242 J. GREMMEN 



DUTCH RESEARCH 



To support his words van Vloten (1939) started research on blister 

 rust, looking for resistant P. strobus trees to promote their further use 

 in Dutch forests. As a result of his work, he advised foresters to avoid 

 the use of P. strobus seedlings originating from commercial nurseries 

 and, instead, to raise their own plants in the forest by sowing. 



His experiments with eastern white pine and blister rust were demon- 

 strated at the September 1939 meeting of the Dutch Forestry Society. 



In 1941, 24,000 plants, exposed to blister rust in the nursery and 

 representing 43 different provenances, were transferred to the Loenermark 

 Forest. Provisional observations already indicated that all provenances 

 were seriously infected. A difference in the development of the aecidia 

 (0 to 72%) was noticed in these 3-year-old plants, but van Vloten (1941) 

 avoided a definite stand regarding possible resistance. 



After the foundation of the Forest Research Station at Wageningen, 

 of which Dr. van Vloten became Director, work on this theme was continued 

 by the present author under the Director's guidance. For this purpose, 

 9 different lots of P. strobus seed--4 from surviving trees in long- 

 exposed Canadian, Dutch, and Austrian plantations, and 5 from controlled 

 crosses between pairs of phenotypically resistant North American trees 

 found by A. J. Riker and his associates at the University of Wisconsin, 

 the Lake States region of the U.S.A. --were obtained and tested from 1952 

 to 1961. Identity of the 9 lots is shown in Table 1. 



Seed lots were sown in a nursery at the Forest Research Station, 

 "De Dorschkamp", in both 1952 and 1953. Then, at the start of their 

 second growing season, seedlings were transplanted into two rectangular 

 experimental plots. The first plot, containing all 9 sources, was 

 established in 1953; the second plot, with 7 of the 9 sources, was 

 established in 1954. Some seed source lots were larger and produced more 

 seedlings than others. Regardless of size, the seedling lots were divided 

 into basic replicates of 10+ seedlings. Replicates per seedling source 

 varied from 2 to 12 in the plot established in 1953, from 1 to 28 in the 

 plot established in 1954 (Table 1) . There were 52 row-plot replicates 

 of 10+ seedlings each in the 1953 plot, 60 in the 1954 plot. These were 

 assigned random positions and then planted in two rectangular plots. 

 Rows of trees alternated with rows of black currants (Ribes nigrum L.), 

 leaves of which were inoculated with aeciospores of C. ribioola in the 

 early summer prior to the autumn pine infection periods. 



Test seedlings were carefully examined each year after their initial 

 exposure to C. ribicola infection. Noted were those plants showing 

 suspicious rust-like symptoms, those with definite early symptoms of the 

 rust disease, those developing aecia, and those killed by the rust 

 disease. Percentages of seedlings in the 9 seed lots killed by the rust 

 through 1959 are shown in Table 1. By 1961, all seedlings had been 

 killed by the rust and the experiment was terminated. 



Our major conclusions were as follows: (1) No genuine resistance 

 was observed in any of the 9 P. strobus sources investigated; (2) a slight 

 difference in degree of attack between individual seedlings was noticed, 

 some of the more lightly attacked seedlings being characterized by the 

 absence of older needles. According to Spaulding (1925), reduced retention 

 of needles may be the means by which plants escape branch infection. 



