MASS. EXPERIMENT STATION BULLETIN 432 



The occurrence of black root rot and the amount of damage caused before its 

 identity was determined in 1906 (3) cannot be ascertained with certainty. How- 

 ever, the nature of the disease and the farming and cultural practices known to 

 have been used in growing tobacco warrant the presumption that black root rot 

 was prevalent and caused heavy damage in the Connecticut Valley long before 

 its identity was known. Numerous investigators hold to this belief. 



Observation and survey of tobacco diseases each year since 1930, and experi- 

 ence with black root rot in investigational work, have convinced the writer that 

 it has been one of the most damaging diseases of Havana Seed tobacco in the 

 Connecticut Valley during the last fifteen years. It is certain that it causes much 

 more damage and entails heavier economic losses than most growers and other 

 people who work with tobacco realize. 



Cause and Symptoms of Black Root Rot 



Black root rot of tobacco is caused by a soil-borne fungus, Thielaviopsis basicola 

 (3,9), which occurs in most if not all tobacco land in the Connecticut Valley. 

 It is capable of living in decaying v^egetable matter in soils for several years in 

 the absence of tobacco or other suscepts; and, therefore, inoculum is almost al- 

 ways present in tobacco soils in sufificient amounts to initiate attacks of black 

 root rot whenever other factors are favorable for the development of the disease. 



Black root rot may occur both in seedbeds and in fields. However, because 

 effective control measures for seedbeds are known and rather generally used, 

 the disease is principally important as a field disease of tobacco in the Connect- 

 icut Valley at the present time. 



As the name implies, it is a disease of the roots of tobacco plants. The causal 

 organism first attacks the cortical tissues of the roots, either throughout their 

 whole length or only in segments. The first stages of the disease are characterized 

 by a peculiar roughening of the cortex and by a peculiar rusty color of the roots, 

 followed by rotting of the cortical tissue and usually of underlying tissues as well. 

 The smaller fibrous roots may be entirely rotted away while the larger roots are 

 only moderately infected, evidencing lesions underneath which the tissues may 

 or may not be dying or entirely dead. The later stages of the disease are char- 

 acterized by the usual brown color of decaying plant tissues throughout, or by 

 this brown color intermingled with black spots. The black spots are growths of 

 a spore form of the organism and are usually more noticeable on the larger roots, 

 which are often encircled by the black spore masses and girdled. Diseased 

 fibrous roots which might well bear the black spores are usually lost in digging 

 and often are not seen. In severe cases, most of the root system may be rotted 

 away, leaving only stubs of roots or a whisk composed of root stubs and short 

 fibrous roots which have grown above older roots, apparently in the plant's 

 attempt to counteract the effects of the loss of the lower roots. These later- 

 grown fibrous roots may become disea.sed in turn and die and decay. 



The disease disrupts the supply of water and nutrients from the roots to the 

 tops of the plants, with a consequent stunting of the plants. The above-ground 

 symptom usually noticed first is the greater than normal wilting of the plants. 

 Later symptoms are either a yellowish color or, more commonly, a darker than 

 normal green color of plants. Other later symptoms are the growth of narrow , 



