236 PLANT DISEASES 



severely attacked up to that period recover. Plants that 

 become diseased when quite young are usually killed, as 

 the disease appears year after year, if the weather is damp 

 in May and June, suggesting that the mycelium is per- 

 manent in the tissues of the host. 



In a seed-bed or young plantation it is observed that 

 the disease spreads centrifugally from a centre, infection 

 being effected by wind-borne spores, which leads to the 

 conviction that the aecidiospores can perpetuate the 

 disease on pines. Spermogonia and aecidia appear on 

 the leaves or young shoots, the cortex of the latter becom- 

 ing orange-colour. Growth of the branch is checked at 

 the part diseased, whereas it continues at other points ; 

 the result of this unequal growth resulting in a curvature 

 of the branch, and as the tip of the branch tends to grow 

 upwards, double curves are the result. 



Rostrup demonstrated by experiment that the teleuto- 

 spores of a fungus supposed at one time to be a distinct 

 species called Melmtipsora tremulae, TuL, growing on dead 

 leaves of the aspen (Fopulus tremidd)^ when caused to 

 germinate on the leaves or shoots of young pines, gave 

 origin to the condition called Caeoma pinitorqiiiim^ also 

 previously considered as an independent fungus. He also 

 produced the uredospore and teleutospore stages on aspen 

 leaves by inoculation wath aecidiospores formed on the 

 pine. 



Preventive Means. — Badly diseased plants should be 

 removed from seed-beds. Aspens should not be allowed 

 to grow in the neighbourhood of nurseries or young planta- 

 tions of pines. 



During seasons favourable to the growth of the fungus 

 on the aspen, its leaves are often quite yellow with the 



