PRESIDENTIAL ADDRESS — SECTION C. 7,\ 



become infected, probably by the falling into them of infected 

 earth. 



An interesting- case of infection by flood water has been 

 rejjorted by Mr. llobson, our chief citrus canker inspector. 



The Sterkstroom River flows throu.s^h a large orchard in 

 the Rustenburg district which was heavily infected with canker. 

 Three miles further down the river there is an orchard containing 

 500 trees obtained from a nursery where canker has never 

 occurred: five miles down the river there is a second orchard of 

 100, twenty year old seedlings ; all these trees had been carefully 

 inspected three times before January, 1918, and no trace of 

 canker tliscovered. The summer of 1918 was an exceptionally 

 wet one, and during January the Sterkstroom River was in flood; 

 it submerged the lower three rows in each of these orchards, 

 which are on the river bank. The next inspection in March, 

 igiS, revealed canker infection in 40 per cent, of the three sub- 

 merged rows in the second orchard, and 60 per cent, in the first. 

 Five seedling lemon trees, apparently self-sown, growing in dense 

 poplar bush between the two orchards on the river bank, were 

 all found infected at the same time. 



Citrus canker also afYords an interesting example of the 

 so-called hold-over infection, in which case the plant itself 

 harbours the parasite indefinitely. Peltier (3Q), in experimenting 

 with various citrus hybrids, noticed that certain trees which 

 failed to become infected at the time of inoctilation, developed 

 canker on the shoots in the succeeding spring. In the field 

 it has frequently been found that trees from infected nurseries 

 which had been kept under observation as suspects, developed 

 canker only many months after being planted out. An instance 

 recently came to our notice of trees which were derived from 

 an infected nursery four years ago. and which have been 

 repeatedly and carefully inspected without any sign of leaf or 

 bark cankers being found. Recently a number of young cankers 

 showed themselves on one of these trees. In other cases canker 

 has appeared after one. two, or three years. 



Wind-blown rain is. without doubt, one of the most 

 important factors in the field and orchard in the dissemination 

 of leaf s)X)t diseases where stomatal infection is the rule; this 

 is the case in the mango blight (Bacillus maii^s^iferce), citrus 

 canker, bacterial shot hole of the apricot (Bacterium pruni), and 

 the walnut blight (B. juglandis). These organisms all aflfect the 

 fruit and twigs as well as the leaves. 



In making field studies of the mango blight, it was noted 

 (14) that the disease spreads in the orchard during the wet 

 season, and in the direction of the prevailing winds. 



Tv/o interesting papers on the agency of wind-blown rain 

 in disease dissemination have recently been published by Faul- 

 wetter (22-3). who worked with the angular leaf spot of cotton 

 caused by Bacterium malvaccarum. He showed by inoculating 

 one row of plants in a cotton field that the disease is disseminated 

 during wet weather in the direction of the prevailing wind. 



