Effects of Fungicide Seed Treatment on Seedling Death of Pumpkin Caused by Phytophthora capsici

M. Babadoost and S.Z. Islam

Department of Crop Sciences, University of Illinois

Approximately 22,000 acres of pumpkins and 8,000 acres of cucumber, melon, and squash are commercially produced in Illinois. About 90% of processing pumpkins produced in the United States are grown in Illinois. Phytophthora blight, caused by the Oomycete plant pathogen Phytophthora capsici Leonian, has become a serious threat to pumpkin industry in Illinois.

Phytophthora capsici survives as oospores in soil and as mycelium in plant residue. The pathogen can infect all parts of the plant at any growth stage, causing pre- and post-emergence seeding damping-off, leaf spot, stem lesion, foliar blight, and fruit rot. In some years, pre- and post-emergence seedling damping-off caused by P. capsici on processing pumpkins in Illinois is so severe that growers may have to replant the fields for a second, and even a third time. Therefore, protecting plants against P. capsici during seedling emergence from soil and at the early growth stages is essential for successful plant stand establishment. The objective of this study was to develop effective seed treatment to protect pumpkin plants against P. capsici during stand establishment.

Materials and Methods

Greenhouse studies. Effects of mefenoxam (Apron XL LS) and metalaxyl (Allegiance FL) on seedling damping-off of pumpkin, caused by P. capsici, were studied in the greenhouse using a naturally infested soil and artificially infested soil mix (field soil:sand; 3:1).

Inoculum for soil infestation was prepared by culturing an isolate of P. capsici, collected from a processing pumpkin seedling in 2000, on oat meal-V8 juice broth substrate in 1-L conical flasks. The flasks were then incubated at 75 °F. After 6 weeks, the colonized oatmeal was added to a steamed-soil mix and mixed thoroughly. The inoculum density of P. capsici in naturally infested field soil and artificially infested soil mix was determined by the soil dilution-plate method using a Phytophthora selective medium (PARPH). The inoculum density in the naturally infested field soil was 90 cfu/g soil, and the inoculum density in artificially infested soils was 600 cfu/g soil.

Seeds of three processing pumpkin cultivers Dickinson, Hybrid-401, and Hybrid-698 were treated with Apron XL LS (0.64 fl oz/100 lb seed) and Allegiance FL (1.5 fl oz/100 lb seed). Plastic pots were filled with P. capsici-infested soil. Pots with non-infested soil were included as a control. The experiments were conducted in a greenhouse maintained at 64-77 °F and pots were watered daily beginning the first day of seeding. Seedling emergence was assessed 10 days after sowing seeds and seedling stand was determined 3 weeks after seedling emergence (31 days after seeding). Diseased seedlings were examined using light microscopy and infected tissues were plated on PARPH for isolating P. capsici.

Field studies. An experiment was conducted in an irrigated processing pumpkin field with a history of Phytophthora blight. The experiment was performed in a randomized complete block design with 3 replications, each consisting of a 25-footlong row. The plots were spaced 3 feet apart. Fifty seeds were planted in a single row in each plot. Seeds, either treated with Apron XL LS (0.64 fl oz/100 lb seed) or Allegiance FL (1.5 fl oz/100 lb seed), or not treated (control), were sown approximately 2 inches deep. Soil samples were collected from the upper 4 inches of soil in the field, at the time of planting, using a soil auger, and mixed together. The population density of P. capsici was determined by dilution plating of soil samples on a PARPH selective medium and was 100 cfu/g soil. The seedlings were also sprayed with a P. capsici zoospore suspension (10⁵ spores/ml; 150 ml/5 square feet area) 1 week after seedling emergence.

Seedling emergence was assessed 10 days after sowing seeds and seedling stand was determined 25 days after seedling emergence (35 days after seeding). Post-emergence damping-off was determined by counting plants showing girdling stem lesions with or without falling-over and/or death of seedlings. Diseased seedlings were examined using light microscopy and infected tissues were plated on PARPH to isolate P. capsici.

Results and Discussion

Greenhouse studies. Seed treatment with either Apron XL LS or Allegiance FL significantly increased seedling emergence from naturally and artificially infested soils (Table 1). There was no significant difference between Apron XL LS and Allegiance FL in percentage of seedlings emerged from soil. Seedling emergence from the untreated control treatments decreased significantly as inoculum density of P. capsici was increased from 90 to 600 cfu/g soil.

Twenty-one days after seedling emergence (31 days after seeding), with an inoculum level of 600 cfu/g soil, the mean value of the seedling stands for three cultivars combined was 88.3, 85.8, 45.7, and 97.5% respectively for the Apron XL LS, Allegiance LS, untreated control with P. capsici, and untreated control without P. capsici infestation.

Field studies. In the field trial, both Apron XL LS and Allegiance FL significantly reduced pre- and post-emergence seedling damping-off compared to the untreated control (Table 2). Post-emergence damping-off started within 1 week after seedling emergence in some of plots. Twenty-five days after seedling emergence (35 days after seeding), the average seedling stand for three cultivars combined were 76.7, 74.7, and 44.9% respectively for the Apron XL LS, Allegiance FL, and untreated control.

Both Apron XL LS and Allegiance FL are Acylalanine fungicides (Phenylamides) and used for control of Oomycete pathogens. These two fungicides contain the same active ingredient. Metalaxyl is a 50% solution of mefenoxam and 50% of an inactive isomer. Both fungicides are systemic and translocated upward to new growth (apoplastic) in the. When applied to seed, mefenoxam and metalaxyl are translocated to the shoots and protect the seedling against soilborne Oomycete pathogens.

Inoculum density of P. capsici in commercial pumpkin fields has been determined to be approximately 100 cfu/g soil. Both Apron XL LS and Allegiance FL effectively prevented seedling damping-off in soil with 600 cfu/g soil in the greenhouse studies. Thus, seed treatment with either Apron XL LS or Allegiance FL is expected to effectively control seedling damping-off caused by soilborne inoculum of P. capsici in commercial pumpkin fields.

Table 1. Fungicide seed treatment effects on seedling stand of pumpkin caused by Phytophthora capsici in the greenhouse.

Cultivar	Inoculum density (cfu/g soil)^w	Seedling growth (%)	Treatment^x
			Mefenoxam	Metalaxyl	Control
Dickinson	0	Emergence^y	100 a^z	98.9 a	95.8 a
		Stand^y	100 a^z	98.9 a	95.8 a
	90	Emergence	97.1a	87.3 a	52.3 b
		Stand	97.1 a	87.3 a	52.3 b
	600	Emergence	94.4 a	88.9 a	48.1 b
		Stand	85.2 a	79.6 a	33.3 b
Hybrid-401	0	Emergence	100 a	100 a	97.9 a
		Stand	100 a	100 a	97.9 a
	90	Emergence	95.3 a	82.0 a	60.0 b
		Stand	95.3 a	82.0 a	60.0 b
	600	Emergence	92.6 a	92.6 a	64.8 b
		Stand	88.9 a	88.9 a	57.4 b
Hybrid-698	0	Emergence	95.3 a	98.4 a	98.9 a
		Stand	95.3 a	98.4 a	98.9 a
	90	Emergence	89.0 a	86.7 a	53.3 b
		Stand	89.0 a	86.7 a	53.3 b
	600	Emergence	92.6 a	90.7 a	55.6 b
		Stand	90.7 a	88.9 a	46.3 b

^w Soil with 90 cfu was from a commercial pumpkin field naturally infested with P. capsici. Soil samples with 600 cfu were prepared by adding oatmeal substrate containing P. capsici.

^x Each value represents the mean of treatments in three experiments.

^y Emergence = percent of seeds germinated and emerged from the soil 10 days after sowing seeds; Stand = percent seedlings without infection 31 days after sowing seeds.

^z Values in each row with a letter in common are not significantly different from each other according to Fischer’s protected test (P = 0.05).

Table 2. Fungicide seed treatment effects on seedling stand of pumpkin caused by Phytophthora capsici in the field^w.

Cultivar	Days after seeding^x	Seedling stand (%)^y
		Mefenoxam	Metalaxyl	Untreated check
Dickinson	10 days	80.0 a^z	85.3 a	64.0 b
	35 days	63.3 a	66.0 a	36.0 b
Hybrid-401	10 days	96.0 a	90.7 a	86.7 b
	35 days	84.7 a	75.3 a	43.3 b
Hybrid-698	10 days	90.0 a	92.0 a	72.7 b
	35 days	82.0 a	82.7 a	55.3 b
Dickinson, Hybrid-401, Hybrid-698 (combined)	10 days	88.7 a	89.3 a	74.4 b
	35 days	76.7 a	74.7 a	44.9 b

^w A field infested with P. capsici (100 cfu/g soil). The seedlings were also sprayed with a P. capsici zoospores suspension (10⁵ spores/ml; 150 ml/2.25 m ²) one week after seedling emergence from soil.

^x Seedlings emerged from soil 7 to 10 days after sowing seed.

^y Each value represents the mean of treatments in three experiments.

^z Values in each row with a letter in common are not significantly different from each other according to Fischer’s protected test (P = 0.05).