4 BULLETIN 1417, U. S. DEPARTMENT OF AGEICULTUEE 



progresses, reaching its maximum severity during the critical 

 months of July, August, and September, and continuing well into 

 October, depending on weather conditions and on the maturity of 

 the plants. During certain years root rot may appear in May, but it 

 usually makes little headway until the plants are beyond the seedling 

 stage. 



In alfalfa fields root rot begins its activity earlier in the season 

 and persists later in the fall than in cotton fields. At Sacaton, Ariz., 

 root rot usually begins its activity in the latter part of April or iai 

 May, and may remain active until October. Eoot-rot activity seems 

 to oe closely related with the period of rapid growth of alfalfa. 



Small fruit trees begin dying from root rot about the first of June 

 and may continue to die any time during the rest of the growing 

 season. Most of the fruit trees, however, die during June and July, 

 both at the station at San Antonio, Tex., and at the United States 

 Field Station at Sacaton, Ariz. 



AGE OF PLANT ATTACKED 



Root rot attacks cotton only rarely at the seedling stage, occasion- 

 ally before the squares are formed, but most often from the time the 

 first squares are formed until the plants mature. As the season 

 advances, usually increasing percentages of plants are killed until a 

 maximum is reached during July, August, and September. No 

 doubt the plants are more susceptible as they grow older and more 

 active mycelium is present in the soil, but the limiting factor during 

 the critical months appears to be the rainfall or the supply of 

 irrigation water. As a rule, early-planted cotton shows a larger 

 percentage of dead plants at the end of the season than late-planted 

 cotton. This would indicate that the age of the plant is also an 

 important factor. 



Alfalfa seeded in the fall does not seem to be subject to root rot 

 until the plants are well established in the late summer or early fail 

 of the following year. Plants 2 years old are apparently attacked 

 with much more vigor. Root rot attacks plants from the above- 

 mentioned young stage until the entire field is destroyed or until it 

 is so badly diseased that it is plowed up as of no further value. 



In the case of fruit trees, root rot can kill trees of any age. The 

 mortality of replanted trees in infested areas is especially high. 



DEVELOPMENT OF THE DISEASE 



In order to determine the development of the disease on the under- 

 ground parts of susceptible plants the following method was em- 

 ployed. A short row of cotton plants showing all stages of root-rot 

 symptoms together with a number of apparently healthy plants 

 was chosen. These plants were then fastened together near the 

 surface of the ground by means of a cord which was tied securely 

 to stakes at each end of the row. A trench about 4 feet deep was 

 dug on each side of the row, care being taken to dig the trench so 

 far away that none of the lateral roots would be cut off. Then, by 

 means of an ice pick, the soil was dug from around the roots with- 

 out injuring them severely. The cord above held the plants securely 

 in about the same relative positions. By this method all the larger 



