Factors affecting local control of White Pine blister rust.

On one area, burned over 26 years before planting with P. strobus, rust was effectively controlled by one eradication of Ribes spp. ; 12 years later, only 6.4% of trees had cankers. On a second area, burned over 1-2 years before planting, control by Ribes eradication was ineffective because of continual regeneration of the Ribes, and 27.5-67.5 of the trees on different sites had cankers. Planting of Pines and eradication of Ribes are both inadvisable on recently burned areas. Removal of cankers by pruning as an additional control measure was also tested. In young planted trees, infected branches were cut off when the canker was 6 in. from the trunk, branches were removed and the bark round the stub excised when the canker was nearer the trunk, and the top of the tree was lopped below the canker margin when the canker was on an upper main stem. after 3 years, 100% of trees in the first group, 69-100% in the second and 95% in the third were canker-free. On sapling-size natural forest trees, diseased limbs only or all lower branches and those with cankers in the upper 5 whorls were removed; observations after 6 years indicated that the second method was more effective. In both planted and natural stands where Ribes can be effectively and cheaply removed, the removal of cankers may be economically justifiable. KEYWORDS: Blister rust control \ f