292 Diseases of Economic Plants 



While the wilting is most likely to occur at flowering time, 

 it may appear at any stage of growth when the plants are 

 25 cm. or more in height. In mild cases the lower leaves 

 wilt first, while in severe cases all wilt at one time. The 

 death of the plant may occur in four days after the first 

 signs of disease, or may be delayed a month, possil^ly with 

 recovery and relapse intervening. 



The roots remain normal, l^ut the veins in the stems 

 appear as yellow streaks, in older cases black,' instead of 

 their normal color. Such stems, if cut crosswise, shortly 

 exude a yellow, viscid drop at the ends of the veins. This 

 is the most distinctive character, and infallibly indicates 

 the presence of the wilt. Death is caused by the plugging 

 of the water passages with this viscid substance, which con- 

 sists mainly of bacteria. 



In the fields the diseased plants, representing young and 

 old stages of infection, grow side by side with normally 

 healthy plants. Health and disease may appear, even in the 

 same hill; yet there appears to be no direct plant-to-plant 

 infection, nor any of that centrifugal spreading from a 

 diseased center that is so noticeable in most wilt or soil-borne 

 diseases. 



The causal organism is found in the seed from affected 

 plants, and it has been proved experimentally that such 

 seed carries the disease to the offspring. 



As a preventive measure resistant varieties should be 

 selected for planting, and all seed from plants which are at all 

 diseased should be avoided. The disease may be carried from 

 one locality to another by any of the means suggested under 

 soil diseases, and especially by manure infested with stalks of 

 diseased plants. 



Mmor diseases 



Brown-spot ^^^ (Physoderma zexv-maydis Shaw) was first 

 noted in India in 1910, in Illinois in 1911. Its distribution 

 is shown in the accompanying map. (Fig. 154.) Considerable 

 damage is done in the region indicated by the darker shading, 



