MAC.9. This rootstock is very similar to 

 Mark, the only difference being that Mark has 

 been virus indexed. It is thought to perform 

 nearly identically to Mark. In this trial, it was 

 the most precocious and productive combina- 

 tion. Tree size was between M.9 EMLA and 0.3. 

 Based on these data, it is the best rootstock in 

 this group; however, it suffers from a few prob- 

 lems. In this planting, it overfruited early, and 

 growth slowed and fruit quahty began to de- 

 cline. It nearly "runted out", but with heavy 

 pruning we were able to restore some shoot 

 growth for renewal of frmting wood. Other 

 problems have been observed in other locations, 

 particularly related to its sensitivity to drought. 

 At approximately three years after planting, 

 trees develop a noticeable swelling at and below 

 the soil line. The water transport system in this 

 part of the tree is very disorganized (as seen by 

 research in Michigan) and is inefficient in water 

 transport. If moisture is limiting, trees on Mark 

 (or MAC.9) will suffer more than those on other 

 rootstocks. It appears that in locations where 

 water is not hmiting, trees on Mark (or MAC.9) 

 perform very well, such as in our trial in 

 Belchertown. The cause of this swelling is 

 unknown, and there is no known cure for the 



problem, except possibly frequent irrigation. 

 The future of Mark is in great jeopardy because 

 of this problem, and many nurseries have re- 

 moved most of their Mark stoolbeds. Hopefully, 

 we wiU not lose a rootstock that can be very good 

 in some locations. It should still be considered 

 seriously for sites that have good moisture lev- 

 els throughout the season. 



OARl. This rootstock produced a tree simi- 

 lar in size to M.26 EMLA; however, it was not 

 productive and fruit size was very small. There 

 is no reason to consider OARl for commercial 

 planting. 



Overall, the rootstock picture is changing 

 rapidly. In this trial, M.9 EMLA, 0.3, and Mark 

 were the ones that performed best. From the 

 1984 NC-140 planting, others will be added to 

 the hst of good rootstocks, including C.6, B.9, 

 P. 2, and MAC. 39. A planting will be established 

 this year that includes new potentially good 

 rootstocks, such as B.146, B.469, G.65, and 

 several strains of M.9. As we move into the next 

 century, many rootstocks will be bred and se- 

 lected; however, it is likely that not much will be 

 gained in terms of productivity. Pest resistance 

 and site adaptability likely will be the major foci 

 of the future breeding programs. 



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Fruit Notes, Spring, 1994 



