410 HETERODERA RADICICOLA. 



The Powers of Resistance of II et erode ra radicicola. 



The ova and larvae are the most hardy stages in the life history 

 of Heterodera radicicola. 



In a moist ambient the ova and larvae are able to exist for 

 a prolonged period, providing temperature conditions are not 

 extreme. On the other hand comparatively small variations from 

 the natural conditions which obtain in the roots are fatal to 

 the parasitic stages such as the developing and mature females. 

 When immersed in water they often burst after an hour or so 

 owing to osmotic pressure within the body, while they shrink 

 beyond recognition when allowed to dry in a watch-glass. The 

 mature males appear to be equally susceptible to desiccation but 

 can remain alive in water for nearly two weeks. They would 

 seem capable of living in moist sandy soil for a longer period, but 

 I have not been able to ascertain this period as yet. 



The isolated eggs hatch within a few days in water. The 

 free living larvae have been kept alive for thirty-two days, the 

 temperature during that period varying between 16°C. and 

 27°C. Drying was fatal in a very short time to isolated ova and 

 young larvae. In the gall worm of the potato, where the ova 

 are devoid of that protective covering or " eirsack " described 

 by Bespey for //. radicicola, the ova shrivel up soon after desicca- 

 tion in the open air and their protoplasmic contents become 

 amorphous. These eggs cannot be revived by moistening and 

 are incapable of further development. The free-living larvae, 

 although more resistant as a result of the tolerably thick cuticle, 

 are not invulnerable. After being exposed in a watch-glass for 

 but a day, large vacuoles appear in the intestine. Larvae dried 

 in this way for only about forty-eight hours could not be revived 

 by immersion in water for a week. 



Duration of the Life History of the Parasite. 

 The period necessary for the full development of the egg 

 and the liberation of the larva therefrom is from five to six days 

 in average mild spring weather in Johannesburg. Eggs removed 

 from a gravid female were placed in a glass vessel with water 

 and observed from day to day. By the fifth day many larvae 

 were found free in the water, while on the sixth day no eggs 

 remain unhatched. From observation made in the field it ;S 

 evident that the larvae are able to survive for a considerable 

 time, probably two years or more, if conditions in the soil are 

 not adverse. However, should the larvae be able to gain access 

 to the roots of a suitable host plant galls will usually appear 

 within four or five weeks, showing that this period is sufficient 

 for sexual differentiation, the fertilisation and subsequent swelling 

 of the female. Experiments demonstrating this were carried out 

 in seed-testing boxes, one side of which was of glass. Healthy 

 tomato seedlings, beans and peas were planted in these boxes, 

 which had previously been heavilv infected with Heterodera egsrs 

 and larvae. After a few days the roots of the plants began to 

 be attracted by heliotropism towards the glass side of the box 



