[Chap. XLV PLANT DISEASES 577 



occur. Sprays are applied often enough to protect all voung growing 

 parts from invasion. This protection is generally secured by some such 

 program as the following: (1) a delayed dormant spray is applied as 

 soon as the leaves which surround the young flowers have emerged 

 about 1/4 inch; (2) a pre-pink spray before the slightly emerged 

 petals have become colored; (3) the pink spray before the flowers are 

 in full bloom; (4) the calyx spray just after petal fall and before the 

 blossom end of the fruit has been covered by the calyx; (5) another 

 spray ten days after the calyx spray; and ( 6 ) later sprays applied when 

 necessarv. 



A well-informed plant pathologist, with the aid of weather forecasts 

 and infoiTnation from orchardists concerning the condition of the buds, 

 can intelligently modify the above spray program according to weather 

 conditions. In some states pertinent information concerning sprav pro- 

 grams is broadcast daily by radio for all sections of the state during 

 the spring months. 



Damping-ofl. Seedlings of plants often are infected at the surface of 

 the ground in such a way as to cause the killing of the stem and conse- 

 quent death of the whole plant. Gardeners and nurser\'men are es- 

 pecially troubled by this damping-off disease in seedbeds. Sexeral 

 species of fungi cause damping-oft, and they have been reported from 

 all parts of the world. Practically all species of plants are susceptible 

 to these fungi. Sometimes the plant is destroyed before the seedling 

 emerges from the ground; in other cases the disease may not be appar- 

 ent until after the seedlings or cuttings are transplanted. The fungi 

 may live for long periods of time in the soil as saprophytes. It is doubtful 

 if highly organic soils are ever free from such organisms. Abundant 

 moisture on the plant or in the soil, and a fairly high temperature are 

 fa\'orable to the growth of these fungi and their inxasion of green 

 plants. 



Control measures for damping-off lie largely in soil disinfection and 

 the proper regulation of temperature and humidity. Hot water, steam, 

 sulfuric acid, and formaldehyde have all been found effective means 

 of disinfecting soils. Dusting seeds with copper or mercury compounds 

 is also an efficient control. 



Downy mildew of grapes. This disease is probably native to North 

 America, having been reported in the United States in 1834. It was not 

 known in Europe until 1875. The presence of the downv mildew on 

 grapes in France led more or less accidentally to the discovery about 



