68 DISEASES OF ECONOMIC PLANTS 



ficially resembling the root tubercles of the legumes, are 

 common upon a large variety of crops and, though they 

 do not properly fall within the province of this book, a 

 word is due to them on account of their extreme destruc- 

 tiveness. They are due to a microscopic worm which enters 

 the root from the soil and by irritation of the root causes 

 the gall to develop. Soil infected with these worms will 

 produce disease symptoms upon susceptible crops, and con- 

 siderable diminution in the vigor of the plant results. The 

 cotton, cowpea, watermelon, and tobacco are especially 

 susceptible, though the list to include all susceptible species 

 would extend into the hundreds, including many prominent 

 ornamentals. The only remedies are those suggested 

 under soil diseases, with especial stress upon crop rotation 

 and the use of resistant varieties. 



