84 FUNGOID PESTS OF CULTIVATED PLANTS. 



traced upwards into the centre of the head. In the leaves the symptoms 

 usually begin at the margins, with yellowing of all the affected parts 

 except the veins, which become brown or black. 



The disease is caused by a yellow bacterium, which enters the plant 

 above ground, and usually at the margins of the leaves. Slugs and 

 caterpillars may spread the disease by going from diseased to healthy 

 plants. 



Rotation of crops is recommended to rid the soil of the pest. Cruci- 

 ferous weeds should not be permitted in the neighbourhood to harbour 

 the disease. Removal of infected leaves in the early stages of the disease 

 would be beneficial. It should be noted that when diseased cabbages 

 have been converted into manure, such cabbages as have been manured 

 with this material have exhibited the disease. 



Smith, U.S. Dep. Agri. Butt. No. 68 ; Mass. PL Dis. 340 ; Journ. 

 E.H.S. xxviii. 1904, p. 627 ; xxix. p. 759. 



TURNIP WHITE MOULD. 

 Oidium Balsamii (Mont.), PI. VIL fig. 102. 



Turnip leaves, and other garden produce, suffer from the incursions 

 of this white mould, which makes its appearance in the manner usual to 

 all of its kind, by spreading a thin white film of mycelium and conidia 

 over the subjects of its attack, like a hoar frost. 



It first attracted attention on Turnips in 1880, and since that time has 

 not been uncommon. The lowermost leaves are those which are first 

 attacked. 



From the coating of white mycelium which soon covers both surfaces 

 of the leaf arise the club-shaped branches, or fertile threads, the lower 

 portion usually consisting of three superimposed cells, surmounted by 

 the maturing conidium, or spore, which is of a barrel shape : that is to 

 say, it is cylindrical, swollen a little in the centre, like a barrel, and 

 truncate or flattened at the ends. When mature, this conidium falls, 

 and, pursuant to the custom in this genus, the next joint pursues its 

 growth and conversion into a conidium, in order to follow its predecessor. 



These conidia germinate very readily when kept moist, the germ tube 

 projecting at one angle. It is most common when a moist September 

 follows a dry August. 



Dusting with sulphur is one of the most effectual remedies in this kind 

 of disease, which cannot but remind us of the Oidium Tuckeri of the vine. 



Smith, Field Crops, 76, 77, figs. 27, 28 ; Gard. Chron. Sept. 25, 1880. 



HORSERADISH LEAF- SPOT. 

 Phyllosticta Armoracia (Cooke), PI. VII. fig. 103. 



A number of specimens of this parasite were collected in a garden, fully 

 a quarter of a century ago, and distributed, under the name of Septoria 

 Armoracia, when the distinctions between Septoria and Phyllosticta were 

 not recognised. The spots and their disposal upon the leaves appear to 

 be precisely the same as in Septoria. 



The receptacles, or perithecia, are minute and immersed in the spots 



