Influence of Root-Knot Nematodes on Verticillium Wilt of Maples

Materials and Methods

Own-rooted plants (about 30 cm tall) of four Norway maple cultivars (‘Crimson King’, Greenlace’, ‘Jade Glen’, ‘Parkway’) had been propagated and donated by J. Frank Schmidt & Sons Nurs., Boring, Oregon, in 1985. Some of the plants from this shipment had been used by Townsend et al. (11), in their determinations of Verticillium tolerance. Susan E. Bentz, Horticulturist, U.S. National Arboretum, had also attempted (in 1989) a number of interclonal grafts using these plants and some of those individuals on which the grafts had failed were provided for our tests. The cutting back and pruning of these plants resulted in some above-ground growth irregularities, but the plants generally had one or two main leaders.

The red maple cultivars (‘Autumn Flame’, October Glory’, ‘Red Sunset’) were two-year-old rooted cuttings that had not been pruned back to a single stem. Seedlings of A. macrophyllum, A. negundo, A. platanoides, A. pseudoplatanus, A. saccharum, and A. velutinum were grown from seed collected in 1989 and represented the same progenies utilized in our studies of nematode susceptibility (9).

The inoculum for all of the nematodes was prepared from root-galls on a susceptible Salix alba clone (NA44016) growing in containers in the greenhouse. The inoculum dosage for the larger clonal maples consisted of two 50-ml aliquots of a chopped gall-media mixture, but only one aliquot for the smaller seedlings.

The isolates of Verticillium dahliae used for inoculation were supplied by Susan E. Bentz. One isolate, from eggplant, had also been used by Townsend et al. (11) in mixed inoculations of Norway maples. Two new isolates, from the roots of infected ‘Crimson King’ and ‘Greenlace’ maples, also resulted from the experiment. These three isolates were grown on potato-dextrose agar for about 30 days, at which time the 100 mm petri dish was completely covered with the fungus.

The Verticillium inoculum was prepared by blending two plates of the eggplant isolate and one each of the Norway maple isolates in 800 ml water. The larger maples (clones) were inoculated by inserting a chisel (2.5 cm wide) into the growing media to one-half the depth of the container on four sides of each plant and pouring 25 ml of the inoculum into each cut (100 ml per plant). The smaller seedling maples were inoculated with a total of 50 ml inoculum in two chisel cuts into the medium on opposite sides of each plant. This was not considered a high level of inoculation especially since there was no conscious attempt to sever roots before adding the inoculum.

Four plants each of the seven Norway maple and red maple cultivars (listed in Table 1) were inoculated with each of four different nematodes on 18 April 1991. The nematodes were M. arenaria-Race 1, M. arenaria-Race 2, M. incognita, and M. javanica. Thus, 16 plants of each cultivar were inoculated with nematodes. On 17 May 1991, two plants of each cultivar-nematode combination were treated with Verticillium and an additional two plants of each cultivar were treated with Verticillium alone. Two plants of each cultivar served as controls with no nematode or Verticillium treatment.

Likewise, only two seedlings of the other maple species were used for each treatment. I noculations were made with both races of M. arenaria and with M. incognita on 15 May 1991, and the Verticillium inoculations were made on 27 June 1991.

Results and Discussion

Data on the response of the Norway and red maple clones to inoculation are presented in Table 1. Neither of the wilt-susceptible cultivars of Norway maple (‘Crimson King’ and ‘Greenlace’) were parasitized by any of the nematodes, but ‘Crimson King’ appeared to be more susceptible to Verticillium than ‘Greenlace’. Of the two wilt-tolerant cultivars, ‘Jade Glen’ proved to be susceptible (galls and egg masses produced) to three of the four nematodes and five of the six plants subjected to these nematodes and the Verticillium fungus exhibited “total wilt.” On the other hand, the wilt-tolerant ‘Parkway’, although galled by two of the nematodes, did not suffer any leaf loss or wilt. Thus, it appeared that although the successful reproduction of root-knot nematodes on Norway maples were not necessary for the expression of wilt symptoms, certain “wilt-tolerant” but nematode-susceptible clones could suffer severe wilt as result of the synergistic action of both pathogens. Interestingly, on three of the five ‘Jade Glen’ plants that exhibited total wilt, one or more symtomless epicormic branches developed near the base of the plants. Such behavior would indicate a tendency of this cultivar to be able to “compartmentalize” the Verticillium infection.

With the exception of the ‘Autumn Flame’ - M. incognita combination, all three red maple clones were highly susceptible to all of the nematodes. However, none of the red maples exhibited any wilt symptoms (Table 1).

Of the maple seedlings inoculated with Verticillium and root-knot nematodes, only Norway maple showed evidence of a synergistic effect of the two pathogens. One each of the two plants inoculated with M. arenaria-Race 1, M. incognita, and M. javanica were killed by the fungus-nematode combination. It was not possible to determine the level of nematode infestation on the dead root systems of these plants. The potentially high degree of variability in nematode susceptibility among seedlings of the same progeny noted earlier (9), would suggest, however, that the dead seedlings had been more extensively parasitized than those that had survived.

The Oregon maple (A. macrophyllum) seedlings inoculated with Verticillium alone were leafless and virtually dead 30 days after treatment. Terminal growth on both of the seedlings that had been inoculated with Verticillium and M. arenaria-Race 1 or M. incognita was also killed within 30 days, but none of the leaves below the terminal area were lost during the next three months. The seedlings inoculated with Verticillium and M. arenaria-Race 2 did not exhibit any wilt symptoms.

No seedlings of the other maple species lost any leaves during the four-month period following Verticillium or Verticillium-nematode inoculation. The seedlings of A. negundo and A. saccharum were practically symptomless and, while there were minor variations among treatments in A. pseudoplatanus and A. velutinum, none of the nematode-inoculated plants exhibited more foliar symptoms than those inoculated only with the Verticillium fungus.

Conclusions

This is the first report of synergism between root-knot nematodes and Verticillium wilt in woody plants. The host range of Verticillium wilt is extensive and we are just beginning to expand our knowledge of root-knot nematodes. Atthis juncture, such synergism would appear to play a minor role among the many maladies of urban and landscape trees, but the door is now open for more detailed studies on a wider range of woody plants.