Some Fungous and Bacterial Diseases of tKe Potato 



Douglas "Weir, B.S.A., Biological Department, Macdonald College 



tt^t' I ^ H E potato crop of the past 

 I^K I few years has been very unsatis- 

 I^B factory in Canada. The mischief 



l^Bcaused by certain fungi and bacterial 

 1^1 diseases seems to have increased, sup- 

 l^f plementing the already extensive rav- 

 I ages of the Colorado potato bettle. In 

 Ontario during the season of 1906, the 

 losses caused bv diseases of this kind 

 were so marked, that it was impossible, 

 in certain districts, to supply even the 

 local markets, and potatoes had to be 

 imported. 



It may be useful to describe briefly 

 some of the pathological aspects of 

 these sources of injury to the potato, as 

 the gravity of the evil sufficiently war- 

 rants every rational effort to mitigate 

 it. We subjoin some notes on the early 

 blight, the late blight, the potato scab, 

 and the fungous and bacterial wet rots. 



THE EARLY BLIGHT 



The Potato Leaf Blight or Early 

 Blight, Alternaria solani, was especially 

 widespread and destructive. The dis- 

 ease attacked the leaves and green shoots 

 of the plant, spreading rapidly and 

 checking the growth of the tubers. The 

 first indication of the disease was the 

 appearance of brownish spots on the 

 leaves about the time of blossoming, 

 resulting in the characteristic curling 

 and withering of these parts, in the de- 

 struction of the stem, and later, of the 

 young tubers themselves. 



It will be readily appreciated that an 

 arly destruction of the leaves must of 

 necessity result in greatly diminishing 

 the size of the tubers, for in the absence 

 of leaves the highly elaborated plant 

 food so essential to growth cannot be 

 obtained. 



Although the early bhght is most fre- 

 quently observed about the time of 

 blossoming, it may also attack plants at 

 an earlier stage, and not uncommonly 

 has been observed on plants scarcely six 

 inches high. 



The common flea-beetle is believed to 

 distribute this fungus extensivelv. It 

 establishes excellent places for infection, 

 jby burrowing holes in the leaves, and 



ay even carry the spores along with 

 ,t from diseased plants. The remedial 

 iineasures are the same as suggested for 

 " te blight. 



THE LATE BLIGHT 



The Late Blight or Fungus Rot, Phy- 

 fiuphthora injestans, has caused even 

 : greater loss than the early blight. Moist 

 iwarm weather is especially favorable to 

 [its development, and under such con- 

 fditions it spreads with singular rapidity. 

 [During the warmer, humid weather of 

 [July, 1906, which prevailed in many 

 [sections, the disease spread so suddenly 

 [that fields, appearing healthy and green 



I 



one day, became withered and blackened 

 the next. 



The infected areas show, in the early 

 stages, a well-defined limiting line, but 

 during such weather as just described, 

 these soon extend over the whole leaf, 

 becoming soft and emitting a very un- 

 pleasant odor. 



GENERAL MORPHOLOGY 



If we examine the brownish spots 

 originating on the tmder side of the 

 leaves with a hand -lens, we find them 

 composed of many delicate white branch- 

 ing threads, which protude through the 

 siomata or breathing pores, and produce 

 (conidia) spores. These spores are 



Late Blight— Final Stage 



Note the scorched-like appearance. (From the New 

 York Experiment Station. Geneva, Bnlletin 241.) 



somewhat egg-shaped, colorless, and are 

 blown about by the wind and washed 

 by the rain, until they fall on neighbor- 

 ing leaves or are washed through the 

 soil to the tubers. After falling on a 

 moist leaf or being washed by rain to a 

 young tuber, the spore gives rise to a 

 number of minute swimming spores 

 (zoospores), which move actively about. 

 These eventually settle down and emit 

 slender germ-tubes, which may enter the 

 leaf through a stoma or directly pene- 

 trate the epidermis. Once within the 

 plant, the fungus develops rapidly and 

 sends its branching root-threads (my- 

 celium) in every direction. 



AN EXPERIMENT DESCRIBED 



Many experiments are to-dav being 

 carried on by plant pathologists in the 

 endeavor to obtain accurate data as to 



the life-cycle of the late blight, so that 

 remedial measures may be taken when 

 they are most applicable and effective. 

 Prof. George Massee describes one of 

 special economic interest which was 

 conducted at Kew. In this experiment 

 three potatoes showing the brownish 

 stains so characteristic of the late blight, 

 were cut in half and planted in pots. 

 Three of the pots were placed in a hot- 

 house at a temperature of about 70° F., 

 where the humidity occasionally reached 

 saturation point, and the remaining 

 three put in a room having no artificial 

 heat and where the air was kept as dry 

 as possible. Equal quantities of water 

 were supplied in each case. The results 

 were, in brief, as follows: 



DISCUSSION OF RESULTS 



The plants grown under- the warm, 

 moist conditions developed rapidly, but 

 the fungus also developed, appearing 

 when the plants were six weeks old, and 

 by the end of the succeeding two weeks 

 the three plants were completely black- 

 ened and killed. On the other hand, 

 those grown under dry conditions did 

 not develop as rapidly as the others, 

 but were apparently free from the fun- 

 gus; when, however, they were removed 

 to the warm, moist surroundings the 

 dormant mycelium developed so ex- 

 ceptionally, that within a fortnight these 

 also were killed. 



CONCLUSIONS 



From this experiment we may con 

 elude that epidemics of potato blight 

 are influenced largelj' by weather con- 

 ditions, and the potato grower is advised 

 to take as much care in selecting his 

 seed potatoes, by obtaining them from 

 districts free from blight, as he now 

 commonly does in selecting his peas to 

 insure freedom from the much dreaded 

 pea-weevil. It should also be remem- 

 bered that infection by the early and 

 late blights is due, not only to the hy- 

 bernating mycelium, but also to the 

 spores produced on the leaves. 



(To be continued in next issue.) 



DlacK Rot of the Tomato 



W. T. Macoun, Horticulturist, Ottawa 



This disease did much damage to 

 tomatoes in some parts of Canada in 

 1906, a large percentage of the fruit 

 being rendered useless in some planta- 

 tions. When the disease begins to 

 spread on the fruit, small, roundish 

 spots may be seen usually towards the 

 ^blossom end. These rapidly increase in 

 size, and the tomato becomes discolored 

 and rotten at the parts affected. The 

 spores are given off from dark mould- 

 like masses on the surface of the fruit, 

 and these being scattered re-infect the 



