Ultrastructure of wilt syndrome caused by Verticillium dahliae. VI. Interpretive problems in the study of vessel coatings and tyloses

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Abstract
When leaves of Verticillium dahliae infected chrysanthemums and sunflowers show symptoms, many of the xylem vessels in the petioles are occluded by tyloses or, partially, by vessel coating materials. Four different cytological techniques are used to study these occluding structures: (1) SEM observation of bulk material fixed in glutaraldehyde and OsO4; (2) TEM observation of thin, monitored cross sections fixed in glutaraldehyde and stained and stabilized with FeCl3; (3) TEM observation of bulk material fixed in glutaraldehyde–acrolein and OsO4; (4) SEM observation of bulk material fixed in glutaraldehyde and stabilized with FeCl3. Three different kinds of coating materials are described: (1) smooth coating (type 1), (2) fibrillar coating (type 2), and (3) bubbly coating (type 3). A possible fourth type of coating is tentatively described as irregular coating (type 4). It is concluded that accurate identification of tyloses and coating materials is impossible with currently used techniques. The interpretational problems encountered at the various levels of microscopy (i.e., LM, SEM, and TEM) are summarized.

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