Evaluation of Disease Management Strategies for Control of Phytophthora Crown Rot of Summer Squash

G.J. Holmes

Department of Plant Pathology, North Carolina State Univ., Box 7616, Raleigh, NC 27695

M.E. Lancaster

N.C. Coop. Ext. Service­Henderson County Center, 740 Glover Street, Hendersonville, NC 28792

P.B. Shoemaker and D.W. Pollard

Department of Plant Pathology, North Carolina State Univ., Box 7616, Raleigh, NC 27695

Additional index words. Phytophthora capsici, mefenoxam, dimethomorph

Abstract. Phytophthora crown rot (PCR) caused by Phytophthora capsici is one of the most destructive cucurbit diseases in North Carolina and the eastern United States. PCR has become serious in North Carolina during the last 5 to 10 years. Resistant cultivars are not available to growers. Cultural practices such as water management, crop rotation, use of raised beds and plastic mulch may provide a partial solution. Our research shows that when the most promising fungicides are used intensively and under a variety of application methods, they are not adequate to control the disease under conditions of high disease pressure. Fungicides such as Ridomil Gold, Acrobat, and mancozeb could be valuable components of an integrated disease management program under conditions of moderate to low disease.

 

Phytophthora crown rot (PCR) caused by Phytophthora capsici is one of the most destructive cucurbit diseases in North Carolina and the eastern United States. The disease was recorded in North Carolina as early as 1954, but has become a serious concern over the last 5 to 10 years. The disease usually starts in low, poorly drained areas of a field, especially after excessive rains. The pathogen can be dispersed in soil, with surface water, and via splash dispersal from soil to foliage. If both mating types of the fungus occur in the same field, the pathogen can produce oospores and persist in the soil for years. In summer squash the disease appears as a crown and fruit rot and occasionally infects the foliage. The pathogen has a wide host range including pepper, tomato, eggplant and cucurbits. Summer squash appears to be the most susceptible of the cucurbits.

Fungicides alone have not proven effective at limiting economic losses of summer squash under heavy disease pressure. Ridomil Gold EC (mefenoxam) (Novartis Crop Protection, Inc.) is registered for preplant and early postplant application to control phytophthora blight in pepper.

Ridomil Gold EC is labeled for use on cucurbits to control pythium damping-off, but not PCR. Thus, the manufacturer makes no claim of efficacy against PCR in cucurbits. Georgia, Michigan, and South Carolina have obtained emergency exemptions (Section 18s) in 1998 for application of Acrobat MZ (dimethomorph) (American Cyanamid Co.) to control diseases caused by P. capsici in cucurbits. Because the disease is so destructive and so prevalent in many states, and because no effective controls exist, the emergency exemptions were apparently granted without adequate field performance data.

Our objective was to evaluate the field performance of several disease management strategies with known or suspected efficacy against PCR in summer squash.

Materials and methods

Three field performance trials, including a summer planting and two fall plantings, were conducted in 1998 in the mountainous, western region of North Carolina. Both fields had several consecutive years of summer squash pro

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duction and high PCR pressure resulting in near total crop loss. Both fall tests were planted immediately following crop failures of summer squash due to PCR. The soil type at both sites was a Delanco loam (fine-loamy, mixed, mesic Aquic Hapludults).

Summer experiment. The experiment was conducted in a commercial grower's field in Henderson Co., N.C. (GPS coordinates: N 35o18.047; W 082o23.872). The field was selected based on the observation that PCR caused 70% loss in a summer 1997 squash crop. Treatments (Table 1) were randomized in four complete blocks. A preplant broadcast application of pulverized lime (1000 lb/acre) and fertilizer (500 lb/acre of 10­20­20) were made on 9 May. At planting soil pH was 6.6 and soluble salts were 0.26 mmhos. Yellow crookneck squash (var. Supersett) was direct seeded on 22 May into flat (nonridged) soil and thinned to a 24-inch in-row spacing upon emergence. Fertilizer was side-dressed (230 lb/acre of 15­0­14) on 13 June. No irrigation was used. Plots were 35-ft single rows with 5-ft end borders, a 36-inch between-row spacing, and separated by a nontreated row on each side. Fungicide treatments were applied using a manual, diaphragm-type Solo backpack sprayer equipped with a single nozzle (Conejet TXVS-26) and operating at 40 psi (90 gal/acre). Fungicides were applied from both sides of the plot (two passes per plot) to insure adequate coverage. Treatments began when plants had 2

to 3 true leaves and were applied on a 7-day interval with applications on 11, 18, and 25 June and 01, 09, and 15 July. Disease incidence, expressed as percent mortality due to PCR, was determined weekly (11, 18, and 25 June and 01, 09, 15, and 23 July). Percent mortality was calculated from the number of plants without symptoms of PCR compared to the original stand. Fruit were harvested weekly (1 to 23 July).

Fall experiments. Results from the summer experiment indicated that Acrobat and Ridomil Gold were good candidates to use in exploring different fungicide application methodologies (Shoemaker and Ray, 1996). With this in mind, the summer crop was followed with another experiment (at the same location) and replicated at a second site where near total crop loss due to PCR had just occurred. Hence, both fall experiments were planted into soil containing fresh inoculum. Application methodologies included preplant incorporated (PPI), postemergent drench (PED) and foliar applications against PCR. Treatments (Table 2) were randomized in 4 complete blocks. Plots were 35-ft single rows separated by a nontreated row on each side and a 54-inch between-row spacing. PPI treatments were applied on 21 Aug (site 1, same as summer experiment) and 25 Aug. (site 2; GPS coordinates: N 35o23.560; W 082o23.580) in a 16-inch band and tiller incorporated to a 3-inch depth. Yellow crookneck squash (var. Supersett) was direct seeded into ridged soil (18-inch-wide, 8-

Table 1. Summer experiment: percent mortality of summer squash on different dates as influenced by treatment.

Treatmentz Ratey 18 June 25 June 1 July 9 July 15 July 23 July AUDPCWx

Untreated --- 28.4 51.3 64.3 78.9 88.2 92.1 26.0 ab

Untreated (w/o Captan) --- 28.0 33.5 76.7 91.7 95.8 98.6 27.4 a

Ridomil Gold Bravo 1.53 1.10 13.0 27.8 31.3 50.1 61.2 11.1 c

Acrobat MZ 1.55 4.5 24.5 53.5 62.7 54.9 70.8 18.1 abc

Acrobat MZw 1.55 10.1 17.2 32.9 40.0 91.4 69.4 14.3 bc

Acrobat WP + Kocide 2000 1.61 20.7 37.3 51.1 60.6 65.4 81.9 19.6 abc

Dithane 75DF 2.40 7.5 19.0 35.0 47.0 53.0 71.0 14.1 bc

Aliette WDG 4.00 ­2.0v 17.4 60.3 92.2 91.4 100 22.8 abc

Tattoo C 0.90 ­2.5v 22.1 55.1 77.0 88.5 89.8 20.6 abc

Quadris 0.20 25.2 42.9 63.8 74.9 82.2 88.7 24.1 ab

Biocontrol seed treatment --- 38.9 54.4 81.0 89.3 98.2 100 29.5 a

zAll seed treated with Captan 400, except untreated, and untreated w/o Captan where it was rinsed off under running tap water for 10 min. This served as a control for the biocontrol seed treatment.

yEqual to lb a.i./acre.

xArea under the disease progress curve.

wFirst application was directed at the lower stem and alternated with a foliar application at 7-day intervals.

vNegative numbers are due to additional germination after initial stand count.

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Table 2. Fall experiments: percent mortality of summer squash on different dates as influenced by treatment.

TreatmentZ Ratey 11 Sept. 15 Sept. 18 Sept. 22 Sept.

Untreated --- 33.8 a 53.0 a 58.9 a 64.6 a

Ridomil Gold EC PPI 1.0 3.6 a 16.5 a 26.3 a 50.0 a

Ridomil Gold EC PPI+PED 1.0+1.0 5.0 a 15.3 a 26.1 a 33.9 a

Ridomil Gold MZ (foliar) 1.7 26.9 a 44.8 a 50.9 a 56.0 a

Acrobat WP PPI 2.0 23.9 a 43.8 a 57.5 a 63.4 a

Acrobat WP PPI+PED 2.0+2.0 33.6 a 55.3 a 63.1 a 68.1 a

Acrobat MZ WDG PPI+PED+foliar 2.0+2.0+1.6 14.7 a 32.6 a 44.4 a 56.2 a

Acrobat MZ (foliar) 1.6 25.2 a 44.3 a 55.8 a 67.8 a

zPPI = preplant incorporated; PED = postemergent drench.

yEqual to lb a.i./acre.

inch-high) at 1.5 seed per foot of bed. Sprinkler irrigation was used for stand establishment. Stands were not thinned. PED treatments were applied 7 and 14 Sept. in an 8-inch band centered on emerged plants at site 1 and site 2, respectively. Fertilizer was sidedressed (250 lb/acre of 15­0­0) on 19 Sept. at both sites. Fungicide treatments were applied using a manual, diaphragm-type Solo backpack sprayer equipped with a single nozzle. PPI and PED treatments used a Floodjet TK-SS5 nozzle (116 gal/acre) and foliar treatments were applied through a Conejet TXVS-26 nozzle (90 gal/acre, 40 psi). Foliar treatments began 7 days following PED treatments (14 and 22 Sept. for sites 1 and 2, respectively). Percent mortality was determined as in the summer experiment.

Results

Summer experiment. PCR was detected in an adjacent field of scallop squash before emergence of plants in the experimental plots. The original stand in experimental plots was excellent, but plants began dying shortly after the first treatments were applied (20 days after planting). Disease distribution was not uniform, but occurred in scattered clusters. Disease pressure was considered very high and conditions were conducive for disease development, especially during the first half of the experiment when rainfall was more frequent. Due to clusters of high disease incidence within plots, variability between replicate treatments was high, yielding significant block effects and reducing the ability to separate treatment effects. Differences in fruit yield were not significant.

Included in the summer trial was SSX P210 (Harris Moran Seed Company), a grey zucchini cultivar with tolerance to the crown rot phase of the disease. Of 49 plants, only 6 (12%) succumbed to crown rot while untreated guard rows on both sides completely collapsed from PGR by the conclusion of the trial. Unfortunately, fruit were quite susceptible to the disease and did not possess marketable characteristics (Infante et al., 1998).

Fall experiments. Note: At the time of manuscript submission these experiments were only partially completed. Complete data will be presented at Cucurbitaceae '98. Although site 2 was not ready for evaluation, PCR was beginning to infect plots.

At site 1, disease pressure was high. While there were obvious fungicidal effects noted in the Ridomil Gold PPI and PPI+PED treatments, variability between replicates was such that no statistical separation of treatment means was detected. Ridomil Gold PPI and PPI+PED treatments resulted in a much lower percent mortality in the first few ratings. However, by 3 weeks after application, treatment effects appeared to diminish. No other treatments showed any effects.

Discussion

There is a tremendous need for effective PCR management strategies. The disease is very destructive and prevalent along the eastern United States and elsewhere. Some cultivars are resistant to crown rot, but not to fruit rot. Moreover, they lack commercially desirable fruit characteristics (Infante et al., 1998). Cultural practices such as water management, crop rotation, use of raised beds and plastic mulch may pro

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vide a partial solution. Our research shows that when the most promising fungicides are used intensively and under a variety of application methods, they are not adequate to control the disease under conditions of high disease pressure. Resistance to Ridomil Gold in P. capsici may partially explain the reduced efficacy of this fungicide (Parra and Ristaino, 1998). Fungicides such as Ridomil Gold, Acrobat and mancozeb could be valuable components of an integrated disease management program under conditions of moderate to low disease.

Literature cited

Infante, M.L., S.A. Johnston, and S.A. Garrison. 1998. Evaluation of squash cultivars for phytophthora crown rot and powdery mildew, 1997. Biol. Cultural Tests 13:183.

Parra, G. and J. Ristaino. 1998. Insensitivity to Ridomil Gold (mefenoxam) found among field isolates of Phytophthora capsici causing phytophthora blight on bell pepper in North Carolina and New Jersey. Plant Dis. 82:711.

Shoemaker, P.B. and J.H. Ray. 1996. Evaluation of Acrobat 50WP for tobacco black shank in Burley, 1995. Fungicide Nematicide Tests 51:275.

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