14 PROTECTION OF PLANTS — 1923-24 



pointed out that there is a tendency in wheat to keep many of its stomata 

 closed and it is suggested that the secretion by the appressorium renders the 

 mechanism of the stomata inoperative and they remain closed, thus excluding 

 the fungus. It is also pointed out that in Kanred wheat with its smaller 

 stomata, these peculiarities of stomatal behaviour might be more effective in 

 excluding fungi than in varieites of wheat with larger stomata (1). It has 

 been shown that the resistance of Citrus nobilus to citrus canker is due to a 

 broad ridge overarching the outer chamber of the stomata, which practically 

 excludes water from them, thus preventing the entrance of the canker bacte- 

 ria (2). Appel states that the resistance of some Remontant carnations to 

 rust is due to the formation of the stomata which makes it impossible for the 

 hyphae to penetrate (3). Neger, studying the biology of the Erysiphaceae and 

 making cross inoculations, found that in some cases in which host plants 

 showed resistance, that many of the penetrating haustoria become eveloped 

 in a gummy deposit secreted by the epidermal cells, which prevents them enter- 

 ing the plant (4). The resistance of certain varieties of barley to rust is due 

 to the bloom (waxy coating) on the leaves which prevents drops of water 

 adhering to them, consequently the rust spores are unable to germinate on 

 them. It is also recorded by Appel that the more waxy varieties of raspberries 

 and grapes are those less damaged by Coniothyrium (5). Lutman from his 

 investigations regarding the resistance of potato tubers to scab (Actinomyces 

 scabies) states "that at the head of the resistant class stand the potatoes of the 

 russet type and that thickness of skin determines resistance of tubers to scab." 

 He also records that close texture lenticels partly buried under the skin surface 

 and filled with small cells are also associated with the russet type of potatoes. 



It is thought that the hairs on the leaves may influence resistance. Appel 

 states that varieties of potatoes with small hairy leaves and open habit of 

 growth dry quickly after wetting and are less liable to infection by Phytoph- 

 thora infestans than other kinds. It is probable that the open habit of growth 

 and not the abundant hairs is the chief factor with such varities! It is also 

 asserted that "hairs also seem to save the under surface of the apple leaves 

 from infection with the apple sacb fungus (Venturia inecqualis), while Venturia 

 Pyrina readily attacks the smooth under-surface of the pear leaves" 7. This 

 statement does not agree entirely with my own observations in regard to the 

 scab on apple. I have frequently observed that most, if not all primary 

 infection by apple scab is on the lower surface of the leaves. It is also a matter 

 of common observation that the young fruits, while still covered with down 

 are very susceptible to scab. It is interesting to note in this connection that 



1. Allen R. F. Jour. AgrL Res. 24: 12, D. 1923. 



2. McLean Forman L. & Lee. H. Atherton. Phytopath, 11: 109-114. Mar. 1921. 



3. Appel, D. Science, N. S. 41, No 1065: 773-782, May 28, 1915. 



4. Niger, F. W. Flora CXVL 3, 331-335, 1923. 



5. Lutman, B. F. Vir. Agr. Expt. Sta. Bui. 215, Je. 1919. 



6. Appel. O. I.e. 



7. Butler Edwin John, I.e. 



