THE MEANS OF CONTROLLING PLANT DISEASES 



49 



ance of the spring crop, this result being brought 

 about by rust which early developed on the winter 

 crop and fell on the immature spring crop. For 

 similar reasons, mixtures of varieties should be 

 avoided when possible. This is especially true of 

 cereals, but applies equally to fruit and vegetable 

 culture. When attempting to control crop diseases, 

 it is a matter of the greatest concern that in the 

 crop there should be an evenness of development 

 and maturing. One can often protect plants or 

 crops of the same grade of growth or maturity, 

 but it is difficult to avoid damage when there is no 

 uniformity in these features. 



Saving seed. — ^After purity of variety, there are 

 no features of caring for the seed of greater impor- 

 tance than those which insure proper harvest, curing 

 and storing. Aside from conditions that may caase 

 weakened vitality of the seeds, there are many 

 features of these processes that may introduce or 

 multiply the chances of introducing infectious dis- 

 eases. Each crop and its special diseases must be 

 studied with these points in mind. 



Vitality or initiative growth power in the seed 

 or cion is of great importance. It is of much 

 moment that the growth period from seed-time to 

 maturity shall be as short as possible. This applies 

 especially to annual crops. This initiative seed 

 power can be gained and maintained only by per- 

 sistent seed selection, cleaning and grading. With 

 this point in mind, one selects to secure varieties 

 and individual types which are the least susceptible 

 to disease, cleans them thoroughly to free them 

 from possible disease-bearing parts, and grades 

 them to get rid of diseased seeds, those that are 

 predisposed to disease and those that are not up to 

 the standard of excellence. Note Figs. 72-74. 



Treatment of seed. — Proper seed treatment pre- 

 supposes a proper selection of seed, proper cleaning 

 and grading. Seed thus prepared is then ready for 

 treatment or disinfection. The theory of seed dis- 

 infection rests on the principle that some plant 

 diseases, indeed many, are transmitted by way of 

 the seed either to the soil or to the new plant 

 directly by way of the embryo. 



Fig. 74. Treating seed grain by spraying and shoveling, as 

 practiced on large farms in the Northwest. 



Taking up this feature of the question, it is 

 necessary to consider just what diseases are to be 

 prevented. Some are known to be directly trans- 

 missible by way of the seed, the embryo or germ 



B4 



layers being internally infected as in the case of 

 flax-wilt or anthracnose of beans and loose smut 

 of wheat. By far the greater number of diseases, 

 such as the stinking smut of wheat and onions and 

 numerous diseases of garden vegetables, including 

 potato-scab and potato-rot, however, are easily 

 transmitted to the ground and the new plant 

 because of the presence of external spores, struc- 

 tures that are simply dusted on the seeds, and only 

 await an opportunity to prey on the roots. (Fig. 

 75.) For all such diseases, seed disinfection is an 



Pig. 75. Potato-scab growing on sugar-beets. This illustra. 

 tion is from the original experiment which proved that 

 potato-scab fungus lives from year to year in the ground, 

 and may attack other vegetables besides potatoes. 



easy and direct remedy, and numerous formulas 

 and washes or solutions suited for special diseases 

 have been developed from time to time, among 

 which may be named the following examples :' Cop- 

 per sulfate solution, corrosive sublimate solution, 

 hot water treatment, and the formaldehyde treat- 

 ments. 



Usually the treatment demands that individual 

 seeds shall be subjected thoroughly to the action of 

 the disinfecting medium for a definite period of 

 time. It is well to remember that, as in the case 

 of serving medicine to persons, or administering 

 washes to wounds, only certain strengths are suit- 

 able to particular cases. Therefore, the directions 

 for using must be followed closely if prevention 

 can be reasonably expected, the aim being to pre- 

 vent the disease, and, at the same time, in no way 

 to injure the growth from the seed. It is an inter- 

 esting feature of seed disinfection that, whenever 

 a proper treatment for prevention has been made, 

 the yield may very greatly exceed that from the 

 untreated seed of the same type, even though no 

 particular disease is known to infest the seed. This 

 may be readily accounted for by the fact that dis- 

 infection does away with many unknown or unob- 

 served organisms on the seed that cause trouble to 

 the young plant suificient to be of great detriment 

 to its growth, and yet not sufficient to give results 

 that would ordinarily be characterized as dis- 

 ease. Thus, with seed properly treated by the for- 

 maldehyde method of disinfection, bacteria, yeasts, 

 molds, all types of organisms which readily set up 

 fermentations in the moistened seed, are disposed 

 of, leaving the young plantlet to draw unmolested 

 the full amount of food materials stored in the 

 mother seed. 



The following farm crops are grown with much 

 greater advantage if the seed is first disinfected : 

 Wheat, barley, oats, millet, grass seeds, flax seed 

 and corn. The method of disinfection is now almost 



