Chemical Management of Phytophthora capsicin Pumpkin in Illinois

Phytophthora blight, caused by Phytophthora capsici, is a destructive disease of cucurbits and peppers in Illinois and worldwide. Fungicide use is a significant component of management strategies of P. capsici in cucurbits and peppers in the United States. Studies were conducted in the laboratory and field to evaluate efficacy of selected fungicides for control of P. capsici in pumpkin in Illinois. In the laboratory, sensitivity of five P. capsici isolates from Illinois was evaluated to the fungicides cyazofamid, dimethomorph, and mandipropamid. Development of colonies and production of sporangia of the isolates were assessed at concentrations of 0.0, 0.1, 0.5, 1.0, 5.0, and 10.0µg/mL of cyazofamid, 0.0, 0.1, 0.5, 1.0, 2.5, and 5.0µg/mL of dimethomorph; and 0.0, 0.05, 0.1, 0.5, 1.0, and 5.0µg/mL of mandipropamid in V8 juice agar in Petri plates. In 2014, 2015, and 2017, effectiveness of the fungicides cyazofamid, dimethomorph, and mandipropamid for control of P. capsici in pumpkin was investigated. P. respectively. In 2015, incidence of fruit rot caused by P. capsici was 69% in control plots and 42, 21, and 26% in the plots sprayed with cyazofamid, dimethomorph, and mandipropamid, respectively. Similarly in 2017, incidence of fruit rot caused by P. capsici was 48% in control plots and 7, 27, and 13% in the plots sprayed with cyazofamid, dimethomorph, and mandipropamid, respectively. In all field trials, percent vines infected with P. capsiciin treated plots with cyazofamid, dimethomorph, and mandipropamid was significantly lower than that of control plots.

use of fungicides should be monitored carefully. Over the past 16 years, several fungicides have been developed for management of oomycete pathogens, including Phytophthora spp.
Some of these fungicides are effective against P. capsici. Cyazofamid, dimethomorph, and mandipropamid are commonly used fungicides to manage P. capsici in cucurbits in Illinois and other vegetable growing areas. These fungicides have different modes of action and can be used together. This study was conducted to determine the efficacy of these fungicides for managing P. capsici in pumpkin production.

Phytophthora capsici isolates
Five isolates of P. capsici were selected from the collection maintained in Dr. Babadoost's laboratory at the University of Illinois for this study. The isolates were grown on V8-juice agar in Petri plates. V-8 juice agar was prepared by adding 50ml of V8 juice, 1.0g of CaCO 3 , and 16g of agar to 950mL of distilled water.

Inhibition of mycelia growth
An in-vitro mycelial growth assay was conducted to determine the effective fungicide concentration at which 50% of the fungal growth was inhibited (EC50) for each isolate-fungicide combination. Plugs of 7mm diameter from actively growing cultures of P. capsici were transferred onto V8 juice agar in Petri plates amended with technical grades of the fungicides: cyazofamid at 0.0, 0.1, 0.5, 1.0, 5.0, and 10.0µg/mL; dimethomorph at 0.0, 0.1, 0.5, 1.0, 2.5, and 5.0µg/mL; mandipropamid at 0.0, 0.05, 0.1, 0.5, 1.0, and 5.0µg/mL; and A20941OD at 0, 1, 10, 50, 100, and 200µg/mL. For assays, the stock solutions of fungicides were diluted with methanol, acetone, or water. Four plates were used for each fungicide-concentration combination. Culture plates were incubated at 25 °C in darkness. Two perpendicular diameters of colonies were measured 4 days after incubation. The experiment was repeated once under the same conditions.
The number of sporangia was counted on two of the agar plugs using a light microscope. The fungicide concentration at which 50% of sporangia production was reduced (EC 50 ) was determined. The experiment was repeated once. Percent of suppression of sporangium formation was calculated from the ratio of the number of sporangia produced in the fungicide-amended plates to the number of sporangia in the control plates.

Field trials
In 2014, 2015 and 2017, trials were conducted in an irrigated field near Green Valley City (Tazewell County), Illinois to evaluate efficacy of selected fungicides for control of Phytophthora blight of pumpkin, caused by P. capsici (Tables 1-3). Seeds were sown on 7 May, 14 May and 15 May in 2014, 2015, and 2017, respectively. Due to continuous heavy rains in 2015, the plots were lost and the trial was re-established using 2-week old transplants on 1 August 2015. Seeds of the processing pumpkin Cucurbita moschata Duchesne 'Dickinson' were sown 45cm apart in single-row plots, 6m long. The plots were spaced 9m apart in a randomized complete block design with four replications.   Weeds were controlled by hand-weeding. Cucumber beetles (Acalymma vittatum and Diabrotica undecimpunctata) and other insects were managed by applying permryhrin (Pounce 25WP at 740mL/ha) and acetamiprid (Assail 30 SG at 358g/ha). Spray applications of fungicides were at 7-day intervals, as shown in (Tables 1-3). Fungicides were applied with a backpack sprayer using 469L of water per ha. The plots were irrigated as needed, using a central pivot irrigation line, which delivered 7.6ml water each time. Incidence of Phytophthora blight in the plots was assessed by examining 20 vines per plot prior to harvest and all of the fruit at harvest.

In-vitro sensitivity of Phytophthora capsici to fungicides
Mycelial growth and sporangia production were significantly (P=0.05) reduced by all three fungicides tested in the laboratory. The EC 50 values of the fungicides for mycelial growth ranged from 0.34 to 3.63µg/mL (average 1.03µg/mL); from 0.75 to 4.66µg/mL (average 2.98µg/mL); and from 0.13 to 7.18µg/mL (average 4.03µg/mL) for cyazofamid, dimethomorph, and mandipropamid, respectively. The EC 50 value for sporangia production was 10, 2.5, and 0.05µg/mL for cyazofamid, dimethomorph, and mandipropamid, respectively.

Field trials
In 2014, vine and fruit infection was first observed in untreated plots on 1 July. Percent vines and fruits infected by P. capsici were significantly higher in untreated (control) plots than those in treated plots ( Table 1). The lowest incidence of infected vines by P. capsici (6.25%) was in the plots sprayed with cyazofamid (Ranman 400F) plus Silwet L-77 alternated with mandipropamid (Revus 2.09 SC) plus Activator-90. The lowest incidence of infected fruit by P. capsici (4.45%) was in the plots sprayed with mandipropamid (Revus 2.09 SC) plus copper hydroxide (Kocide-3000 46.1 DF) plus Activator-90 alternated with cyazofamid (Ranman 400 F) plus Silwet L-77 alternated.
In 2015, vine and fruit infection was first observed in untreated plots on 15 September. Percent vines and fruits infected by P. capsici were significantly higher in untreated (control) plots than those of treated plots ( Table 2). Incidence of vines infected by P. capsici was 5% or less in the treated, while incidence of vine infection was 10% in untreated plots. Incidence of fruit infection in treated plots ranged from 17.53 to 31.25% compared with untreated plots with 69.17% fruit infection ( Table 2). Incidence of fruit infection was the lowest (17.53%) in the plots sprayed with Mandipropamid (Revus 2.09 SC) plus copper hydroxide (Kocide-3000 46.1 DF) plus Activator-90.
In 2017, vine and fruit infection was first observed in untreated plots on 15 and 28 July, respectively. Percent vines and fruits infected by P. capsici were significantly lower in treated plots that those of untreated (control) plots ( Table 3). Incidence of vines infected by P. capsici was ranged from 5.00 to 23.75% in treated plots compare to 45.00% in untreated (control) plots. Incidence of fruit infection in treated plots ranged from 2.06 to 26.64% in treated plots compare to 47.73% in untreated (control) plots (Table 3). The lowest vine infection (5%) was in the plots sprayed with mandipropamid (Revus 2.09 SC). The lowest fruit infection (2.06%) was in the plots sprayed with mandipropamid (Revus 2.09 SC) + Activator-90.

Discussion
Phytophthora blight, caused by P. capsici, is one of the most destructive diseases of cucurbits in the world. Management of this pathogen in cucurbits is a challenging task because no resistant cultivar of cucurbits to P. capsici is available. An important component of strategies for management of P. capsici in cucurbits is fungicide application [6]. Developed an effective seed treatment of cucurbits with mefenoxam (Apron XL LS) that prevents plant infection for five weeks from sowing seed in the soil. Over the past 16 years, several effective fungicides were developed for management of P. capsici in cucurbits. Among the newly developed fungicides are cyazofamid, dimethomorph, and mandipropamid.
Virulence of P. capsici isolates in cucurbits varies among cucurbit producing areas [3,7]. Also, P. capsici quickly develop resistance to fungicides [8]. Therefore, testing efficacy of fungicides for management of P. capsici in each cucurbit growing areas is essential. Our in-vitro and field tests showed that all three fungicides (cyazofamid, dimethomorph, and mandipropamid) tested are effective against P. capsici isolates from Illinois. But applications of these fungicides should be alternated to prevent resistance development in the pathogen. Among the three fungicides tested in fields over the past three years, mandipropamid was the most effective for management of P. capsici in pumpkin.
Similar results have been observed in squash and pepper trials in the fields in Illinois (Babadoost, unpublished data). Spray of mandipropamid mixed the Activator-90 and/or a copper compound (e.g., copper hydroxide (Kocide-3000 46.1 DF) is even more effective in reducing the incidence of vine and fruit infection compared to spray of mandipropamid alone. Application of mandipropamid alternated with application of cyazofamid is a reliable fungicide management of P. capsici in pumpkins, and likely in other cucurbits crops in Illinois.