POTATO DISEASES 



1717 



insects, some thought it due to atmos- 

 pheric influence or electricity, others said 

 wet weather, and some were right in 

 ascribing it to fungi. Many farmers still 

 believe wet seasons to be the cause of 

 this disease. The wet warm season does 

 not cause the blight, but these conditions 

 are essential to the growth and dissemina- 

 tion of the causal fungus. 



The scientific name for this causal fun- 

 gus is Phytophthora infestans. It has 

 been the subject of a great deal of in- 

 vestigation, and much literature regard- 

 ing it has been published from time to 

 time. The group to which Phytophthora 

 belongs shows a great deal of variation. 

 Other more or less common forms which 

 are closely related are the mildew of 

 onions and the one frequently occurring 

 on lettuce. 



fue 



Fig. 2. Early Stage of the Blight on Tuber. 



It has been shown by several investiga- 

 tors that the fungus seldom, if ever, runs 

 back from the blighting foliage to the 

 tubers through the stem, but that the 



spores carry it to the exposed tubers and 

 those near the surface. This same fungus 

 causes a blight of tomatoes (Fig. 3). It 

 is favored somewhat on the tomato by the 

 fact that the fruit is not only easily in- 

 fected, but also furnishes a great abun- 

 dance of moisture and food, so that great 

 numbers of spores can be produced. 

 Leaves and stems are also affected. 



Treatment 



It has been very clearly demonstrated 

 that late blight and the rot of tubers 

 caused by the same fungus can be pre- 

 vented by proper spraying. Several of 

 the Eastern states have carried on spray- 

 ing experiments for a number of years, 

 and in every case the results show a 

 marked advantage. 



It has been demonstrated that the vigor 

 of the plant is held up in this way and 

 an increased yield results, even when no 

 blight appears. 



The only satisfactory spray to use for 

 late Dlight is Bordeaux mixture. Several 

 applications during the season will be 

 necessary, and the time for spraying will 

 depend on a number of factors, namely, 

 time of planting, weather conditions, and 

 other pests. If the spray is to be put on 

 for blight alone it will be safe to put on 

 the first application when the blossoms 

 of the late plantings are well out, unless 

 rainy weather sets in earlier, in which 

 case spray as soon as possible after the 

 rain. Follow this with later applications 

 every two weeks. The first application 

 should be with 44-50 Bordeaux mixture, 

 and the later with 6-6-50. In the case 

 of an epidemic of blight it is necessary to 

 spray oftener, once every week or ten 

 days being advisable. 



Other pests, especially flea beetles, re- 

 quire an earlier application. 



There is no practical method of seed 

 treatment for this trouble. The fungus 

 is carried over in the seed but under 

 proper storage remains dormant and often 

 cannot be detected. Seed should be care- 

 fully sorted so that no decaying or dis- 

 colored tubers are used in order to se- 

 cure good germination and later depend 

 on proper spraying to prevent the infec- 

 tion from outside sources. 



