Shank Tissue Proliferations in Apple Rootstocks: Effects on Tree Growth and Correlation with Site Factors

Abstract

A factorial planting of three scion varieties ('Gala,' 'Fuji,' and 'Braeburn') on each of four rootstocks (Mark, M.26, M.7a, and MM.111) was established at two locations in Maryland in 1990. In 1993, trees were scored for trunk circumference (TC), extension growth, leaf color, burrknots (BK) and root mass proliferations (RMP) both above and below ground, and presence of pests on the shanks. Mark was the only rootstock with RMP; 75% of the trees examined had these tissue proliferations. The % of TC composed of RMP below ground was 2-3X higher in Mark grafted with 'Fuji' than was observed with other scion varieties. Across rootstocks, % TC composed of BK below ground was significantly affected by location. In each rootstock, regressions between growth parameters and % of TC with BK or RMP were almost all negative and many had significant correlations. In Mark, % of TC with RMP below ground (but not above) was significantly correlated with reduced TC at 25 cm. The occurrence of RMP was much greater below ground than above. Orchard surveys of trees on Mark were also conducted at several commercial sites in MD and NY The percentage of trees showing RMP ranged from 0 to 90%. There was a significant reduction in TC at 25 cm with increasing levels of RMP. The percentage of trees with RMP and mean % of TC with RMP significantly correlated with percentage of trees having visible woolly apple aphids or their distinctive galls.