62 DISEASES OF CROP-PLANTS 



Antilles, and the iron cow pea is a variety adopted for its im- 

 munity from the attacks of Heterodera radicicola, which are ruinous 

 to other varieties. 



The life histories of plant-infesting nematodes and the nature 

 of the diseases they produce may best be illustrated by the 

 following summarized description of typical examples. 



Heterodera radicicola, Greef. 



This is the best known of the nematode enemies of plants, 

 owing to its cosmopolitan distribution and its great range of 

 hosts. It is abundant in both temperate and tropical countries, 

 but in those with a severe winter it is mainly or entirely a green- 

 house pest. It infests very many common vegetable crops, 

 various trees, and a large variety of weeds and plants growing 

 wild. As regards crops more specially tropical it acts as a serious 

 check to the cultivation of the Chinese banana in Egypt, has 

 been credited with great destructiveness to coffee (concerning 

 which see remarks under diseases of that crop) and has been 

 recorded from Hawaii and Java on young and soft roots of sugar- 

 cane. In the British Antilles its status is mainly that of a garden 

 pest. 



The effect of Heterodera attack on roots is the production of 

 galls, which differ in size and shape according to the nature of 

 the infested plant. On the lower stem and collar of trees the 

 infested bark becomes swollen and irregular, often presenting 

 a rough and scaly appearance on the outside for a foot or two above 

 the soil. 



The eggs are laid in the galls or swellings, and hatch in situ, 

 or in the soil if they are liberated by the death and decay of the 

 galls. From the soil the tiny larvae, which possess an oral 

 spear, force their way into tender roots and proceed to develop. 

 The body at first swells out to the shape of a bolster, from which 

 state the male worms later return to the ordinary slender eel- 

 worm form, which they develop inside the old skin as in a cocoon. 

 The female continues to broaden out until it reaches the form 

 of a gourd, broad and round at the posterior end and tapering 

 off towards the head ; its length is about one millimetre and its 

 breadth more than half of this. The time occupied from entering 

 the root to reaching maturity is about 6 weeks. As the develop- 

 ment of the ovaries proceeds the female is reduced to a lifeless 

 bag fully occupied by the 300-500 eggs. The males meanwhile 

 perish after copulation. 



The principal effect of infestation on the plant appears to be 

 due to root interference arising from malformation and from the 

 premature death of infected roots. The plant attacked is liable 

 to become stunted and sickly, or even to die outright. The 

 secondary effects may be equally or more important, since access 

 to other parasites, especially fungi, is afforded directly by the 

 galls and indirectly by the debility induced. 



