Aspen Bibliography

Screening aspen for resistance to Hypoxylon canker

J.R. French

Document Type

Article

Journal/Book Title/Conference

Dissertation Abstracts International

Volume

37

Issue

9

Publication Date

1977

Abstract

Two methods of screening aspen (Populus tremuloides and P. grandidentata) for resistance to Hypoxylon canker were tested. These included inoculation of naturally existing clones of aspen with pathogenic isolates of Hypoxylon mammatum, and assay of excised leaves from various host genotypes with host-selective metabolites produced in vitro by the fungus. The inoculation method was evaluated by comparing length of cankers resulting from inoculations with the amount of natural infection in each clone.

I. Pathogenic single spore isolates of H. mammatum were identified by inoculating branches of trees in 4 clones of P. tremuloides with 12 isolates. The two most virulent isolates and one isolate with low virulence were used to inoculate 100 naturally occurring clones of trembling aspen (Populus tremuloides) and 13 clones of bigtooth aspen (P. grandidentata) in 12 geographic areas of Michigan during late April and early May. The resulting cankers were measured 70 days after inoculation, and canker length among clones located within the 12 areas was subjected to analysis of variance. The two highly virulent isolates caused cankers on 95% and 96% of inoculated branches, and one isolate Produced significantly longer cankers than the other (mean length 59 vs. 45 mm over 100 clones). Cankers ranged from 12 to 14 mm in length. P. tremuloides clones in northern areas produced significantly shorter cankers than those in some southern areas. Significant differences in length of cankers among trembling aspen clones in 10 areas were observed. Amount of natural infection of Hypopylon canker in each clone was determined at the same time that cankers were measured; infection level per clone ranged from 0% to 58%. Length of cankers resulting from artificial inoculation was not correlated with amount of natural infection among clones in 11 of the areas. Lack of correlation may be ascribed to absence of suitable levels of natural inoculum within some of the areas, so that genetic susceptibility was not expressed.

Branches of P. grandidentata inoculated with H. mammatum became infected, and developed cankers similar in size to those on P. tremuloides. Large amounts of callus were observed on inoculated branches in some clones. Size of cankers following indculation also varied among clones of this species, and mean length ranged from 29 to 69 mm after infection with the most pathogenic isolate.

II. H. mammatum produced toxic metabolites in culture which were host-selective. Toxic preparations caused spreading necrotic lesions when applied to puncture wounds on leaves of P. tremuloides. The preparations also affected leaves of P. grandidentata and P. maximowiczii. Leaves of 10 other woody species were not affected.

Toxic metabolites from one isolate of H. mammatum were partially purified, and at least two host-selective components were evident. Lesion diameter was plotted against toxin concentration, using leaves of a sensitive clone of P. tremuloides. The response was linear over a 2,000-fold toxin concentration gradient. Toxic compounds were probably less than 1000 d in molecular weight, and did not lose activity after heating to 120 C. Toxic metabolites were produced under all tested conditions which allowed growth of the fungus.

Isolates of H. mammatum differed in ability to produce host- selective toxic metabolites in culture. Non-pathogenic isolates and those with low pathogenicity in inoculation tests produced the lowest amounts of toxin in culture.

Attempts were made to correlate clonal sensitivity to H. mammatum toxin with susceptibility to infection by the fungus. Leaves from 29 clones of P. tremuloides were assayed with toxin preparations, and stems of young trees in the same clones were inoculated with H. mammatum. In general, sensitivity to toxin in leaf assays was not correlated with length of cankers developing from inoculations with the fungus. The lack of correlation in these tests may be ascribed to the juvenile nature of tissues which were inoculated. Additional attempts to correlate sensitivity to toxin with genetic susceptibility to H. mammatum infection are suggested, because it is not clear whether inoculation with the fungus or tests with its host-selective toxin are good indicators of susceptibility to the disease.

Comments

This thesis was submitted towards fulfillment of the requirements for Ph.D. degree in Botany and Plant Pathology.

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