Reduction of Phytophthora Blight of Madagascar Periwinkle in Florida by Soil Solarization in Autumn R. J. McGovern, University of Florida, IFAS, Gulf Coast Research and Education Center, Bradenton 34203; R. McSorley, Department of Entomology and Nematology, University of Florida, IFAS, Gainesville 32611; and R. R. Urs, Southwest Florida Research and Education Center, University of Florida, IFAS, Immokalee 34142
ABSTRACT McGovern, R. J., McSorley, R., and Urs, R. R. 2000. Reduction of Phytophthora blight of Madagascar periwinkle in Florida by soil solarization in autumn. Plant Dis. 84:185-191. Three field experiments were conducted in southwest and west-central Florida in 1993 through 1995 to evaluate the effectiveness of soil solarization during autumn in reducing Phytophthora blight of Madagascar periwinkle (Catharanthus roseus) caused by Phytophthora nicotianae. Plots (3.6 by 3.6 m) were infested by incorporating winter wheat seed containing P. nicotianae in the upper 15 cm of soil. Solarization was then conducted for 21 to 41 days, primarily during October, using clear, 25- or 50-µm low-density polyethylene mulch. The progress of Phytophthora blight, monitored for 31 to 42 days following planting, was significantly reduced by solarization in all experiments, and final blight incidence was reduced in two of three experiments. Solarization also reduced population densities of P. nicotianae. Additional keywords: cool-season soil solarization, diseases of ornamentals, Dolichodorus heterocephalus, flooding, low-impact agriculture, Paratrichodorus minor, plant-parasitic nematodes, subtropical climate, sustainable landscape pest management
Madagascar periwinkle (Catharanthus roseus, also called “vinca”) is a bedding plant widely used in Florida and the southern half of the United States because of its drought tolerance, adaptability to full sun or partial shade, and ever-increasing array of colors. Although actually a perennial, periwinkle is most often grown as a fall and winter annual in central and south Florida. One of the most damaging diseases of C. roseus in the state is Phytophthora blight caused by the fungus Phytophthora nicotianae (syn. = P. parasitica). Symptoms of the disease include a grayish-brown discoloration of shoot tips and foliage, followed by wilting, necrosis, and rapid plant death. One shoot or the entire plant may be initially infected. Total losses of landscape plantings are not uncommon and considerable losses in the production of periwinkles as transplants and potted plants have also been observed (R. J. McGovern, unpublished data). P. nicotianae may survive in the soil as chlamydospores or in association with plant material (17). Naturally occurring inoculum densities of the pathogen were Corresponding author: R. J. McGovern E-mail:
[email protected] Florida Agricultural Experiment Station Journal Series No. R-06945. Accepted for publication 22 October 1999.
Publication no. D-1999-1221-01R © 2000 The American Phytopathological Society
found to be highest in the upper 10 cm of soil, although the pathogen has been detected at a depth of 60 cm (20,37). Phytophthora blight in periwinkle is favored by high humidity, surface moisture, and temperature (24). Hence, landscapers avoid growing periwinkle during the rainy summer months in Florida because of their inability to control Phytophthora blight during this highly disease-conducive period. Blight of C. roseus and other Catharanthus spp. caused by P. nicotianae has occurred in other states (21,24,25, 28,33) and internationally (4,14,36). This pathogen routinely causes severe losses to other landscape plants, such as petunia and snapdragon, in Florida and has a broad host range which includes many other important horticultural crops (1,27). Systemic fungicides such as metalaxyl (Ridomil) and fosetyl-al (Aliette) are applied in commercial production areas and landscape sites for control of Phytophthora blight of periwinkle. However, resistance to metalaxyl has been reported in P. nicotianae isolated from periwinkle (22), and neither fungicide is directly available to homeowners or other unlicensed personnel. In addition, metam sodium (Vapam) was designated several years ago as a restricted-use pesticide by the Environmental Protection Agency, thereby eliminating the last available preplant fumigant available to unlicensed landscapers and homeowners for control of soilborne pests, including Phytophthora spp. Soil solarization has been shown to be an effective, low-impact, and sustainable
method for reducing many soilborne pathogens and other pests of primarily food crops (32). Application of solarization to diseases of landscape ornamentals has been limited to management of Pythium collar rot and dieback of periwinkle in India (34). Soil solarization has almost exclusively been used during summer in hot, arid to semiarid temperate or tropical regions with limited rainfall and cloud cover (38). Solarization conducted in summer reduced Phytophthora nicotianae on tomato and carnation in glasshouses in Egypt and Italy, respectively (23,47). Summer solarization also reduced P. capsici in pepper in Italy and Turkey (45,52) and buried inocula of P. cactorum and P. citricola in California (26). P. nicotianae was eliminated from a nursery potting mix in Australia through solarization (16). A distinct and intense rainy season with frequent afternoon showers is the norm from June through September in peninsular Florida, the period during which previous solarization experiments have been conducted in the state. Solarization carried out during the summer in Florida was effective against some plant pathogens but ineffective or produced inconsistent results against others. Summer solarization in north Florida significantly reduced naturally occurring populations of the reniform nematode (Rotylenchulus reniformis) and buried packets containing P. nicotianae, but produced variable results with the bacterium Ralstonia solanacearum, the root-knot nematode (Meloidogyne incognita), and the fungi Fusarium oxysporum f. sp. lycopersici and F. oxysporum f. sp. radicis-lycopersici (9–11,13). The technique consistently decreased incidences of Verticillium wilt caused by Verticillium dahliae and increased tomato yield, but was inconsistent against the root-knot nematode (M. incognita) and failed to reduce the incidence of Fusarium wilt caused by F. oxysporum f. sp. lycopersici in westcentral and south Florida (40,42,44). On the other hand, summer solarization in west-central Florida reduced population levels of the awl (Dolichodorus heterocephalus) and stubby-root (Paratrichodorus minor syn. P. christiei) nematodes in comparison with a summer cover of native weeds (42). Populations of the sting nematode (Belonolaimus longicaudatus) were also reduced and yield increased in strawberry (43) by summer solarization in Plant Disease / February 2000
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west-central Florida, but it was ineffective when used against Fusarium crown and root rot in tomato (F. oxysporum f. sp. radicis-lycopersici) in southwest Florida (39). Our objective was to evaluate the effectiveness of soil solarization to reduce Phytophthora blight of periwinkle in peninsular Florida when conducted in autumn, a drier, yet still relatively hot period. MATERIALS AND METHODS Inoculum production. An isolate of Phytophthora nicotianae from periwinkle was used to produce inocula for in vitro thermal inactivation studies and field experiments. For field studies, 100 5-cm plugs from a 1-week-old corn meal agar (CMA) culture of the fungus were aseptically mixed with 907 g of winter wheat seed previously autoclaved with an equal amount (vol/wt) of deionized water. Flasks containing the inoculum were resealed with aluminum foil and Parafilm to prevent contamination, and incubated at room temperature (approximately 24°C) for 1 month. In vitro thermal inactivation. Because of the lack of data on the thermostability of P. nicotianae, a preliminary experiment was conducted to examine the survival of the test isolate of P. nicotianae at a range of temperatures, including those typically achieved by soil solarization. Time/temperature relationships for thermal inactivation at 38, 40, 45, 47.5, 50, 55, and 60°C utilized a hot-water bath and glass test tubes containing potato dextrose broth. Five 4-mm agar plugs from the margins of 7-day-old CMA cultures of P. nicotianae
were placed into test tubes in the water bath. Two test tubes were removed at preset times, which ranged from 24-h intervals at 38°C to 1-min intervals at 60°C. The agar plugs from the retrieved tubes were then placed on CMA plates. The plates were incubated at 26°C for 4 days and monitored for the viability (+ or – growth) of P. nicotianae. Thermostability experiments were generally conducted two or three times at each temperature. Experiment 1—1993. A landscape site in southwest Florida (Naples) formerly grown in bahia grass, was rototilled, cleared, graded, and divided into six plots (3.6 by 3.6 m) separated by 1.5-m buffers. Infested wheat seed (0.56 kg dry weight) was uniformly dispersed over the surface of each plot and incorporated in the upper 15 cm of soil on 27 September. The site was then irrigated to a depth of approximately 30 cm. The experiment utilized a randomized complete block design with three replications of the two treatments (solarization versus bare soil). Three plots were covered with 50 µm of clear, stabilized polyethylene mulch (Polyon Barkai, Kibutz Barkai, Israel). The mulched soil was solarized for 3 weeks; the remaining three plots served as nontreated controls. Maximum soil temperatures at 5, 15, and 23 cm were measured daily in single solarized and nonsolarized plots between 3:00 and 4:00 P.M., using soil thermometers, to estimate the thermal inactivation potential of solarization. The mulch was removed on 18 October and 10 500-cm3 soil samples were obtained from each plot
Fig. 1. Time and temperature exposures required to inactivate 100% of the propagules of Phytophthora nicotianae in nutrient broth. 186
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to a depth of 15 cm for enumeration of P. nicotianae. Samples were pooled, mixed, and assayed for Phytophthora spp. propagules using 13 plates per plot of a selective medium (29) and a soil-plating procedure (41). An application of fertilizer (10N:4.4P: 8.3K) at a rate of 0.454 kg/0.09 m2 was broadcast on each plot, and 58 to 64 2month-old transplants of C. roseus cv. Peppermint Cooler were planted using a 46-cm spacing between plants immediately following mulch removal. Irrigation was applied as necessary to the plots using four overhead sprinklers positioned at each corner of the experimental site to ensure uniform coverage. Plants were initially monitored for incidence of Phytophthora blight every 2 to 3 days during the first 3 weeks of the experiment, and once per week thereafter. A final count of blighted plants was made on 29 November, 41 days following planting. Representative plant samples were taken from both solarized and nonsolarized plots for confirmation of infection by P. nicotianae using the Phytophthora-selective medium. Experiment 2—1994. A field at the Southwest Florida Research and Education Center (Immokalee, FL), previously grown in sugarcane, was cultivated, cleared, graded, and divided into 10 plots (3.6 by 3.6 m) separated by buffers of at least 8 m. A higher inoculum level of P. nicotianae was used than in the first experiment to further challenge the effectiveness of solarization by lessening the chance of escape by individual plants from infection. On 30 September, 1.6 kg (dry weight) of wheat seed infested with P. nicotianae was uniformly dispersed over the surface of each plot and incorporated in the upper 15 cm of soil. The experiment utilized a randomized complete block design with five replications per treatment. Five plots were covered with 25 µm of clear, stabilized, low-density polyethylene mulch (Dow Agrosciences, Indiannapolis, IN), and the remaining five plots served as controls. Maximum soil temperatures were measured daily using soil thermometers as previously described. The mulch was removed after 4 weeks on 31 October and 10 250-cm3 soil samples were taken from each plot to a depth of 15 cm and analyzed individually (instead of pooled) for Phytophthora spp. densities, in an attempt to improve recovery over the previous year. On 2 November, 64 2-month-old transplants of C. roseus cv. Peppermint Cooler were planted in each plot using a spacing of 30 cm between plants. A closer spacing was used than in the previous experiment to promote greater plant-to-plant spread of P. nicotianae. Plants were irrigated through management of a perched water table (semiclosed, seep irrigation) which was maintained at a relatively shallow depth (approximately 25 cm) to enhance disease
development. Although this subirrigation method is not used in a landscape setting, other subirrigation techniques, such as drip irrigation via “soaker hoses”, are often employed. No supplemental fertilizer was used. Phytophthora blight incidence was monitored daily for 30 days and representative plant samples were taken from both solarized and nonsolarized plots for confirmation of infection by P. nicotianae. Experiment 3—1995. A field was used at the Gulf Coast Research and Education Center (Bradenton in west-central Florida) which had been cropped in vegetables for the previous 20 years. The drainage and cropping history of the site had allowed the buildup of a wide diversity of phytoparasitic nematodes (42,44). The field was cultivated, cleared, graded, and divided into 10 plots (3.6 by 3.6 m) separated by buffers of at least 8 m. On 29 September, 1.6 kg (dry weight) of infested wheat seed was uniformly dispersed and incorporated as previously mentioned. The experiment utilized a randomized complete block design with five replications and the same solarization methods as in experiment 2. Soil temperatures were monitored as before. The mulch was removed after 41 days on 9 November, and 10 250-cm3 soil samples were taken from each plot to a depth of 15 cm for enumeration of P. nicotianae propagules. Using a spacing of 30 cm between plants, 36 to 49 2-month-old transplants of C. roseus cv. Peppermint Cooler were planted in each plot. Plants were irrigated by subirrigation (seep irrigation) without supplemental fertilizer. Phytophthora blight incidence was monitored for 31 days, and representative plant samples were taken from both solarized and nonsolarized plots for confirmation of P. nicotianae. In addition, soil samples for analysis of naturally occurring nematodes were collected from each plot on 28 September (before solarization), 9 November (at planting), and 13 December (end of experiment). Each soil sample consisted of six soil cores (2.5 cm in diameter by 20 cm deep) collected in a systematic pattern from an individual plot using a soil sampling cone (18). The cores comprising a sample were mixed and stored in a plastic bag at 10°C for 2 to 3 days prior to extraction. Nematodes were then extracted from a 100-cm3 subsample using a modified sieving and centrifugation procedure (30). All individual specimens extracted were then identified and counted using an inverted microscope. Data analysis. The incidence of Phytophthora blight was plotted versus time for each experiment and the development of the disease was evaluated by calculating the areas under the disease progress curves (AUDPCs) using a computer program created by R. D. Berger (Plant Pathology Department, University of Florida, Gaines-
ville). Mean recovery of P. nicotianae, AUDPCs, final disease incidence, and nematode densities were compared using analysis of variance and Fisher’s least significant difference test (P ≤ 0.05) following arc sine transformation of percentage data where appropriate. RESULTS In vitro thermal inactivation. The relationship between the time required for in vitro inactivation of the isolate of P. nicotianae tested and temperature was linear (y = –0.17x + 10.3, r2 = 0.96, P = 0.001) when plotted using a log/linear scale (Fig. 1). The thermostability of the isolate ranged from 2 to 3 min at 55 to 60°C, 1 to 3 h at 45 to 50°C, and 70 to 120 h at 38 to 40°C. Experiment 1—1993. Rainfall and mean ambient air temperature during the solarization period in October were slightly above the 30-year historical means recorded at the Southwest Florida Research and Education Center, Immokalee (Table 1; 46). Soil temperatures recorded between 3:00 and 4:00 P.M. were consistently increased by solarization at all three depths (Fig. 2). Typical soil temperature decreases resulting from cold fronts or significant rainfall were observed in this and subsequent experiments. Maximum temperatures observed at 5, 15, and 23 cm under clear mulch and in bare soil were 41.6, 36.1, and 32.2°C, and 35.0, 30.2, and 27.8°C, respectively. Average daily maximum soil temperatures at 5, 15, and 23 cm in solarized and bare soil were 37.5, 33.7, and 29.9°C, and 32.4, 28.3, and 27.1°C, respectively. The post-plant period (November) was drier than normal, but normal temperatures prevailed (Table 1). Solarization reduced mean recovery of P. nicotianae; 0.0 CFU/g soil were detected in solarized plots compared to 53 CFU/g soil in nonsolarized plots (Table 2). However, reduction of P. nicotianae was not significant (P ≤ 0.05) because the microorganism was only recovered in one of three nonsolarized plots. Nevertheless, a high incidence of Phytophthora blight was rapidly observed in all periwinkles grown in control plots (Fig. 3). Disease incidence among nonsolarized plants reached 50%
within 14 days after planting and 69.5% by the conclusion of the experiment. Solarization reduced (P ≤ 0.05) both the progress of Phytophthora blight (AUDPC) and final blight incidence (Table 2). P. nicotianae was consistently recovered from symptomatic tissue in this and subsequent experiments. Experiment 2—1994. During the period of solarization, rainfall was slightly below and mean ambient air temperature slightly above the historical means recorded at the Southwest Florida Research and Education Center, Immokalee (Table 1). Soil temperatures recorded between 3:00 and 4:00 P.M. were consistently increased by solarization at all three depths (Fig. 2). The maximum temperatures observed at 5, 15, and 23 cm under clear mulch and in bare soil were 47.8, 37.7, and 33.3°C, and 37.7, 32.7, and 28.3°C, respectively. Average daily maximum soil temperatures at 5, 15, and 23 cm in solarized and bare soil were 41.7, 34.9, and 30.7°C, and 31.7, 29.2, and 26.1°C, respectively. Slightly higher-thanaverage temperatures prevailed during the post-plant period (Table 1). Precipitation during the post-plant period averaged 112% higher than normal, but almost all of the rainfall (96%) resulted from a tropical storm which occurred on 13 through 16 November. Rainfall greatly exceeded field capacity at the experimental site, resulting in the presence of surface water for at least 12 h. Although only 15.2 CFU/g soil were detected in solarized plots compared to 109.2 CFU/g soil from nonsolarized plots, this reduction in P. nicotianae was not significant because of variability in the data (Table 2). Initially, the progression of Phytophthora blight was quite similar to that seen in the previous and subsequent experiments. Blight increased rapidly in control plots, reaching 50% within 11 days after planting, while remaining minimal in the solarized plots (Fig. 3). However, the rate of blight increase greatly accelerated following flooded field conditions and all controls were dead 22 days after planting. Solarization significantly reduced (P ≤ 0.05) the progress of Phytophthora blight (AUDPC), but not final disease incidence (Table 2).
Table 1. Weather data recorded during solarization (October) and post-plant (November) periods, 1993 to 1995y Year, location 1993 Southwest Florida 1994 Southwest Florida 1995 West-central Florida y z
Period
Rainfall (cm)z
Mean ambient air temperature (°C)z
Solarization Post-plant
7.1 (6.6) 1.8 (4.9)
25.0 (24.4) 21.0 (21.0)
Solarization Post-plant
5.9 (6.6) 10.4 (4.9)
25.1 (24.4) 22.9 (21.0)
Solarization Post-plant
13.0 (7.4) 10.0 (4.9)
26.1 (23.8) 21.6 (20.5)
Weather data were recorded at the Southwest Florida Research and Education Center, Immokalee, in 1993 and 1994, and at the Gulf Coast Research and Education Center, Bradenton, in 1995. Numbers in parentheses represent 30-year (Immokalee) and 45-year (Bradenton) historic norms. Plant Disease / February 2000
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Experiment 3—1995. Although cumulative precipitation in Bradenton during October was 75% above the 45-year average (48), detectable rainfall occurred on only 17 of 42 days during the solarization period (Table 1, Fig. 2). Soil temperatures recorded between 3:00 and 4:00 P.M. were consistently increased by solarization at all three depths (Fig. 2). Maximum soil temperatures at 5, 15, and 23 cm under clear mulch and in bare soil were 49.0, 36.8, and 32.7°C, and 35.0, 29.4, and 28.3°C, respectively. Average daily maximum soil temperatures at 5, 15, and 23 cm in solarized and bare soil were 38.4, 33.6, and 29.8°C, and 28.8, 26.8, and 25.8°C, respectively. The post-plant period was wetter than normal, but precipitation occurred on only 6 of 40 days and field flooding never occurred (Table 1). Solarization dramatically reduced mean recovery of P. nicotianae; 0.0 CFU/g soil were detected in solarized plots versus 290 CFU/g soil in nonsolarized plots (Table 2). This reduction was not significant because the pathogen was not recovered from one of the nonsolarized plots. However, further analysis of the combined propagule data from all three experiments (1993 to 1995), which were not significantly different from each other (P ≥ 0.3), indicated that solarization had significantly lowered (P ≤ 0.05) the average number of P. nicotianae propagules from 150.7 CFU/g soil to 3.06.
A high incidence of Phytophthora blight was rapidly observed in periwinkle grown in control plots (Fig. 3). Disease incidence of plants in nonsolarized plots reached 50% within 14 days after planting and 99% by the conclusion of the experiment. Solarization significantly reduced both the progress of Phytophthora blight (AUDPC) and final blight incidence (Table 2). Reductions in certain nematode populations also were observed after solarization. On 9 November, immediately following solarization, population densities of awl (Dolichodorus heterocephalus) and stubby-
root (Paratrichodorus minor) nematodes were lower (P ≤ 0.05) in solarized plots than in non-solarized control plots (Table 3). Population levels of these two nematodes remained suppressed in the solarized plots until the end of the experiment on 13 December. The other nematodes present were not affected by solarization. However, these nematodes showed natural declines in their population levels in fallow control plots from 28 September to 9 November and did not increase on periwinkle, with final population levels of
