ROOT NECROSIS AND HISTOLOGICAL-CHANGES IN SURVIVING ROOTS OF WHITE ASH INFECTED WITH MYCOPLASMALIKE ORGANISMS

Root necrosis and histological changes in surviving roots of white ash (Fraxinus americana) infected with mycoplasmalike organisms that cause ash yellows were evaluated in graft-inoculated 3-yr-old plants grown in hydroponic culture and in soil, and in naturally infected trees. Dead and dying lateral roots on diseased plants in hydroponic culture were first noted at 34 days after inoculation. At 58 days, root necrosis in both hydroponic culture and soil was extensive. Leaves on some diseased plants growing in soil began wilting at 68 days. In secondary phloem of living, diseased lateral roots, starch accumulation was reduced, mild hyperplasia developed, many sieve tubes and companion cells collapsed, and phloem parenchyma adjacent to collapsed cells developed thick lignified walls (pathological sclerenchyma). Taproots and stems remained alive. After vernalization, diseased plants grew feebly. The roots of naturally infected white ash in the field displayed dieback and scarce feeder roots or abnormally tapered and bushy form. These roots contained pathological sclerenchyma and scattered nonfunctional sieve tubes that autofluoresced under UV. Diseased roots of green ash (F. pennsylvanica) saplings in the field had autofluorescent sieve tubes and pathological sclerenchyma but no dieback or abnormal form. Uninfected control plants of both species had none of the above symptoms. Root damage in white ash infected with mycoplasmalike organisms precedes other symptoms and perhaps triggers branch dieback in the ash yellows syndrome.