560 INSECTS AFFECTING VEGETATION 



grow gradually larger and larger, finally turning black and a little 

 later decomposing and emitting a disagreeable but characteristic 

 odor. If one of these infested areas be examined closely, it will be 

 found to be bordered by a grayish-white mildew. 



For the development of the mycelium, that is, for the growth 

 of the fungus within the potato plant, moderately cool weather seems 

 the most favorable. For this reason this disease rarely proves trou- 

 blesome where high temperatures prevail for considerable periods 

 of time. Spore production, however, seems to be hastened and enor- 

 mously increased when a few days of warm, cloudy and muggy 

 weather alternate with the longer cooler periods. Under such con- 

 ditions, a field showing but slight infection, may in a few days look 

 as if it had been swept by fire or frost. It rarely attacks early potatoes, 

 mostly appearing upon the late varieties during the tuber-forming 

 period. 



Various experiment station workers have tried different remedies 

 for holding this disease in check. At some stations these experi- 

 ments have been carried on for many years. While several have 

 given results which were of value, no treatment has been uniformly 

 successful as the application of Bordeaux mixture. The universal 

 experience is that spraying with this fluid will so nearly control the 

 late blight as to make it possible to secure a crop even in those years 

 when this disease is most prevalent. It requires, however, that the 

 spraying should be begun in time and continued at intervals through- 

 out the growing season. As already stated, it must be a precaution- 

 ary measure. If not begun until after the blight is evident in the 

 field only partial control can be expected. If the spray is applied 

 thoroughly from the beginning, not only will the blight be con- 

 trolled, but the rot of the crop which usually follows a severe attack 

 is altogether prevented. (Wyo. E. B. B. 71.) 



Fusarium Disease of Potatoes. On badly infected soils the dis- 

 ease is characterized as follows: The stand is uneven, though few 

 of the hills are missing. The early growth is somewhat slow. When 

 the plants reach a height of 10 to 14 inches, there is an apparent ces- 

 sation of growth. The first indication of the disease is usually con- 

 spicuous at this time. The preliminary symptoms are a light green 

 color of the foliage, particularly the lower leaves ; this is accompanied 

 during the heat of the day by a partial wilting and an inward and 

 upward rolling of the upper leaves. The color gradually changes 

 to a sickly yellow, which slowly and evenly covers the affected vines. 

 The wilting and rolling of the leaves extends to all parts of the af- 

 fected plants. In the varieties with heavy dark-green foliage the 

 wilting and rolling-in symptoms are not always accompanied by 

 marked changes in the leaf coloration. 



As the disease progresses the field takes on a mottled yellow to 

 light green color. The growth of the foliage is greatly restricted. 

 The wilting becomes more pronounced ; there is associated much tip- 

 burn owing to the failure of the leaflets to fully recover at night 

 from the severe wilt of the mid-day. The lower leaves of the affected 

 plants are first to die ; they fall into a vertical position along side the 



