60 ROOT-KNOT AND ITS CONTROL. 



enon often observed that a sloping field may have nematodes at its 

 upper or middle portion and be free from them at the lower end where 

 the soil is water-soaked part of the year. 



DRYING. 



Laboratory experiments by the wi'iter seem to show that the root- 

 knot nematode can not withstand the drying out of the soil. Thus, 

 two pots of badly infested earth, containing badly knotted plants, 

 were allowed to remain without watering from June 4 to September 

 22, 1908. The soil became very dry and dusty. It was then watered 

 and seeds of susceptible plants were sown. These remained entirely 

 free from root-knot. It is certain that tlie adults are killed by drying 

 out, they being, indeed, very susceptible to injury of tliat kind. The 

 foregoing experiments led the writer to the conclusion that tliorough 

 drying was fatal to larvse and eggs as well. This was strengthened 

 by the observation that in his cross-inoculation work where carefully 

 washed root-knot roots of various plants were planted in sterilized 

 pots of soil and seeds of the desired plants sown in the pots, infection 

 was obtained wherever the roots used were fresh, while whenever 

 they were somewhat wilted, not even dry, no infection was obtainable. 

 Frank ^ and Stone ^ were also of the opinion that drying out was fatal 

 to these nematodes. 



On the other hand, there are several recorded observations which 

 would seem to indicate that the opposite is true, at least sometimes. 

 Thus, Goldi^ dried the roots of coffee affected with root-knot, both in 

 the sun and in the shade. After two months he wet them up and soon 

 found, with the aid of the microscope, numerous nematode larvse, 

 which he considered to be those of the root-knot nematode. A second 

 case was as follows: Prof. P. H. Rolfs, of the Florida Agricultural 

 Experiment Station,^ kept some sandy soil in the laboratory for 10 

 months. It became dry long before the expiration of that period. 

 The soil w^as watered and tomato seeds were sown. The radicles of 

 the seedlings became swollen and oedematous in a manner reseml)ling 

 the work of the root-knot nematode. No nematodes were found 

 within the roots, but clinging to the outside were found nematodes 

 which he identified as Heterodera radicicola. 



Goldi's conclusions may have been erroneous, for there are many 

 nematodes, almost indistinguishable from Heterodera radicicola in the 

 larval state, that endure drying out for long periods. If they were 

 examined only with the microscope and not tested in connection with 

 living plants on which they could be grown to maturity, it would be 

 almost imjiossible to tell whether those seen by Goldi were the one or 

 the other. Prof. Rolfs, on the other hand, is not likely to have made 



1 Frank, 1885. ' Stone, 1899. « G6Idi, 1892. * Rolls, 1894. 



217 



