Graduate Thesis Or Dissertation

 

The relationship of phloem graft union responses to pear decline Public Deposited

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  • Since 1948, pear decline has been responsible for the devitalization and death of many pear trees in the Pacific Coast states and British Columbia. Use of anatomical abnormalities of the graft union as a diagnostic technique to separate pear decline from other disorders exhibiting similar symptoms was evaluated. Modifications in technique and seven abnormal graft union responses of the phloem are described. The ability to relate the distinct graft union response of french-oriental scion-rootstock combinations to decline expression makes the technique an effective tool for these combinations. However, the non-distinct union reaction of french-domestic scion-rootstock combinations was difficult to interpret because of the inability to trace previous phloem injury and the slow rate of decline of the trees sampled during the course of this investigation. The distinct graft union responses were non-repetitive and could be followed by an annual increment of phloem which reacted in one of the described categories. This is probably related to the gaps observed in previous increments where no apparent replacement phloem was produced. Occasionally a normal annual increment of phloem could follow a positive reaction in which no replacement phloem was produced. Samples taken mid-August to September 30th were effective in demonstrating the reaction. If replacement phloem formed, initial stimulation of cambial activity occurred in July or August after the original increment had ceased to function at the graft union. The brown line symptom at the graft union was distinct and coincidental to the presence of necrotic phloem. Histological evidence for decline was found in the Hood River area, the Willamette valley and the Rogue River valley. Quick decline was induced one year after initial psylla infestation of two year old Bartlett-P. serotina scion-rootstock combinations. Trees which collapsed in October had shown severe graft union phloem abnormalities in July, while those which had purple and red leaf symptoms in October had less severely injured or normal phloem in July. Feeder root deterioration may play an initial role in decline expression. A proposed relationship of graft union responses and decline expression is based on the tolerance of a given rootstock, titer or toxic substances and the stresses imposed on the tree by other factors.
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