DISEASES AND INSECTS 



DISEASES AND INSECTS 1027 



membranes are formed which remain for a time sus- 

 pended in the liquid. When sprayed upon the plants 

 the water soon evaporates, leaving a coating of these 

 dried membranes. The active fungicidal principal 

 in these bordeaux membranes is the copper. When 

 leaves or fruit are rewetted by rains enough of the 

 copper in these membranes comes into solution to 

 prevent the germination of the spores of the parasite 

 that may have been deposited thereon. (See under 

 Bordeaux, p. 1028.) 



Bordeaux, however, is sometimes injurious to such 

 plants as peaches, plums and apples, and has, within 

 the last few years, been largely replaced as a summer 

 spray, especially for apples. Lime-sulfur, unlike bor- 

 deaux, is a solution. It is made by boiling together 

 in water, lime and sulfur. A concentrated solution of 

 certain poly-sulfides of calcium, chiefly penta- and tetra- 

 sulfide, is thus obtained which, when properly diluted 

 is applied in the same way as the bordeaux. (For 

 method of preparation, see Lime-sulfur, p. 1028). When 

 this solution dries on the leaves and fruit, it is rapidly 

 converted by the action of the atmosphere into other 

 calcium compounds and free sulfur. The sulfur is in 

 a very finely divided state and is the active principal 

 of lime-sulfur. It becomes oxidized in the presence of 

 moisture probably as sulfuric or sulfurous acid, which 

 prevents the germination of the spores of the pathogen. 

 Flowers of sulfur and sulfur-flour, when very finely 

 ground and applied dry by dusting or sprayed on in 

 suspension in water, alone or with lime-milk (the so- 

 called self-boiled lime-sulfur) are also quite effective 

 against certain diseases. Dusting with sulfur is em- 

 ployed in combating powdery mildews of grapes, 

 hops, roses and the rust of asparagus. 



Lime-sulfur may not be used on potatoes and grapes, 

 as it dwarfs the plants and reduces the yield, while 

 bordeaux has just the opposite effect on these crops. 

 Bordeaux, as already pointed out, is, however, injurious 

 to leaves and fruit of the apple and to the foliage of 

 peaches and certain varieties of plums. It will thus be 

 seen that there is no universal 

 fungicide and also that both the 

 effect on the host and on the 

 parasite must be considered. It 

 is now known for example that 

 while lime-sulfur is very effective 

 against the apple-scab fungus, it 

 has little fungicidal effect on the 

 spores of the bitter-rot pathogen. 



To be effective, fungicides must 

 be applied before the disease ap- 

 pears. As the spores of most 

 parasitic fungi germi- 

 nate during the period 

 of rainy cloudy weather, 

 the fungicide, to be effec- 

 tive, must be applied 

 before and not after such 

 periods. They must not 

 only be thoroughly applied to the sus- 

 ceptible parts but also at the proper 

 stage of growth or development of the 

 plant. To illustrate: the only effective 

 periods for spraying apple trees for 

 scab are: just before the blossoms open 

 (not dormant); just after the petals 

 fall; ten days or two weeks later; and 

 again in late summer just before the 

 late summer rains, to protect the 

 rapidly developing fruit from late 

 infection. 



4. Immunization consists in estab- 

 lishing within the plant itself some 

 condition which renders it immune or 

 resistant to the attacks of the patho- 

 gens. Immune crops may be developed 



by selection and propagation of individuals naturally 

 immune, whose immunity has been evidenced by their 

 coming through an epidemic unscathed. Immune 

 varieties may be crossed with susceptible ones having 

 other especially 

 desirable quali- 

 ties and then 

 by segregation 

 and propagation 

 strains of the 

 crop may be 

 developed com- 

 bining the resist- 

 ance or immu- 

 nity of the one 

 parent with the 

 desirable quali- 

 ties of the other. 

 Some striking 

 results have 

 been obtained in 

 this line of dis- 

 ease control as 

 witness the wilt- 

 resistant cotton, 

 cowpeas and 

 watermelon, the 

 nematode-free 

 Iron cowpea, 

 rust-resistant 

 wheat, barley, 

 and asparagus, 

 and the anthrac- 

 nose-resistant 

 clover. Never- 

 theless, this 

 method of con- 

 trol, while the 

 most ideal, is 

 beset with many 

 difficulties and 

 uncertainties. 

 That pathogens, 

 as well as crops, vary, giving strains capable of attack- 

 ing host plants immune to other strains of the same 

 pathogen, has generally been overlooked by breeders, 

 and doubtless accounts for the frequent failure of sup- 

 posedly resistant varieties when transferred to new 

 localities. The production of artificial immunity by the 

 injection of some substance into the plant or by the 

 application of certain substances (fertilizers, etc.) to the 

 soil is at most only in the preliminary stages of experi- 

 mentation and as yet offers but little of practical value 



1290. A blight of grapes due to some 

 constitutional disorder. Notice that the 

 leaves die first at the edges. ( X H) 



to the grower. 



H. H. WHETZEL. 

 Fungicides. 



1289. Strawberry leaf 

 rolled up from the attack 

 of the mildew. 



A fungicide is any material 

 or substance that kills fungi or 

 their spores. The word is used 

 particularly for those substan- 

 ces employed in the warfare against parasitic 

 fungi. 



A satisfactory fungicide must be one that does 

 not injure the plants and at the same time is 

 effective against the parasite. For spraying, 

 additional requirements are imposed: it should 

 not dissolve readily in rain-water; it should 

 adhere to foliage and fruit; in some cases it 

 should be colorless in order not to make orna- 

 mentals more unsightly than when diseased. 

 The fungicide which has been used most for 

 general purposes is bordeaux mixture. Lately 

 some other preparations, particularly lime-sulfur 

 combinations, have come into use, and in many 

 cases are supplanting bordeaux. There are in 



