434= 



IXJURIOUS INSECTS AND FUNGI. 



[March 189f. 



INJUKIOUS INSECTS AND FUNGI. 



The Turnip Mildew {Oidmm halsamii, Mont.}. 

 Fig. 1. 



X- 400 



The figure shows a mass of spores upon the surface of a leaf, A A. 

 B. A spore germinating. 



Turnip mildew w^as so very prevalent last summer upon swedes 

 and other plants that it has been thought desirable to give 

 a further description of it, accompanied by an illustration of 

 this troublesome fungus, which covers the leaves of swedes 

 with thick congeries of white threads, and injures the plants by 

 stopping or checking their assimilation of plant food from the 

 air, hindering transpiration, and stopping their growth. Some- 

 times the upper and lower surfaces of the leaves are as white 

 as snow from innumerable dust-like spores, which adhere to the 

 clothes of sportsmen and others walking through the swedes. 

 Mildew is usually more prevalent in hot seasons, or in those 

 that, after having been hot and dry for a long period, are fol- 

 lowed by thunderstorms and heavy showers. Early sown swedes 

 that have been checked by drought are especially liable to be 

 attacked by Oidium halsamii, yihich must not be confused with 

 another turnip mildew, Peronospora parasitica, frequently found 

 on swedes, other kinds of turnips, and cabbages. The latter, 

 although resembling Oidium halsamii upon casual examination, 

 is quite different in its habits and action upon the plants, inas 



