Verticillium Wilt of Maples: Symptoms Related to Movement of the Pathogen in Stems

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Abstract
Saplings of A. rubrum in which the main stems were wound inoculated in Sept. with V. dahliae did not develop foliar necrosis or wilt. Infections were compartmentalized within xylem present at the time of inoculation. In the first 10 mo. V. dahliae grew only an average of 45 cm upward and 10 cm downward from the wound; no additional growth occurred in the next 14 mo. Xylem discoloration extended beyond the fungus. Greenhouse-grown, 2 yr old seedlings of A. platanoides were inoculated by a split-root technique that provided infected, nonwounded stems on healthy roots. In these plants, V. dahliae moved from the xylem sheath of one growth period to that of the next. This movement occurred both by direct radial growth and by growth into bud traces and from there into new shoots and the new xylem sheath. In naturally and experimentally infected maples, acute symptoms (wilt and intercostal necrosis) occurred only if V. dahliae was present in xylem of the current growth period. The fungus extended into shoots and petioles of sprouts on main stems, but not into those on topmost or outermost twigs. The extent of branch dieback caused by V. dahliae between growing seasons was not related to previous symptoms. Wilt occurred in consecutive years in about half of the naturally infected A. platanoides, A. rubrum and A. saccharum that were observed. Trees in remission or showing only chronic symptoms (scorch and suppressed growth) had infections confined to xylem at least 1 yr old. V. dahliae was isolated from xylem up to 3 yr old from trees in remission. Infections in stem xylem may enlarge during several seasons before death of the tree or dieback of major branches occurs, and that after each season an infection is subject to compartmentalization that can lead to remission of acute symptoms.