Cypress Canker Control With Fungicides

Materials and Methods

Conidia were produced on propyleneoxidesterilized needles of Sequoia sempervirens imbedded in 1.5% water agar in petri dishes. Conidia were washed from the plates and adjusted to give 1 × 10⁵/ml. Fungicides were incorporated into melted potato-dextrose-agar (PDA). One ml of conidial suspension was applied to the surface of the hardened agar in 9 cm plastic petri dishes. Germination counts were made 24 hr following inoculation. Circular discs 4 mm in diameter cut from 10-day-old PDA plates were placed in the center of 9 cm plastic petri dishes containing solidified PDA in which the fungicides were incorporated. The diameter of growth was measured 10 days later. Incubation of all plates was in a fluorescent lighted laboratory where the temperature varied from 21-22 C.

Cypress trees were grown in a sand-peat potting mixture in 20 × 22 cm plastic containers. Greenhouse-grown plants were fertilized daily with the irrigation water and the plants grown out-of-doors were fertilized using 14-14-14 slow release fertilizer. Plants were 70 to 90 cm high when sprayed and inoculated. Trees were wounded in the main stem by making ten incisions 1.0 to 1.5 cm long in the stem extending into the xylem. After wounding, 5 plants for each treatment were sprayed to run off with the fungicides. Chlorothalonil was applied at 1.35 g/liter, benomyl at 0.3 g/liter and tribasic copper sulfate (53% Cu) at 6.0 g/liter. The control trees were sprayed with water.

In the first trial conducted in February 1981 with C. leylandii, wounds were inoculated by spraying a conidial suspension 24 hours after fungicide application. The trees were out-of-doors and were covered with polyethylene bags for 48 hours following inoculation. Disease evaluations were made in May, 90 days following inoculation.

The second trial was initiated in September 1982 in the greenhouse where the temperatures ranged from 20 to 26 C. C. macrocarpa trees were wounded as before and the same three fungicides were sprayed following wounding. The sprays dried in one hour and the wounds were then inoculated as before. The plants were covered with polyethylene bags for 48 hours. Disease evaluations were made 77 days after inoculation.

Results

Chlorothalonil prevented spore germination at all levels tested and retarded mycelium growth (Table 1). Benomyl allowed germination of conidia but the germ tubes were short and distorted and the spore germination process did not proceed further. No radial growth of mycelium occurred on benomyl-amended PDA (Table 2). Tribasic copper sulfate had no effect on spore germination and radial growth of mycelium was not appreciably affected except at 1000 g/liter.

Benomyl and chlorothalonil provided a high degree of control of cypress canker (Table 3). Tribasic copper sulfate was no better than the control in the greenhouse test and the amount of cankering was greater than the control in the out-of-doors trial with C. leylandii (Table 3).

Discussion

Wagener and Dimock (8) did not present data to support their recommendation for spraying bordeaux mixture, although they stated that the recommendations were based on a series of laboratory and field tests extending over a period of 2 years. Marchetti and D’Aulerio (3) also recommended a copper fungicide (copper oxychloride) as a treatment to prevent canker of C. sempervirens.

In this trial, tribasic copper sulfate failed to prevent spore germination and growth on PDA, and failed to prevent infection. It seems obvious that the fungus used in this trial is not sensitive to cupric ions. It would be of interest to learn if the fungus used by the Italian researchers (2,3,4) was sensitive to cupric ions.

Since benomyl was effective in all of the trials in which it was used, it would seem to be the best fungicide to use in a control program.