Phytophthora Root and Crown Rot of Franklinia Trees

Materials and Methods

Initial isolations of Phytophthora spp. were carried out on potato dextrose agar (PDA). Two-cm root segments (healthy roots exhibiting only a few lesions) and wood from advancing cankered areas were surface sterilized in 5.25% sodium hypochlorite for 0.5 to 1.5 min, placed on PDA and incubated at room temperature. Repeated isolations and reisolation from inoculated plants were subsequently carried out on a selective medium for pythiaceous fungi (5).

Studies of optimum, minimum and maximum growth temperatures were conducted on V-8 juice agar (V-8A) from 4-36C at 4-degree intervals. Plates were inoculated with 1 -cm diameter mycelial plugs from margins of actively growing colonies and incubated in the dark. The amount of linear growth was measured at 2-day intervals on four plates per temperature setting.

Mycelial characteristics were determined from young, vigorously growing portions of colonies on malt agar (MA) and from older mycelium grown on PDA, corn meal agar (CMA) or V-8A media (6).

Induction of sporangial production was attempted by different methods, e.g., by placing mycelial plugs and baited fruit flies in distilled water, nonsterile pond water (pond: distilled, 1:2) (6), Petri’s mineral solution (6), and by Hwang’s method (2). Induction of sexual reproductive organs was carried out on V-8A containing 200 ml V-8 juice with 3 g CaCO₃ cleared by filtering through a cake of celite 545 (Fisher Scientific Co.), 200 ml; 0.1 g B-sitosterol dissolved in 20 ml of hot 95% ethanol; 20g agar and 800 ml distilled water. The isolates were also paired with each other and with two different sets of known A¹ and A² compatibility types of P. cinnamomi and incubated at room temperature in the daylight. The Phytophthora isolates were then identified from keys and descriptions by Waterhouse (6,7) and Novotelnova (3).

To establish the pathogenicity of the Phytophthora isolate, rooted Franklinia cuttings were inoculated. The fungus was grown in 250-ml flasks, containing 50 ml of V-8 broth, (100 ml V-8 juice, 3 g CaCO₃ and 1 liter distilled water) for 6 weeks. Forty mycelial mats drained free of medium were homogenized for 20 sec at low speed, and the volume was brought up to 1 liter with distilled water. Nineteen healthy, rooted cuttings grown in 10-cm pots were inoculated by gently scraping away the top layer of soil around the crown, applying 40 ml of the inoculum (approximately one mycelial mat per pot), replacing the soil and watering. An uninocualted check of 19 cuttings was treated in a similar manner with distilled water substituted for inoculum. Reisolation of the organism from wilted cuttings in this and subsequent inoculation experiments was accomplished and described previously.

The comparative ability of a Phytophthora isolate from Franklinia and a virulent P. cinnamomi isolate from rhododendron to cause typical wilt of rooted Franklinia cuttings was also investigated. Inoculum was grown and applied as described above. Five rooted cuttings were used per isolate and per uninoculated check.

Crowns of 3-year old-Franklinia trees were inoculated by making a 1-cm longitudinal incision through the bark with a sterile scalpel and inserting an agar-mycelium block of the Phytophthora isolate into the wounds. The wounds were then covered with cotton and moistened with distilled water to prevent premature dehydration of the inoculum plugs. Check plants were wounded in the same manner with a small ball of moistened cotton substituted for inoculum. Crown inoculation without wounding the bark was attempted by placing an agar mycelium block directly on the bark surface and covering it with moistened cotton. Five trees were used per isolate and check.

Two experimental fungicide treatment series to control Franklinia wilt were conducted in 1975-1976 before the causal organism was definitely determined. The fungicides used and their respective rates/liter were: 5-ethoxy-3-tri-chloromethyl-1,2,4-thiadiazole (Truban 25EC), 0.7 ml; 5-etho×y-trichloromethyl-1,2,4-thiodia-zole, 15% and dimethyl 4, 4-0-phenylenebid (3-thioallophanate), 25% (Banrot 40WP), 1.0 g; 2-chloro-6-methoxy-4-(trichloromethyl) pyridine (Nurelle, Dow Chemical Co., Moorestown, N.J.), and methyl 1 -(&butylcarbamoyl)-2-benzimidazole-carbamate (Benlate 50W), 1.2 g. Fifty 3-year-old Franklinia trees, growing in 7.6-liter containers, received the first series of treatments on 5 May, 23 July and 5 September 1975. The treated trees were selected from a group of trees with a wilt history and arranged randomly. There were 10 plants per trėatment. Each fungicide solution was applied at 1 liter/container and watered in after treatment. A nontreated water check was also included.

A second group of 50 trees from a relatively disease-free block was treated on the same dates, but also inoculated on August 8, 1975 with a Phytophthora isolate from Franklinia. The fungus inoculum was grown on V-8 broth for 6 wk, then 50 mycelial mats were homogenized in a blender and the volume was brought to 2.5 liter with distilled water. Inoculum was applied at the rate of 50 ml/7.6-liter container and watered in. Disease ratings were taken monthly until the end of the growing season and again in June of the following year. Winter damage was also noted in each treatment.

Results

Discussion

Franklinia root rot and crown canker has become increasingly important in propagating beds and in container-grown trees in nurseries but as yet has not been observed in well-established field-grown plantings. The disease problem has been reorganized by growers in the past, but the cause has not been previously reported. Phytophthora was consistently isolated from diseased plants and, when inoculated into healthy plants, produced typical disease symptoms.

The root rot phase of the disease is more common than the canker phase. In general the disease is most prevalent on plants grown in heavy or poorly drained soil, which favors the root rot phase. Also rooted cuttings appear much more susceptible to the root rot phase than 3-yar-old container-grown trees. Evidence that Franklinia crowns were susceptible to this fungus by wound inoculations, but resistant without wounds, would indicate that the crown canker phase of the disease would not likely occur in nature unless trees were mechanically damaged.

Evidence from these tests and general observations of other Franklinia plantings would indicate a relation between wilt and susceptibility to winter damage. Usually, trees showing wilt late in the summer but living through the winter were more subject to dieback attributed to winter damage. Conversely trees that exhibited winter damage were more susceptible to wilt. Apparently physiologically weakened plants are more susceptible.

The general morphology of the Phytophthora isolates from Franklinia when grown under the test conditions reported, was similar to known isolates of P. cinnamomi. Although few cultural or pathogenic differences were noted among the Franklinia isolates, some strain differences may occur between these isolates and P. cinnamomi from other plants. Since an isolate of P. cinnamomi from rhododendron was also pathogenic to Franklinia, there was no host specificity indicated, although some differences in virulence between the isolates from the two sources was indicated.

Disease control was achieved with Truban, Banrot and Nurelle (an experimental compound now withdrawn from testing). Benlate was included originally in the experiment because of the nature of the root damage and in the event organisms other than pythiaceous fungi were involved in the root rot phase. Benlate was ineffective and appeared to enhance the disease when compared with the untreated check.