the interior valleys is limited to early spring and 

 late autumn. In the U.S. a similar state of affairs exists, with the disease prevalent 

 only in the humid areas east of the Mississippi. It is very rare in the Rocky Mountains 

 and Pacific coast (Heald, 1933). Walker (1950) tells us that it is entirely excluded 



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