The change from furrow irrigation to overhead sprinkling must also be considered 

 if one is to obtain a clear picture of the value of arid region farming. The general be- 

 lief in Israel is that the rapid spread of late blight (Phytophthora infestans) is due to 

 the abandoning of the old furrow method for the intensive use of overhead sprinkling. 

 In the case of the cucumber disease already mentioned, this has been experimentally 

 established. (Duvdevani, Reichert & Palti, 1946). Sprinkling changes the microclimate 

 of the plants by increasing the moisture on the leaf surface and the moisture over a 

 greater area of soil. Consequently a greater opportunity for parasitic activity presents 

 itself. 



It is noteworthy that in arid regions, root and stem diseases are more prevalent 

 than leaf diseases, and may also appear in irrigated crops, but never to the extent 

 found in humid areas. 



In xerothermic climate there are, therefore, two primary components which operate 

 in the inhibition of plant diseases — high temperature and low humidity. The relative 

 importance of each factor depends upon the origin of the disease. Uith those derived 

 from a cooler climate, as Tilletia tritici, Sclerotinia sclerotiorum, Phytophthora infes- 

 tans, etc. the high temperature is the limiting factor, but in the case of pathogens ori- 

 ginating from tropical countries, such as Bacterium colanacearum, scab of citrus 

 (Sphaceloma fawcetti), and citrus canker (Phytomonas citri), the low air humidity be- 

 comes decisive in the inhibition of the diseases. 



References ' 



^^on, 1938 Government of Palestine. Meteorological Observ. :23pp. 



Ashbel, D. 1949 Bio- climatic atlas of Israel. Met. Dept. Heb. Univ. : 151pp. 



Avizohar- Hershenson, Z. 1947 The inoculation of rye with Claviceps purpurea as related to 

 environmental conditions. Palest. J. Bot. Rehovol. 



Duvdevani, S. Reichert, I. & Palti, J. 1946 The development of downy and powdery mildew of 

 cucumbers as related to dew and other environmental factors. Palest. J. Bot. Reh. Ser. 5 : 

 127- 151. 



Fawcett, H.S. 1936 Citrus diseases and their control. 656pp. New York. 



Goldsworthy, M.C. & Smith, R.E. 1931 Studies on a rust of Clingstone peaches in California. 

 Phylofjath. 21; 133- 168. 



Heald, F.D. 1933 Manual of plant diseases. 953 pp New York. 



Jaczewcki, A. 1935 Bacterial plant diseases (in Russian) 750pp. Moscow. 



Littauer, P., Volcani, Z., & Temkin, N. 1947 Brown rot of potatoes in Palestine. Palest. J. Bot. 

 Rehovot Ser. 6, : 2 19 -'20. 



Peltier, G.L. & Frederick, V^'.G. 1926 Effects of weather on the world distribution and preva- 

 lence of citrus canker and citrus scab. ]. Agric. Res. 32: 147- 164. 



Perlberger, J. 1943 The rust disease of stone fruit trees in Palestine. Bull. Agric. Res. Sta, 

 Rehovol. 34: 9pp. (In Hebrew with English Summary). 



Perlberger, J. 1944 The occurrence of apple and pear scab in Palestine in relation to weather 

 conditions. Palest. J. Bot. Rehovot. Set. 4 : 157- 16 1. 



Reichert, I. 1927 Downy mildew (Plasmopara viticola) of the vine in Palestine. Yedeoth, Tel 

 Aviv, 7-8 : 28pp. (In Hebrew with English Summary). 



74 



