A comparison between Scolecobasidium constrictum Abbott and Heterosporium terrestre Atkinson shows them to be conspecific. The genus Scolecobasidium is preferred over Heterosporium for the disposition of this species. S. terreum Abbott is redescribed, and S. humicola and S. variabile described as new species.
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.
Photoperiod was found to control both initiation of tubers and susceptibility of potato to Verticillium wilt. Plants grown in a short-daily photoperiod (long dark period) formed tubers and such plants, when inoculated with Verticillium albo-atrum, developed moderate to severe wilt symptoms. Plants grown in a long-daily photoperiod did not form tubers and developed no disease symptoms when inoculated with Verticillium. When a 2-hour light period was interjected in the middle of the long dark period, no tubers were formed and plants developed no symptoms of disease when inoculated with Verticillium. The resistance to Verticillium exhibited by plants was not limited to the root but was present throughout the plant.
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