258 C. HEIMBURGER 



Earlier work with resistance breeding of white pine, particularly 

 in Wisconsin, has been amply reviewed by Patton (1966) and will not 

 again be reviewed here. Because of limitations of time and space, only 

 the most important data on rust resistance of P. strobus and some of 

 the related species and hybrids will be discussed in the following. 



The white pine breeding project initiated in 1946 at the Southern 

 Research Station of the Ontario Department of Lands and Forests at Maple, 

 Ontario, Canada, includes the breeding of white pine for resistance to 

 blister rust. The breeding for resistance to this rust was greatly stimu- 

 lated by the findings of Riker and Kouba (1940), and Hirt (1948). In 

 Canada, forest pathologists discovered a heavy attack of blister rust in 

 a white pine plantation at the Seigniory of Mr. Joly de Lotbiniere at 

 Pointe Platon, on the south shore of the St. Lawrence River, about 30 

 miles west of Quebec City. This plantation was established in 1908 with 

 about 400 plants imported from a nursery in Germany. After heavy infec- 

 tion with blister rust and removal of all infected trees by 1945, there 

 were 35 trees left that were completely free from any infection. These 

 remained free from the disease during the following 10 years and have 

 been used as a source of seeds and scions since then. Seedlings from 

 this plantation have also been used to compare the resistance to blister 

 rust of white pines of different origins (Heimburger, 1956) . These studies 

 indicated that the seedlots from Pointe Platon contained a greater propor- 

 tion of healthy plants after heavy natural infection in a nursery than 

 comparable seedlots from unselected trees. 



The following method of inoculation with blister rust has been used: 

 A seed bed is surrounded by a frame of lumber, the soil is moistened and 

 ribes leaves with telial columns are stuck into the soil rather densely 

 among the seedlings, with their petioles about 1/2 inch (1.2 cm) in the 

 soil. The ribes are then watered down. Sections of ribes shoots with 

 suitable leaves are used in the same manner as the above to inoculate older 

 seedlings and grafts. The bed is covered with a lath screen and double 

 thickness of wet burlap which is held down by another lath screen. The 

 whole is kept moist for about a week with plastic hose sprinklers. 



Black currant (Ribes nigrum L.) leaves are used as a source of 

 inoculum. The currants are often infected so heavily by the rust that 

 many leaves drop off during the hot weather of July-August and very few 

 are available at the time of inoculation in September-October. This was 

 remedied about 15 years ago by raising a few thousand seedlings of 

 commercial varieties from berries bought on the market and by selection 

 among these for rust susceptibility and good leaf retention during the 

 summer drought period. About a dozen selected seedlings are being propa- 

 gated by cuttings as a clone mixture. The currant bushes are cut down to 

 the ground in the fall at the time of shoot and leaf collection for 

 inoculation. This produces strong shoots with good leaves in the following 

 year. 



After 2 to 3 years in the inoculation bed, the healthy seedlings are 

 set out in a nursery compartment at a rather wide spacing. After another 

 4 to 5 years in the nursery, the remaining healthy plants are set out in 

 test areas. The climate at Maple is not favorable for inoculation with 

 blister rust. Occasionally a short period of moist and cool weather, 

 suitable for inoculation is available at about the end of August. After 

 this there is usually a fairly long period of "Indian Summer" with warm, 

 dry days, later followed by frosty nights when conditions are again 

 favorable for inoculation in September-October. 



