JUI.V 



IRISH GARDENING 



103 



given is probably correct in this case, particu- 

 larly as the plants were covered with bell-jars, 

 and therefore the risk of infection by aerial 

 " spores " was somewhat reduced. At the same 

 time the experiment does not prove that infec- 

 tion took place through the stem by means of 

 mycelium. If plants grown from healthy tubers 

 had remained healthy in the same house, then 

 the proof, in so far as such a small experiment 

 can prove anything, would be much inore com- 

 plete. It is not quite clear if it is necessary to 

 assume in this case that the fungus mycelium 

 lay dormant during the six weeks which elapsed 

 between planting and the first visible appear- 

 ance of blight. According to Massee's theory, 

 dormancy lasts onl}^ until the atmospheric con- 

 ditions become favourable, whereas there were 

 six weeks of favourable "weather" before 

 Phytophthora made its appearance externally 

 in this experiment. If the mycelium was not 

 dormant it must have been growing exceedingly 

 slow through an uninjured stem. Such a 

 phenomenon in the case of Phytophthora in- 

 festans is unknown and quite unlikely, and until 

 it has been proved we may be allowed to dis- 

 believe its existence. Consequently we have to 

 fall back on the assumption that the mycelium 

 lay dormant most of the time in spite of the 

 extremely favourable atmospheric conditions 

 during the whole period. One of the advan- 

 tages of the theory of direct mycelial infection 

 over that of aerial infection by means of spores 

 is, according to Massee, that it accounts more 

 easily for the outbreaks of severe and simul- 

 taneous attacks over large areas, with the result 

 that whole fields of potatoes, " under certain 

 climatic conditions," are reduced to a blackened, 

 fietid condition within twenty-four hours. If 

 the necessary climatic conditions mean six 

 weeks, or even three weeks, of weather similar 

 to that of the experiment it is only in abnormal 

 seasons that we would have such visitations as 

 that just described. 



To return to the experiment. The three plants 

 grown in the cool, dry house remained healthy 

 as long as they were left there. This is ac- 

 counted for, according to Massee, by assuming 

 that the mycelium remained dormant in the 

 parent set (?) owing to the lower temperature 

 and comparative absence of moisture in the air. 

 When, however, two of the plants were re- 

 moved to the hot, damp house — one eight and 

 the other ten weeks after planting — they suc- 



cumbed to Phytophthora in seven and nine 

 days respectively. This, we are asked to be- 

 lieve, was due to the dormant mycelium spring- 

 ing into activity immediately it was placed in a 

 favourable environment, and rapidly growing 

 from the set to the foliage. Why it should 

 take only a week to destroy a vigorous plant 

 and eight weeks to kill one that was " attenu- 

 ated and weak " is not very clear. If we 

 assume, on the other hand, that these two 

 plants were infected by "spores" from the 

 three plants which had just died of the disease 

 in the same house we have a complete explana- 

 tion of the more sudden and virulent attack. 

 Such an assumption may be borne out by ex- 

 periment, as will be shown in the further course 

 of the paper. 



A further experiment, similar in all essentials 

 to Massee's, was carried out by Dr. Pethybridge 

 in Ireland,* with the addition, however, of an 

 equal number of pots containing sound sets. 

 Thus, in the warm house there were six pots of 

 sterilised soil, three being planted each with 

 half a blighted tuber and three with half a 

 healthy tuber. In the cool house there were 

 six similarly treated pots planted with the other 

 halves of these tubers, three healthy and three 

 blighted. In this house there were also twelve 

 pots of unsterilised soil, six planted with healthy 

 sets and six with blighted ones ; thus making 

 a total in the cool house of nine pots of healthy 

 seed and nine of sets diseased with Phytoph- 

 thora. The sterilisation or otherwise oi the soil 

 does not seem to affect the results. 



One of the diseased sets in the warm house 

 failed to produce a plant ; the other two pro- 

 duced plants which remained free from blight 

 till the end of the experiment, a period of five 

 months. The three control plants grown from 

 sound tubers also remained healthy. In the 

 cool house things went differently. All the nine 

 healthy sets produced healthy plants ; but only 

 six plants grew from the nine diseased sets, the 

 remaining tubers rotting in the soil. Of these 

 six one quickly became blighted when only two 

 or three inches high, the attack spreading from 

 below upwards, and doubtless originating 

 directly from the parent set. The plant was 



* ■• Cousideralions and ExperiineiUs 011 llu- supposed 

 Infection of the Potato Crop with the Blight Fungus 

 (Phytophthora infestaus) by means of Mycelium, derived 

 directly from the Planted Tubers."— Scientific Proceed- 

 ings of the Royal Dublin Society. 



