Abstract
Little is known about the influence of N, P, and K on the growth and movement of Verticillium in the vascular systems of woody plants. Nutrient-depleted Acer saccharum trees were fertilized with six treatment preparations containing varying concentrations of N, P, and K. Following the response of the trees to the fertilization, the trees were inoculated with Verticillium dahliae. Growth response of the trees to fertilization and the extent of colonization of the trees by the pathogen were determined. Growth of the trees was correlated to the concentration of nitrogen in the fertilizer and was described by the regression equation Y = 76.71 + 1.16X (r = .306). Phosphorus was not correlated to new growth of the trees. The effect of potassium was negatively correlated to growth and was described by the regression equation Y = 129.98 - 2.25X (r = -.456). The concentration of potassium was significantly correlated to the colonization of inoculated trees with the relationship described by the regression equation Y = 100.7 - 2.141X (r = -.579). Nitrogen and phosphorus concentrations were not correlated to pathogen colonization of inoculated trees.
Verticillium dahliae Kleb., a pathogen of Verticillium wilt, is extremely destructive to Acer saccharum Marsh, and numerous other tree species in landscape sites and nurseries. In nurseries, plants with symptoms are rogued out and destroyed, but symptoms frequently develop in infected plants after being transplanted into the landscape. Most homeowners and landsape managers prefer to treat infected trees rather than replace them. However, other than the cultural practices of watering, pruning and fertilizing, no methods of control have been effective in the treatment of Vert/c////um-infected trees. Even those methods have met with limited success. Fertilization to a balance of major nutrients (4) and fertilization with urea to promote vigorous growth (3) have been successful under some circumstances.
Little is known about the growth and movement of Verticillium in the vascular systems of woody plants. Sinclair, et al. (5) determined the movement of V. dahliae in inoculated A. rubrum stems in relation to the recurrence of acute symptoms and the ability of the trees to compartmentalize the pathogen. Smith and Neely (6) determined the extent of linear colonization of inoculated twigs by V. dahliae to assess the relative susceptibility of several species to the pathogen.
Dwinell (2) has shown that major nutrients escalate the localization of Verticillium in infected trees. Sinclair, et al. (5) suggest that remission of acute symptoms in infected trees, following pruning and fertilization, results from the trees’ enhanced compartmentalization response.
The objective of this study was to determine the effect of nitrogen, phosphorus, and potassium levels on the growth of A. saccharum trees and the resulting influence of the nutrient levels on the growth of V. dahliae in inoculated stems of differentially fertilized trees.
Materials and Methods
Bare-root one-year old sugar maple (A. saccharum) trees were selected for this study. They were potted into three-gallon plastic containers using a 60-20-20 soilless mix of graded pine bark, sphagnum peat moss, and sand. Lime was added to the medium to adjust the pH to 6.8. Thetrees were grown two years without fertilization. Each year they were maintained in a green-house and watered as necessary during the spring and summer, then moved outside to a cold frame to overwinter. The trees were pruned to produce a central leader and reduce side branching. At the beginning of the third growing season, the trees were moved into a greenhouse with 53% shade and watered with an automatic drip irrigation system to provide uniform watering of all the plants.
Tree growth response
Twenty trees were selected at random from a uniform group for inclusion in each of seven fertilizer treatments. The trees were fertilized with one of six liquid fertilizer preparations. Water served as a control treatment. The nutrient concentrations of the treatments were (1) 20-19-1 8, (2) 15-10-30, (3) 15-30-15, (4) 12-6-6, (5) 10-52-8, and (6) 33-6-6 (Table 1). The six fertilizer preparations were applied five times at 3-4 week intervals. Each aplication consisted of 14.2 grams of fertilizer dissolved in one liter of water and uniformly distributed over the surface of the media in each container. Treatments were applied in a completely randomized design. Following the fifth fertilization, the trees were measured for length of new growth of the central leader.
Fertilizer treatments applied to Acer saccharum trees at 3-4 week intervals.
Pathogen growth response
Twelve trees from each treatment were selected for inclusion in an inoculation study to determine the effect of differential fertilization on the growth of V. dahliae. Trees were selected that exhibited the greatest response to fertilization and appeared to be the healthiest in each treatment. The trees were inoculated with V. dahliae three days after the last fertilization treatment. A hole was drilled through the central leader of each tree approximately ten cm above the soil line with a 5/32 in. (3.96 mm) bit. Two drops of inoculum suspension were placed into the hole, and the hole was covered with Parafilm to prevent dessication. The inoculum suspension was a mixture of conidia and mycelial fragments that was produced by comminution of three isolates of V. dahliae in 500 ml of distilled water in a Waring blender. The isolates were grown on potato dextrose agar at 24C. The inoculated plants were arranged in a completely randomized design on benches in the greenhouse.
The linear distal growth of the pathogen in the central leader of each inoculated tree was determined using a two-stage isolation procedure 40 days after inoculation. The trees were excised at the point of inoculation and the main leaders were marked at 5 cm intervals. In the first stage of the procedure, isolations were attempted from the stems at 15 cm intervals. The stems were dipped in 95% alcohol and flamed. The bark was stripped away using a flamed knife. Cross-sections of the stems were asceptically made with flamed pruning shears and placed in Petri dishes containing a medium selective for Verticillium (1). The culture dishes were incubated at 18C for 12-14 days. They were observed with a dissecting microscope to determine the presence or absence of Verticillium. Immediately following the first stage isolation the stems were cut into five cm sections, marked for reference, and stored at 4C for use in the second stage isolation procedure.
The first stage isolation located the extent of the pathogen growth within a 15 cm interval. When this location had been determined, isolations were attempted on sites five cm proximal and five and ten cm distal to the most distal first stage location. The isolation technique described above was used.
Results and Discussion
Tree growth response
Growth response of A. saccharum to the fertilizer treatments was determined by the length of new growth of the central leader of each plant in each treatment. Measurements of new central leader growth lengths were subjected to analysis of variance.
Trees receiving treatment number six, which contained the highest concentration of nitrogen (33-6-6), had the greatest mean new growth, 132.3 cm (Fig. 1). However, the new growth mean was not statistically different from those of trees receiving treatments five (10-52-8) and three (15-30-15). Trees receiving the control treatment, number seven, had the smallest mean new growth, 29.0 cm, but this was not significantly different from the mean new growth of trees in treatment two (15-10-30). The new growth means for all treatments formed an overlapping array with no isolated values.
Growth response of A. saccharum to fertilization. Plants were fertilized five times at 3-4 week intervals.
A simple analysis of the influence of the individual nutrients on new growth was conducted, ignoring the effect of the other nutrients or the interaction of the three. The extent of new growth was correlated (.05 level) to the nitrogen concentration in the treatments (r = .306) and was described by the regression equation Y = 76.71 + 1.16× (Fig. 2). Phosphorus concentration was not correlated to new growth. However, potassium concentration was correlated (.05 level) to new growth (r = -.456) and was described by the regression equation Y = 129.98 - 2.25X (Fig. 3). A multiple regression analysis of the effects of all nutrients combined did not provide more information about the growth of the trees in this study. Increasing nitrogen concentrations resulted in corresponding increases in growth of the fertilized trees. The negative correlation of potassium concentration on growth can be understood from the standpoint that this nutrient affects metabolic processes not directly related to linear growth of the trees.
Effect of Nitrogen Concentration on the Growth of A. saccharum.
Effect of Potassium Concentration on the Growth of A. saccharum.
Pathogen growth response
The length of pathogen colonization of the central leaders of the inoculated trees was determined. The values were subjected to analysis of variance with significance stated at the .05 level.
V. dahliae colonized the trees in treatment number three (15-30-15) to a greater extent, 80.4 cm, than the trees receiving the other treatments (Fig. 4). However, the mean colonization distance was not significantly different from those for treatments one (20-19-18), six (33-6-6), five (10-52-8), or four (12-6-6). Colonization was least in the control treatment trees, 27.5 cm. The pathogen growth in the fertilized trees was significantly less in treatment two (15-10-30) which contained the highest potassium concentration. In all of the inoculated trees, the isolations of the pathogens were continuous along the stems. It was concluded that movement of the pathogen in each case resulted from linear growth.
Linear Colonization of A. saccharum by Verticillium dahliae in response to fertilization.
The pathogen growth response in the fertilized trees, excluding the control treatment, was analyzed for linear regression relating to nutrient concentration. The concentration of potassium was significantly correlated to the growth of the pathogen (r = -.579) with the relationship described by the regression line Y = 100.7 - 2.141X (Fig. 5). Similar analysis of the effect of nitrogen and phosphorus concentrations on pathogen growth resulted in nonsignificant correlation values. The control treatment was excluded from these analyses because the concentrations of the nutrients in the media were not determined. Increasing potassium concentrations suppressed the linear growth of the pathogen in inoculated trees.
Effect of Potassium Concentration on the Colonization of A. saccharum by Verticillium dahliae.
The results of this study indicate that treatment of Verticillium-infected trees should include fertilization with a formulation containing abundant potassium. This study indicates that increased potassium concentrations result in a reduction of host growth and a suppression of pathogen growth. Suppression of pathogen growth in inoculated trees was not related to a reduction of nitrogen concentration since pathogen growth was suppressed in the treatments containing high nitrogen concentrations. The mechanism of pathogen suppression is not evident from this study but it seems that pathogen suppression is desirable, even with an accompanying reduction in tree growth.
Compartmentalization of the pathogen was not examined in this study. However, an increased compartmentalization response would account for the pathogen growth reduction observed. The potassium concentrations used in this study were not sufficiently high to be toxic or inhibitory to the pathogen. More study is necessary to determine the extent of pathogen growth suppression by potassium and the mechanism involved in the suppression.
Acknowledgement
The author wishes to express his appreciation to the International Society of Arboriculture Research Trust which supported this study.
- © 1983, International Society of Arboriculture. All rights reserved.
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