422 V. STEENACKERS 



Larix , the aecia of M. larici-populina are sometimes rather difficult to 

 find. 



Through laboratory experiments, Taris showed that uredospores of 

 Melampsora spp. can maintain viability and pathogenicity for 10 months. 

 In this way, he reported, the uredospores assure the perpetuity of the 

 rust species without passing through the aecial host. 



Toole (1967) identified the common rust on P. deltoides in the Lower 

 Mississippi Valley as M. medusae, which year after year, survives without 

 the presence of a larch host. 



Ge (1964) has described the symptoms of M. rostrupii on P. tomentosa 

 Carr. in China, but the alternate host was not found. 



According to the observations of Gremmen (personal communication) , 

 only a few uredospores will survive in nature once the poplar leaves drop 

 and the teleulostage starts. The small number of surviving uredospores in 

 nature will very quickly deteriorate due to changing temperature and mois- 

 ture conditions. However, the number of viable uredospores, on the leaves, 

 grown under artificial conditions is much higher. 



PHYSIOLOGICAL STRAINS OF POPLAR RUSTS 



While there is no information concerning the existence of geographical 

 races of the poplar rusts, van Yloten (1944) has proved the existence of 

 different physiological strains of M. larici-populina in the Netherlands. 

 It is possible to differentiate the strains by comparing the reaction of 

 a series of poplar clones to these strains after artificial infection. At 

 the same time, van Vloten showed that new virulent strains can originate 

 by crossing of two different strains. 



More recently, two physiological strains of M. allii-populina were 

 isolated by Magnani (1965) . 



IDENTIFICATION OF POPLAR RUSTS 



Several authors have made observations and measurements of the 

 color, form, and size of the different fruiting bodies, of mycelium, 

 spores, and paraphyses on both host plants of several rust species. 



Gremmen (1954) and Taris have made many measurements of the dif- 

 ferent spores and especially of 'the paraphyses in the uredosori of M. 

 allii-populina and M. larici-populina. 



Hennebert (1964) prepared two dichotomic keys to facilitate identi- 

 fication of the rust species. One key is based on form and size of the 

 uredospores; the other is based on the teliospores. 



Most authors have concluded that it is difficult, if not impossible, 

 to identify a poplar rust solely by morphological characteristics of the 

 spores . 



Accurate identification of the rust species is normally possible only 

 by means of pathogenicity tests on the two host plants. However, certain 

 morphological characteristics, and eventually the color of certain spores 

 are helpful. In the inoculation tests, it is necessary to use only 



