820 



THE GARDENERS' CHRONICLE OF AMERICA. 



with scientific workers. The result is that we are now 

 developing spraying mixtures which answer well to cer- 

 tain particular diseases. Bordeaux used to be recom- 

 mended as a panacea for all plant diseases. Practical 

 observations, together with recent studies, have shown 

 that Bordeaux cannot be used in every case. It produces 

 spray injury to plants of delicate foliage and tender fruit. 

 There are several other fungicides which may take the 

 place of Bordeaux. 



A spraying mixture besides its fungicidal value must 

 also possess sticking or adhering qualities, otherwise it is 

 of little use, particularly in cases of plants with w'axy 

 coverings on the leaves, stems, or fruits. Moreover, 

 spraying to be effective must be done very thoroughly. 

 Every part of the plant, including also the lower part of 

 the leaves, must be coated with the solution. Last, but 

 not least, for spraying to be effective, we must know 

 what we are spraying for and also something of the na- 

 ture of the parasite which we wish to combat. In this 

 respect many careful growers fail and then the chemicals 

 or the manufacturers are often blamed. To make this 

 point clear I shall illustrate it by a specific example. 



Most of us are no doubt familiar with the apple disease 

 known as scab. This fungus disease injures both the 

 leaf and the fruit. The parasite winters over on fallen 

 leaves. In the early spring, the spores are formed during 

 damp or rainy periods on the old leaves on the ground 

 and carried by the wind to the dormant buds or opening 

 foliage or young fruit. Apple scab may be effectively 

 controlled by from two to three sprayings. The first ap- 

 plication is known as the dormant spray and is given just 

 before the buds open. The second spraying is applied 

 just after the petals fall. In case of continuous wet 

 weather, a third spraying is often beneficial when applied 

 about three weeks later. These facts have been proven 

 time and again. The orchardist who follow's these direc- 

 tions and sprays thoroughly is sure to control scab. Yet 

 we know of cases where spraying has failed. Upon in- 

 quiry it was found that directions were not carried out 

 and the applications were not given in proper time. If. 

 for instance, the first spraying is done too early, the scab 

 spores having not matured, would not be destroyed. In 

 case it is given after the fruits have set, most of the in- 

 fection W'ould have taken place. In either case spraying 

 is of no avail. 



Spraying for combating insect pests is even a more 

 complex proposition than spraying for fungi. Here the 

 l^arasite is not always easily reached. In these pests we 

 have to know whether they are chewing insects like the 

 Colorado potato beetle for instance or a sucking insect 

 like the green aphis. In dealing with biting insects, the 

 proposition is to coat the parts of the plants with a 

 ]'oison such as Paris green. The insect while feeding 

 on tiie plant also takes some of the poison, which kills it. 

 In the case of sucking insects, which insert their beaks 

 in the tissue of the plant to feed on the juice, volatile oils 

 are applied in a dilute form. These sprays are called 

 contact sprays, and they actually destroy life by bodily 

 injury or blocking respiration. A third class of sprays 

 are known as repellents. These are substances with pe- 

 culiar odors which when placed around or on a plant is 

 offensive to the approaching insect. 



This is another form of treatment against plant dis- 

 eases. Seeds are a common source of disease carriers. 

 I 'ere the disease may be carried in the form of adhering 

 sjjores or the surface coat, or it may be carried as hiber- 

 nating fungus threads in the interior of the seed. In the 

 first case all seeds including healthv ones mav carrv ad- 

 hering spores on their surface coat. Treatment of all 

 seeds before planting becomes a necessity. To kill all 

 adhering spores the seeds may be exposed to the fumes 



of formalin. This treatment is applied to seed potato 

 (tubers) for scab or to grains attacked with stinking 

 smut. Soaking the seed in a solution of liquid formalin 

 or of corrosive sublimate will answer the same purpose. 

 Care should be taken that the liquids used be not too 

 concentrated, or the treatment be extended too long. 

 Where this is overlooked the seeds may be injured. Th§ 

 treatment of seeds for fungus parasites which are car- 

 ried internally is a more difficult task and in some cases 

 impossible. Poisons cannot easily reach the threads of 

 a fungus which had once entered the interior of the seed. 

 The loose smut of wheat, the late blight and dry rot of 

 the potato, the anthracnose of the bean and the sweet pea 

 for instance belong to this group. The possible means 

 of control is heat. After soaking for some time the 

 seeds are plunged in hot w-ater the temperature of which 

 is sufficient to kill the fungus in the interior without 

 impairing the germinating power of the seed. This 

 method applies well to the treatment of wheat, oats and 

 barley against loose smut. The main difficulty, how- 

 ever, against the general use of this treatment is that the 

 degree of heat necessary to kill the invading fungus will 

 somewhat injure the germinating power of that seed. 



Soils often become infected with parasitic fungi or 

 bacteria. This is usually caused by growing the same 

 crop too continuous on the same land. Soils may also 

 become infested through accidental agencies such as dis- 

 eased seeds, diseased plants, infected manure, the wind 

 carrying spores with the dirt, drainage, water, hoofs of 

 animals or even man. There is nothing more discour- 

 aging to the grower than a sick soil. Failure then will 

 be certain to face him sooner or later. Soil treatment is 

 both a preventive and a cure. The object in treating 

 sick soils is, of course, to kill the parasites present therein. 

 This may be accomplished in two ways ; first, to treat the 

 infected area with formalin, and, second, to subject the 

 soil to a high steam pressure. The latter method is by 

 far the best as it kills the eggs of the eel worm as well 

 as those of harmful insects which are unhurt by the for- 

 ■ malin. Both of these methods, however, are only prac- 

 tical on small scales such as under greenhouse conditions 

 or in seed beds. Their use on large areas is prohibited 

 because of their cost. Generally speaking, large fields 

 infested with disease producing organisms such as the 

 \ellows of cabbage, the stem blight of the Irish potato, 

 etc., cannot be treated with the above two mentioned 

 methods because of the prohibitive cost. On the other 

 hand, certain sick fields may be treated on a large scale 

 by the use of certain fertilizers. For instance, fields in- 

 fected with the club root of the cabbage may be treated 

 by the application of lime. The scab of the white potato 

 in the land may be controlled by rendering the soil slight- 

 ly acid, a condition which acts injuriously to the fungus. 

 This may be accomplished by heavy applications of acid 

 phosphates or by turning under a green cover crop such 

 as clover or rye. The humus in this case naturally tends 

 to make the soil somewhat acid. 



It is not the purpose here to point out the general ad- 

 vantages of rotation, but only as a means of plant disease 

 control. If a soil becomes sick to a certain crop, rota- 

 tion is indispensable, for if we persist in growing that 

 same crop year after year, w-e simply reach a stage where 

 the soil will refuse to grow that crop any longer. In 

 rotating our crops we grow for a time such hosts as the 

 fungus cannot attack, hence the parasite is starved out 

 and it gradually disappears. In rotation, as with spray- 

 ing, success depends on a knowledge of the parasite with 

 which we deal. .Some disease producing fungi, such as 

 the yellows of cabbage, will persist many years in the 

 soil without its particular host. 



There is no denving but that nature's law is the strug- 



