FIRE-BLIGHT OF APPLE, PEAR, ETC.: HISTOLOGY 



383 



If possible, embed and section on the 

 microtome early stages of blossom blight. 

 Can you make out: a, multiplication of 

 bacteria in the nectary; 6, invasion of the 

 ovary and pedicel? Is there any choice 

 in the tissues invaded ? Study also blight- 

 ing shoots and fruits. The bacterial slime 

 is often abundant enough to drip from the 

 hands when such fruits are handled after 

 cutting. Is the organism always inter- 

 cellular or does it sometimes also enter the 

 cells? There is a difference of opinion on 

 this point. Do not decide too hastily. Is 

 the cell-wall destroyed? What becomes 

 of it? Does the organism commonly come 

 to the surface on attacked stems? or. 

 fruits? Make permanent stained prepara- 

 tions showing relation of the organism to 

 the various tissues. Contrast with No. I. 



Variability. How long does an at- 

 tacked shoot live? Using a susceptible 

 variety, study effect on rapidity of blight 

 of: a, rapid vs. slow growth, which may 

 be correlated with time of year (May- 

 June vs. August-September) ; b, amount of 

 rainfall or water given; c, moderate vs. high 

 temperatures; d, light vs. heavy inocula- 

 tions, e, kind of cultivation and manuring. 

 Rich soils and alkali soils are said to favor 

 the development of the disease (O'Gara). 



If you have opportunity to study 

 blighting orchards look for varietal differ- 

 ences. Some commonly cultivated pears 

 are nearly immune, e.g., the Douglas, a 

 seedling of the Kieffer (V. B. Stewart) ; 

 others are very susceptible. The same is 

 true of apples. Make inoculations on re- 

 sistant and susceptible varieties and record 

 the results. 



FIG. 296. Shoot of 

 "'Blight-proof" pear (hy- 

 brid between the Chinese 

 Sand pear and some com- 

 mon pear). Inoculated 

 5 days with a pure cul- 

 ture of Bacillus nnujlor- 

 orus plated in 1915 from 

 a blighted apple branch. 

 Bark blighted (browned) 

 and beads of bacterial 

 slime oozing from the in- 

 terior through stomata. 

 May, 1915. This variety 

 blighted rather freely as 

 the result of needle prick 

 inoculations. 



