In 1934, when visiting the vast plantations of Pistatia vera in the great steppes 

 east of Aleppo, we noticed that the trees were completely free of any disease, where- 

 as westward towards the seaport of Lattaquieh, we found them heavily infected by 

 Septoria pistaciae. 



Citrus trees. The limiting effect of dry weather conditions upon plant disease 

 is particularly pronounced in the case of citrus diseases, and is evident in Israel and 

 adjacent countries. The arid conditions of this region exclude two diseases that in 

 warm humid countries are considered to be among the worst scourges of citrus. They 

 are 'scab', caused by the fungus Sphaceloma fawcetti, a near relative of the anthrac- 

 nose of vine, and 'canker', a bacterial disease caused by Phytomonas citri (Fawcett, 

 193^). Peltier & Frederick (1922) revealed the dependence upon warm and very humid 

 conditions of these diseases. 



Another bacterial disease, Phytomonas syringae, the causative agent of 'blast' of 

 of citrus, develops in our country only when the autumn is particularly rainy and cool 

 (Reichert & Perlberger, 1928; Reichert, 1939 b). 



Industrial Plants 



Tobacco. Although this country has for many years been importing tobacco 

 seeds from U.S.A. and other countries that grow the crop under warm, humid conditions, 

 there has been no emergence of the very serious diseases that tobacco has been heir 

 to in the countries whence the seed came ; we have not found to date any of the downy 

 mildews such as Phytophthora nicotiana, Peronospora hyoscyamae, nor the various 

 bacterial diseases that affect tobacco. Even Bacterium solanacearum, which was men- 

 tioned above as having once attacked potatoes in the spring here, has failed to appear 

 on tobacco in Israel, The reasor! is that the crop here is grown during the dry summer. 



Other instances of summer crops avoiding fungal attacks are provided by the sun- 

 flower (Helianthus annuus) and safflower {Carthamus tinctoria), both of which, in cool 

 damp countries are prey to Sclerotinia sclerotiorum. Safflower is attacked also by a 

 downy mildew, Plasmopara halsteadii. In Israel these diseases make no headway 

 whatever in the summertime, the optimal temperature for the germination of the asco- 

 spores of Sclerotinia sclerotiorum being 17°C : much lower than the average prevailing 

 temperature of the air during the growing period of the host. Besides, the parasite de- 

 mands a great amount of soil moisture for the fruiting bodies to discharge their spores. 



DISCUSSION 



The accumulated data regarding crop plants escaping disease when grown in hot, 

 arid, regions show clearly how important these regions, including steppe and desert 

 may become for agriculture. All these vast, neglected lands, hitherto considered as 

 'barren', may be transformed into productive, remunerative agricultural areas for grow- 

 ing crop plants since these are here less subject to the attacks of dangerous para- 

 sites than if they are grown in humid areas. Thus free of such troubles, the grower is 

 able to devote his energies to the control of the few xerophilic diseases remaining. 

 For instance, great areas of vineyards in the interior valleys of Israel, in the neigh - 



72 



