422 PLANTS AND MAN 



copper sulphate, and water — are advisable. If the blight is known 

 to exist in a field, a delay in digging the tubers until after frost, 

 and exposure of the tubers to the sun upon digging, prevents 

 increased infection. The development of disease resistant varie- 

 ties and hybrids is being carried on, and ultimately will further 

 decrease the present large annual loss due to late blight. 



Downy mildew of grape is probably native on the wild 

 grape of the Mississippi valley. Although the native American 

 grapes are not notably resistant to the disease, they are not so 

 completely susceptible to its attack as are the European varieties. 

 Downy mildew was introduced into Europe on rootstocks im- 

 ported because of their resistance to attack by a root louse which 

 likewise had been introduced to the vineyards of Europe from 

 America. Its spread was both rapid and destructive, and had it 

 not been for the discovery of the Bordeaux mixture referred to 

 above, it might well have completely destroyed the grape in- 

 dustry of Europe. During the 1880's the disease was so destructive 

 that many vineyards were abandoned, the owners turning to 

 other pursuits for their livelihood. Although spraying helps 

 greatly in combatting the disease, Germany in 1925 lost 40% of 

 her grape crop in spite of all available precautions. Downy 

 mildew attacks the leaves, young stems, and fruit of the grape, 

 initial infection occurring in the summer by means of spores 

 produced on the surfaces of infected leaves, or in the spring by 

 similar types of spores which have arisen from heavy walled rest- 

 ing structures formed in the fallen leaves. The mycelium of the 

 fungus is capable of overwintering in the buds, resuming its 

 infection when growth of the host begins in the spring. Presence 

 of this pathogen is easily detected by the masses of downy, white 

 fungus growth which appear on the lower surfaces of the leaves, 

 and on the clusters of green fruit. Like most fungus diseases, 

 downy mildew is checked in its growth by long dry periods, and 

 favored in its development by shaded, moist situations and 

 humid weather. Spraying with Bordeaux mixture every two 

 weeks during the growing season gives good control of downy 

 mildew, and removal or plowing under of the leaves in early 

 spring prevents infection from overwintering spores present in 

 such refuse. The pruning off of infected plant parts is also ad- 



