Page 14 



BETTER FRUIT 



December 



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tion of tlic so-calleil "liic blight." This 

 i.s true in .seriou.s crown iind root in- 

 fections. The term "collar-rot phase," 

 as used by an occasional writer, is a 

 notabl.v bad one, simply because the 

 germ does not produce a rot. The germ 

 causing pear blight does not belong to 

 the rot-producing group of plant jiara- 

 sites. Even the term "pear blight" is 

 not good; it would be best to make use 

 of the term "pome blight" or, better, 

 "bacterial pome blight." There are cer- 

 tain other diseases of pome fruits 

 which often produce effects somewhat 

 resembling the bacterial blight, and 

 therefore it would be much better, in 

 speaking of blight, to qualify if. If we 

 would use the term "bacterial blight" 

 in the case of pears, apples, quinces 

 and other pome fruits we would not 

 fall into error. 



As stated above, in order to have a 

 general infection or, for that matter, 

 any infection, the blight germ must be 

 present. It has been demonstrated that 

 this germ will live during the dormant 

 season of a tree in the cankers formed 

 by the previous year's infection of 

 limbs, bodies or roots of pome trees. 

 The germ "hobls ovei'" or lives over in 

 no other place. No part of the pome 

 fruit trees may be free from infection. 

 The germ is carried from these centers 

 called "hold overs" by various agencies, 

 principally insects. However, birds 

 and other animals, even man himself, 

 may be distributing agents. A very 

 large number of species of insects and 

 their near relatives have been found to 



be caiTiers of the cantagium vivum, the 

 pear-blight bacillus. Not only flying 

 insects have been foimd to be elfective 

 in spi'eading infection, but also many 

 insects and insect-like species, which 

 do not depend upon flight have been 

 foiiinl lo be particularl.N' dangerous. 

 Wliile the odimn of the fruitgrower has 

 been heaped upon the honej bee, it 

 must be remembered that this beneficial 

 insect is active during a relatively .short 

 period of tiie season, that is, during the 

 period of blooming. Certainly the 

 honey bee spreads infection at this 

 time provided that oozing hold-overs 

 are present in the orchard, or at least 

 in the neighborhood. It has been par- 

 liculaily noted that serious blossom in- 

 fection in pears, especially the early- 

 blooming varieties, rarely occurs. The 

 leason for this is because at this time, 

 when the days are relatively shcnt and 

 moderately cool and the nights rela- 

 tively long and generally cold, the hold- 

 overs are not actively oozing. The im- 

 I)()rtant factor, namely, the presence of 

 active organisms is wanting. Later, 

 when Ihe late-blooming varieties of 

 pears and ap])les are blooming, condi- 

 tions are favorable for the germ and 

 all hold-overs in pome-fruit trees gen- 

 cralh- become active because of the 

 favorable conditions for growth. The 

 number of days in any season when 

 the honey bee is active in the orchard 

 may he known by notin.g the number 

 of days covered by the blooming 

 period. When the blooming season for 

 orchai'd fruits is over, the honev bee 

 will be found to be busy on a different 

 variety of llowering plants. It is well 

 known that bees do not visit more than 

 one group of flowering plants at the 

 same time; that is to say, the bee works 

 wholly upon the apple during its sea- 

 son, then turns to some other plant, 

 and then another and so on. Even the 

 amateur bee-keeper knows that honey 

 of different color, flavor and quality 

 comes from dilTcrent plants. 



It is well known that the greatest 

 amount of infection may occur, and 

 usually does occur, at a time when bees 

 could not have carried the infection. 

 Of course, the primary infection noted 

 in some of the blossoms was probably 

 carried there by the bee, but subse- 

 quent infections of the later blossoms, 

 twigs, shoots and watersprouts are cer- 

 tainly caused by other agencies. Biting 

 and sucking insects are here respon- 

 sible. Aphides are notably bad as car- 

 riers of the pear-blight germ. Upon 

 the e(mtroI of blight, therefore, de- 

 pends in a great measure the control 

 of insects. However, in the first place 

 all sources of infection, namely, the 

 hold-over cankers should be carefully 

 removed. This is a ditlicult thing to do, 

 for the reason that the most careful 

 worker will often miss hold-over 

 blight, especially if the hold-over oc- 

 curs in the body or root system of 

 the tree. 



Resistance and Susceptibility. — We 

 know that there are varieties of pome 

 fruits which are (luite resistant, others 

 that are very susceptible to the pear- 

 blight disease. We often hear of vari- 

 eties being "immune"; but, so far, no 



species of the pome family has been 

 found immune to blight. There are 

 various degrees of resistance, and that 

 is about all that can be said. Very 

 often the attention of the fruitgrower 

 is directed away from resistant vari- 

 eties by the statement that they are so 

 resistant that blight will not hold over 

 in them. However, every fruitgrower 

 knows that our cultivated varieties of 

 pome fruits are not growing on their 

 own roots. For instance, a Bartlett 

 pear is not all Bartlett; a Newtown 

 apple is not all .\ewtown, and the same 

 may be said for any other variety. 

 Until recently no attention was paid by 

 nurserymen to blight-resistant stock 

 upon which to work our commercial 

 varieties. In the main, our commer- 

 cial varieties of pears are all worked 

 on French stock which is very suscep- 

 tible to blight. Our commercial vari- 

 eties of apples are also worked on seed- 

 ling stock which is never selected for 

 its resistance. From this it may he 

 readily seen that, although the variet> 

 top-worked on the stock may be cpiite 

 resistant, the stock or root being sus- 

 ceptible renders the tree unsafe from 

 the blight standpoint. It is often said 

 that a chain is no stronger than its 

 weakest link, and, in the same way, 

 any variety of pome fruit is no more 

 resistant than its least resistant part, 

 and if this part be the root system so 

 much the worse for the tree. If the 

 root system is very susceptible, the tree 

 may be lost, although aijparently there 

 may have been no infection noted 

 above the graft union. I have seen the 

 worst cases of blossom and twig infec- 

 tion in very resistant varieties where 

 not a single hold-ovei- could be found 

 in the orchard itself or in the imme- 

 diate vicinity so far as the examination 

 above the graft union was concei-ned; 

 however, by noting the infection cen- 

 ters, examinations of the growths and 

 roots below the ground showed the 

 presence of hold-over blight in certain 

 trees. In the case of the Xewtown 

 (and I am only using this variety as 

 one example), which is quite resistant 

 under average conditions, I have found 

 the most serious hold-overs in the roots 

 (stocks). Here, then, is an important 

 factor in the control of blight. Not 

 only should the parts above the ground 

 be examined, hut the ci-own and roofs 

 of a tree should he bared and inspected 

 so as to be sure that no hold-over ex- 

 ists there. There is no nnstery about 

 crown and root infection; certain insect 

 agencies work as readily under ground 

 as others do above. Furthermore, in 

 cases of severe blossom, twig or limb 

 infections, these may be carried down 

 to the roots from above by rain. I have 

 been able to demonstrate the presence 

 of blight germs in droplets of rain 

 water trickling down the trunks of in- 

 fected trees. Referring again lo the 

 matter of insect control, it may be said 

 that the persistent use of insecticides, 

 siich as some of the better sulphur com- 

 pounds, arscnicals and nicotine com- 

 pounds, together with sticky bands, will 

 go far to reduce the amount of summer 

 infection. It has been particularly 

 noted in certain districts on the Pacific 



