Nature of Plant Diseases 99 



that have become too alkaline. This is true for 

 soils containing an excess of lime, wood ashes, or 

 magnesia, and especially when nitrate of soda is used 

 in excess. 



Control. Chlorosis when brought about by the 

 lack of available iron in the soil may be remedied 

 by the application of small quantities of iron sul- 

 phate. If the disease is caused by the other factors 

 previously mentioned, a cure may be effected by re- 

 moving tiie cause. 



Blossom Drop 



This is another trouble which may be termed 

 physiological and the cause of which carmot be at- 

 tributed to the work of parasitic organisms. It is 

 often noticed on tomatoes and various other plants. 

 Various causes lead to it. Sudden drops of tempera- 

 ture at blossoming will induce many plants to shed 

 their blossoms. Blossom drop may also be brought 

 about when too much nitrogen is applied to the soil 

 in the form of manure, especially hen manure. To 

 overcome this, the fertilizer in the soil must be bal- 

 anced by the addition of 600 pounds of acid phos- 

 phate and 150 pounds of muriate of potash per acre. 

 Overacidity in the soil may also cause the shedding 

 of blossoms. A sudden checking of the water sup- 

 ply, or overwatering may have the same effect. 

 Finally, improper pollination is often one of the 

 main causes for the blossom drop of greenhouse 

 plants. In the field, |}ollination is favored by both 



