I04 



IRISH GARDENING 



Jll.Y 



removed al once tor tear the others inii^lii 

 beeome iiiteeted trom its spores. Tlie next plant 

 to become afteeted was one tjrow n from a liealthv 

 set, in wliieh ease, k.^{ course, tliere can be no 

 question ol" mycelial inlection direct troni the 

 parent tuber. This plant was also lenuned al 

 once tor tear ol' contaminating; tlic others, but 

 witliout avail. The blij^ht spread trom one to 

 another indiscriminately, aui-l in less than a 

 t'ortnii,^ht all the plants but two had shown sij^ns 

 o[ blii,>-ht and been removed, these two beini; 

 the produce, one of a diseased antl the other ol 

 a healthy tuber. These plants were lelt in the 

 cool house tor a further period ol about live 

 weeks, and at the end of that time they were 

 still absolutely free from riiytophthora. Thex 

 were then transferred to the warm, damp house, 

 covered with bell-jars, and left there for just 

 four weeks. A tiioroui^ii examination o( the 

 foliai,^e was finally made, but no trace o\' blij^ht 

 could be found. 



The results obtained in this experiment were 

 quite dilTerent to Massee's, and Massee's hypo- 

 thesis will not account for them. On the con- 

 trary, the hypothesis which accounts for these, 

 accounts for Massee's equally well and better 

 than does his own. 



It is admitted that tlic m\celium o( I'iiytoph- 

 thora may grow direct throuj,'-h the stem from 

 the set to the foliag^e, bu^ 07ily in very yotmg 

 plants. This, however, does not involve any 

 period of dormancy on the part of the fungus, 

 and the stem is killed as it is traversed by the 

 blight from below upwards. It may be assumed, 

 in the absence of more complete knowledge, 

 that this is an important means by which the 

 fungus establishes itself in the new crop every 

 year. It is conceivable that "spores" pro- 

 duced on such plants early in the season infect 

 isolated spots of the general crop, particularly 

 the lower leaves, which are less subject to dry- 

 ing. These spots are, of course, then over- 

 looked on a superficial examination of the crop ; 

 but with the advent of trained observers it is 

 becoming increasingly clear that little patches 

 of blight may be found here and there in a 

 cj-op long before the farmer announces that the 

 blight has " struck." 



This view is strengthened by the fact that in 

 Dr. Pethybridge's experiment in the warm house 

 the fungus in no case grew up through the 

 stems, with the result that there was no blight. 

 In the cool house, on the other hand, one plant 



succumbed in this wa\, and no ilouin spoies 

 were produced. An intcival. ol Which unlortu- 

 nately we do not know the length, then elapsed 

 beloie the next plant became attacked, so that 

 it is impossible to tell whether or not the 

 "spoies"' weie probably produced from the 

 original iliscased plant or from another source. 

 Hut this is not vitally important, for the plant 

 must have become infected from spores, no 

 matter where produced, because it grew from a 

 healthy set. Although this plant was removed 

 at once tile blight now set in in earnest just 

 like a held epidemic in miniature, seven of the 

 eight remaining healthy plants and four of the 

 live remaining ones which gievv from blighted 

 tubers becoming attacked within twelve days. 

 The former seven plants must have become 

 infected bv means of " spores " since they 

 grew from healthy tubers ; and it is extremely 

 improbable that the latter four contracted the 

 disease from mycelium derived from the sets, 

 because, in the first place, the mycelium had 

 failed to develop in them during the three 

 months which had elapsed since planting, 

 whereas it had grown very much more rapidly 

 in the one plant in the same house, which had 

 undoubtedly become directly infected ; and, in 

 the second place, the only plant grown from a 

 diseased set which failed to take the blight in 

 the cool house also failed to reveal any latent 

 mycelium when given the postulated conditions 

 of moisture and warmth during a period of four 

 weeks. Consequently, in this case also infection 

 was probably by "spores." 



The conclusion that one is inevitably driven 

 to from a consideration of this experiment is 

 that the ordinary attacks of blight which occur 

 in the fields in June, July, and August are prac- 

 tically entirely due to "spores." It is equally 

 clear that isolated plants are killed much earlier 

 in the year, possibly in April and May, by a 

 direct invasion of mycelium from the set to 

 the shoot. How far the former attack depends 

 on the latter, if it does at all, it is yet premature 

 to say. 



Massee's experiment appears to point to a 

 perfectly similar conclusion. In the absence of 

 details it is not possible to say how the three 

 plants in the hot-house became infected, but 

 we may assume, as the experimenter did, that 

 all three were attacked directly from the sets, 

 in which case they would correspond to the 

 plants in the field which show the presence of 



