Abstract
Six cultivars of Gleditsia triacanthos were inoculated with the fungus Thyronactrìa austro-americana in May, July, September, and November. Branches were excised and examined in the laboratory the following spring. Small cankers and extended xylem discoloration were present, but wilting and dieback did not occur. The fungus persisted in the wood of every tree inoculated but caused cankers at only 20% of the inoculation sites. More cankers were present on trees inoculated in September and November than in May or July. Cankers were most frequent on ‘Sunburst’, less frequent on ‘Moraine’ and ‘Skyline’, and least frequent on ‘Imperial’, ‘Holka’, and ‘Shademaster’.
Honeylocust, Gleditsia triacanthos, is planted extensively as a windbreak tree in the Great Plains and as an amenity tree in the midwestern and northeastern states. In the 1980s many honeylocust less than 30 years old began to decline (3, 5, 8, 9). Cankers caused by Thyronectria austro-americana were shown to contribute to the decline in several states (3,4,5, 6). A wood-boring insect (Agrilus sp.) has been associated with cankers and may be a vector for the fungus (5, 11). Another canker-causing fungus, Kaskaskia gleditsiae, was proved pathogenic to Gleditsia triacanthos var. inermis(2), but this fungus is believed to be of the same species as T. austro-americana (1).
Susceptibility of honeylocust to Thyronectria appears to be related to environmental stresses, although no direct patterns have been established. Naturally-occurring cankers caused by T. austro-americana have consistently been associated with trunk and stem wounds (5, 6) and inoculations with the fungus have been successful only when the host tree is wounded (4, 5, 6, 9). Disease development in G. triacanthosvaries with size of stem, size and age of wounds, and temperature (7, 9). This paper reports field trials on the effects of season of inoculation of six commercially available cultivars of G. triacanthos with 7. austro-americana.
Materials and Methods
Six honeylocust cultivars (‘Holka’, ‘Imperial’, ‘Moraine’, ‘Shademaster’, ‘Skyline’, and ‘Sunburst’) were provided as grafted stock (1.0-1.3 m tall) and planted in the spring of 1982 in the Illinois Natural History Survey arboretum at Urbana, in a grid with 2.6 m between trees. In 1987 the saplings were 3-4 m tall with a stem diameter of 3.0-4.5 cm at 0.3 m height. The trees had annual twig growth of 40-70 cm.
The isolate of T. austro-americana used in this study was obtained in 1981 from the margin of an active canker on a honeylocust and was established as a pathogen of G. triacanthos by Stim and Himelick (11). The fungus was stored at 5°C on potato dextrose agar (PDA). In 1987 inoculum was produced by growing the fungus on PDA on petri dishes at 24°C for 5-7 days. The inoculum (conidia and hyphal fragments) was adjusted to 107 conidia per ml of sterile distilled water. Approximately 0.03 ml of inoculum was injected with a hypodermic syringe into 2-mm holes drilled completely through a branch at three sites 25-35 cm apart. Inoculated branches (mostly 1 -yr-old) were 10-15 mm in diameter. The inoculation sites were wrapped with freezer tape to prevent drying.
Trees were inoculated four times during 1987 in May, July, September, and November. Each tree had a different terminal branch inoculated on each of the dates; 15 trees of each of the six cultivars were inoculated.
The inoculated branches were excised and examined in the laboratory in May of 1988. Both discolored external bark and internal wood were measured proximal and distal to each inoculation site. Bark canker discoloration was measured to the nearest mm. Stems were then serially sectioned at 1O-mm intervals; the extent of red-orange discoloration in the xylem was made to 30 mm above and below the inoculation site. Serialsection wood chips from one of the three inoculation sites on every branch were plated onto PDA to verify pathogen presence.
Results
Thyronectria austro-amerícana inoculations into vigorously growing honeylocusts resulted in canker formation. Frequently, the cankers were neither large nor abundant. Of the 1,080 inoculation sites, 206 developed cankers. Most cankers were 6-12 mm long; only seven cankers were more than 12 mm long. The largest canker was 41 mm long. Foliage wilt was not observed on any trees.
Thyronectria canker on honeylocust showing typical bark depression.
Dieback and decline of honeylocust approximately 20 years old. Thyronectria canker contributes to these symptoms.
Inoculations resulted in red-orange discoloration of the xylem that extended well beyond the external bark cankers. In reisolations attempts from discolored wood, the fungus was recovered from 100% of the trees and from 71% of the discolored wood chips.
Honeylocust cultivars responded differently to T. austro-americana inoculations. ‘Shademaster’ had fewer cankers than other cultivars, but was not significantly better than ‘Holka’ or ‘Imperial’. ‘Moraine’ and ‘Skyline’ were intermediate in susceptibility and ‘Sunburst’ was the most susceptible. The xylem tissue on all cultivars became discolored for 60 mm in length at least 50% of the time (Table 1). Thyronectria austro-americana was readily reisolated from the discolored wood in the excised branches.
The incidence of canker formation and internal discoloration in field-grown honeylocust cultivars inoculated in different months.
Season of inoculation influenced canker development. Inoculations in May and July resulted in significantly fewer cankers than inoculations in September and November (Table 1). Discoloration of the xylem was present regardless of season of inoculation.
Discussion
In this study, cankers resulting from artificial inoculation were relatively small and more closely approximated the size of those described by Crandall (4) than the larger cankers reported by other researchers (2, 5, 6, 9, 10). The cankers observed in this study might have increased in size if left through the 1988 growing season, or they could have decreased in size through production of surrounding callus (6, 9). Coalescence of small cankers can result in extensive dieback and decline of honeylocust (4).
More cankers resulted from the September and November inoculations in this study than from inoculations made earlier in the year. This may indicate that the causal fungus is not especially virulent in nonstressed trees or that localization of infection by the host is more effective during the season when there is more active growth. These data on honeylocust are not in agreement with those of Born and Crane who found that inoculations in June and July produced more and larger cankers than those made in September (2).
Selection of honeylocust cultivars for resistance to the Thyronectria canker disease would be an important practice in landscape planning. The results of this study support an observation by Kenna (8) that ‘Sunburst’ and ‘Moraine’ are more susceptible than other cultivars to Thyronectria.Selection of the cultivars ‘Imperial’, ‘Holka’, or ‘Shademaster’ would appear warranted. Other recommendations that could reduce disease incidence are the avoidance of injuries, the maintenance of vigor through suitable fertilization and watering practices, and the pruning out of dead and dying branches.
- © 1989, International Society of Arboriculture. All rights reserved.
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