For June, 1920 



221 



effectiveness, and to the fact that it can be prepared hi the form 

 of a paste. It has recently been suggested that hme arsenate is 

 quite as effective, while being cheaper. 



When an insect feeds by sucking the juices from a plant in- 

 stead of biting it. it is clearly impossible to poison it through 

 the stomach. A considerable number of contact insecticides e.xist 

 and there is room for much work on the precise nature of 

 their action, but they all agree in acting on the insect either 

 through the skin or by stopping up the breathing tubes. These 

 sucking insects do not breathe through their mouths for ihey 

 have none, but through tubes opening in various parts of their 

 bodies. 



The question naturally arises as to what are the essentials 

 for an effective contact insecticide. The very name supplies 

 the answer. Firstly it must make contact and, secondly, it must 

 be msect killing. By making contact it is meant that the insec- 

 ticide must wet the insect and have the power of penetrating 

 the skin, which is frequently woolly. Water will not do this. 

 In some instances water is used for washing off by physical 

 force insects from plants, such as, for example, red spider, but 

 this alleviates the trouble only temporarily as the insects are 

 not killed. To be effective, when ?. drop of contact insecticide 

 falls upon an insect, it must have the power of spreading over 

 it and penetrating its woolly covering. To bring this about 

 various forms of emulsions are used together with nicotine. 

 Nicotine is practically an ideal insecticide since it acts both as 

 a contact and as a stomach poison, but unfortunately it is ex- 

 pensive. 



It does not appear necessary to .go into the question of the 

 manufacture of insecticides as there are proprietary spray fluids, 

 together with the ordinary compounds, which have stood the 

 test of time and which cause it to be not worth while to mix 

 one's own, which in any case is an unsatisfactory procedure, 

 especially in small quantities. There is, however, considerable 

 room for chemical research in the direction of producing simple 

 >ubstance5 having a contact insecticide value. 



.■\s previously mentioned, it is important to attack sucking in- 

 sects at the most vulnerablp period of their lives, which period 

 in connection with outdoor plants is invariably just as the foli- 

 age begins to appear, or in the case of those which flower before 

 the foliage comes out, just as the flowers commence to open. 

 Further, as not only do the eggs of insects hatch out over a 

 somewhat extended period, and also as, in the case of aphis, 

 viparious reproduction goes on more or less continually for a 

 certain period, several sprayings at intervals of a few days are 

 necessary. In connection with the various species of plants the 

 most vulnerable period is not at the same time, but by keeping 

 watch for their pests' first appearance and dealing with them 

 at once they are more easily kept under control, and bad and 

 more damaging attacks are avoided. For greenhouse work fre- 

 quent sprayings are necessary, and by this means even white fly 

 can be exterminated and the dangerous cyanide fumigation 

 avoided. To be eft'ective, especially in the latter connection, the 

 spraying must be thoroughly done and all the underneath parts 

 of leaves covered. While in the case of all plant houses, fumi- 

 gation by burning tobacco in soine form may be effective by 

 reason of the fact that the plants will be entirely surrounded 

 li\- the fumes, dift'erent species of plants vary considerably in 

 the amount of tobacco smoke they will withstand, and it is as 

 easy to kill plants by this method as it is insects. 



The control of parasites belonging to the fungus order stands 

 upon a totally dift'erent fooling to that of insects. In the latter 

 case we can only bring direct treatment to bear after they ap- 

 pear, but in connection with fungi, efforts to successful 

 control tniist lie directed to prevention because there is prac- 

 tically no cure for any fungous disease, that is to say, so far 

 as any part of a plant affected by the disease is concerned. 



Fungi are propagated by means of spores, which are for the 

 most part individually invisible, and which float about in the 

 atmosiihere and are wafted from one district to another by 

 means of .Tir currents. When a spore falls upon a leaf or 

 olher part of a i)lant, it will under suitable conditions germinate 

 and the tube or root l)rought into existence by tlie act of 

 germination will penetrate througii the outer covering of some 

 part of the plant, generally the leaf, or through a wound. .-\s 

 soon as the germinating tube enters the plant it commences to 

 feed upon and spread through the plant tissue, in which stage it 

 is known as mycelium and its effects are invisible to the naked 

 I ye outwardly. When we see the effect of this growth in the 

 forms of rust, black spot, etc.. the harm has already been accom- 

 plished ; the mycelium of the fungus has destroyed that part 

 of the plant ni which it lived and upon which the outward 

 manifestations of it appear, and the latter is really the fruiting 

 stage of the fungus. This fruiting stage upon a large scale is 

 seen in the case of tlie edible mushroom and other fungi of 

 like nature. Their mycelium or spawn must first live and grow 

 in a suitable medium and the mushrooms or toadstools are merely 

 Ihe fruiting sta.ges. 



It is therefore clear that, as above mentioned, our efforts must 

 be mainly along the lines of preventing the germination of the 

 fungus spores in the first instance ; only so far as we can kill 

 spores by spraying is the use of fungicide after manifestation 

 any use. and also of course, as far as the latter operation will 

 prevent any further .germination. 



The underlying principle of spraying against fungi is to look 

 upon it as insurance. .Spray before there is any appearance of 

 trouble and keep it up at frequent intervals. Fungi generally 

 delight in wet cool weather and they give much less trouble 

 in seasons which are dry and warm ; it is therefore necessary 

 to spray with greater frequency in seasons of the former char- 

 acter. 



Some species of fungi produce spores of more than one char- 

 acter, and in most cases there is a special resting spore which 

 will remain dormant through the' Winter and there is no doubt 

 that spores in general are capable of a very extended period 

 of dormant life, awaiting suitable conditions and the presence 

 of their host to commence their activities. 



Many, too, have more than one host, sometimes using one in 

 the Siiminer and another in the Winter, and if one of these hosts 

 is entirely absent they cannot exist at all. In this connection 

 the extermination of all species of Ribcs would exterminate 

 the White Pine Blister Rust. 



Spores of many species are undoubtedly disseminated by seeds 

 and we believe that it will becoine part of the ordinary routine 

 to treat seeds previously to sowing with some preparation which 

 will kill any fungus spores adhering to them. 



Then, too, fungus spores undoubtedly remain in the soil on 

 or near which diseased plants have been growing. In relation 

 to this, sterilization ot the soil where practicable is of great 

 value, as is also the use of a fungicide upon the soil before 

 the plants commence to grow. This latter practice has, for 

 instance, been found to be of great use in connection with Holly- 

 hock rust. Then again many fungi work entirely in the soil 

 and cause disease in some part of the underground plant struc- 

 tures, as for example, that causing club-root in members of the 

 cabbage family always lives in the soil, and may be controlled 

 by heavy dressings of lime to affected soil. 



In cases of the latter character, and in more or less all others 

 a rotation of crops assists materially and considerably in pre- 

 vention. 



As a general rule, weak and unthrifty plants are the first to 

 be attacked by disease, a fact which can be frequently noticed, 

 especially in connection with the peach leaf-curl caused by a 

 fungus known as E.roasus dcforinans. For this, the usual con- 

 trol method is spraying just before the leaves appear. The 

 effect of this disease is to destroy the peach leaves and there- 

 fore to destroy a part of the tree's digestive organs thereliy re- 

 ducing growth and rendering the fruit more or less u.seless. 

 But where peach trees are growing under the best conditions and 

 are making luxuriant growth the effect of the leaf-curl fungus 

 is seen to lie almost nil. In practically all cases a healthy plant 

 supplied with a properly balanced food will to a considerable 

 extent resist disease. We must emphasize the necessity for 

 properly balanced feeding as a factor in healthy growth and fn 

 disease resistance ; the reverse of this, as for instance, supply- 

 ing plants with an excessive aiTKuuit of available nitrogen in a 

 chemical form, increases their susceptibility, esjiecially in wet 

 seasons, to fungous diseases. This has been particularly noticed in 

 the rust of cereals and in the mildew of roses. 



.^ direct way of minimizing the losses caused by fungi is to 

 select and cultivate disease-resistant plants. It is very frequently 

 seen than when many varieties of any plant are grown under 

 the same conditions they show considerable dift'erence in their 

 capacity to resist di.sease ; a few may even be found which are 

 practically immune. Unfortunatclv immuneness to any particu- 

 lar disease is not necessarily associated w'ith other characteristics 

 equally desirable from the point of view of the cultivator. For 

 example, sonic varie'ies of potatoes are known ts possess con- 

 siderable cajiacity to withstand the blight known as Phytophthora 

 mfcxtans. but they are little grown on account of the bad table 

 qualities of their tuijers : and. conversely, some producing the 

 most desirable tutiers are very susceptible to this disease. 



The question here ari.ses as to whether immunity to disease, 

 and its converse susceptibility, are heritable characters. If so. 

 it would seem possible by cross-breeding to associate the valu- 

 able feature of immunity with other characteristics the culti- 

 vator desires. A\'liile some work along the lines of selection 

 and breeding has lieen done by seed .growers and others, and 

 which has resulted in new varieties less susceptible than older 

 ones, more precise knowledge is necessary with regard to the 

 mode of inheritance of this feature before really scientific data 

 can ht obtained. There must be a definite reason why certain 

 plants remain immune when every chance of infection is pres- 

 ent. The selection of and breeding from immune specimens is. 

 while valuable, a "rulc-of-thumb" method. The "why" is the 

 question to be settled, and to tliis end a better knowledge of 



