50 DISEASES OF CROP-PLANTS 



Apparent recovery may take place only in plants not fully 

 susceptible. It is not usual for death to result directly from 

 this type of disease, though more or less necrosis of tissue fre- 

 quently occurs. In spinach blight this feature is unusually 

 marked, and, contrary to the general rule, its development 

 continues until the plant is killed. 



The conditions governing infection by contact and the 

 question of the distribution and characters ot the virus have been 

 closely investigated by Allard, working on tobacco mosaic. The 

 virus permeates all parts of the plant, including the corollas of 

 the flowers, the hairs on the leaves, and the roots. It is still 

 contained in juice expressed through a Chamberland orBerkefeld 

 filter. Plant material dried and ground retained its virulence 

 i| years. The virus preserved in ether, toluene, and glycerine, 

 was infective 4 months later, as was juice which had been allowed 

 to undergo natural fermentation. It is highly resistant to the 

 action of powerful chemicals, withstanding nitric and hydro- 

 chloric acid added to the virus solution until the concentration 

 reaches i per cent., and phosphoric, citric and acetic acids up to 

 2 to 5 per cent. Carbolic acid had no effect in a 2 per cent, 

 solution and the much more powerful germicides cresol, creolin and 

 phenoco showed no apparent increase over carbolic acid in their 

 effect on the virus. Chloral hydrate (10 per cent.) and naphtha- 

 lene, camphor and thymol in excess had no appreciable effects. 

 Mercuric chloride i in 100 affected the virus but little, and 

 quinine bisulphate i in 25 had no appreciable effect after 19 

 days' treatment. Alcohol destroyed the virus in concentrations 

 above 50-55 per cent., copper sulphate showed itself rather 

 toxic, while 4 per cent, formaldehyde had a rapid action and was 

 found suitable for sterilisation purposes. The virus is destroyed 

 by temperatures above 80° C. 



A dilution of the virus to one part in 1000 in water was as 

 effective in producing infection as the original virus ; at one in 

 10,000 some attenuation was shown, and one case occurred even 

 from a dilution of one in 1,000,000. The inoculations for these 

 tests were made with only so much of the diluted virus as could be 

 introduced by the prick of a needle. 



The incubation period of the mosaic diseases is variable. In 

 tobacco the shortest time observed has been 6-7 days, and 

 usually 12-15 days are required. The length of the period 

 depends on the kind and age of the plant, and on such external 

 factors as accelerate or retard growth. 



Infection can be artificially effected by inoculation as above, 

 and by the touching or handling of healthy plants after diseased 

 ones. Spraying the virus on to sound leaves does not readily 

 give rise to infections, but injury to the leaf hairs such as arises 

 from handling is sufficient to open the way for infection to take 

 place. 



It is characteristic of^this type of disease that the infective 



