OZOXIUM EOOT EOT 7 



of the larger lateral roots, so that these dead rootlets serve as an 

 entrance for the fungus to the bark tissues. These depressions can 

 be formed at any point in the region where the main side roots 

 originate. After entrance is effected the taproot and lateral roots 

 rot very rapidly. During periods of cloudy days and high soil 

 moisture, adventitious roots which may sustain the diseased plant 

 for some time form above the diseased portion. Xo pronounced 

 constriction of the . taproot occurs. This type of infection is not 

 common, owing probably to the greater depth in the soil at which 

 it occurs. Duggar (7) 4 lias reported two types of infection and has 

 designated them as centrifugal and centripetal. However, only the 

 centrifugal type of infection was observed by the writers. 



Fig. 1. — A sector of a root-rot spot in alfalfa, showing the different zones formed 

 by the various stages of the disease from aboveground and underground observa- 

 tions. Sacaton, Ariz. 



ALFALFA 



DETAILED EXAMINATION OF UNDERGROUND CONDITIONS 



Detailed notes on the condition of alfalfa roots excavated from 

 a sector of a spot of root rot in an alfalfa field, made on September 

 3, 1924, at Sacaton, Ariz., are here presented (fig. 1 and pi. 2) : 



Zone 1 : Plants dead. This zone consisted of weeds, a few weak live plants, 

 and dead plants killed by root rot. The taproots rotted to about 75 centimeters. 

 Sloughing of the bark occurred for 60 centimeters. No mycelium was present 

 on the roots below 50 centimeters. The entire roots above this point were 

 badly disintegrated. Mycelium was rather difficult to find. Below 75 centi- 

 meters the taproots were sound and fibrous roots were present. The roots of 

 the plants on the outer edge of this zone were not so badly disintegrated. 



Zone 2 : Plants wilting. The foliage was wilted and drying up and the 

 leaves were shedding rapidly. Taproots were rotted for about 60 centi- 

 meters. Below this point they appeared progressively more healthy and were 



4 Serial numbers in italic in parentheses refer to " Literature cited 

 bulletin. 



at the end of this 



