MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Michigan has over 82,000 acres of vegetables that are susceptible to the soilborne oomycete pathogen Phytophthora capsici. The pathogen may overwinter in the soil and persist for many years (greater than 10 years). The roots, crowns, stems, leaves and fruits of peppers can all be infected by P. capsici. This pathogen is favored by rain and warm temperatures and spreads readily via water. P. capsici has been found in irrigation ponds and surface water sources. Integrated management strategies are required to control Phytophthora crown, root and fruit rot of pepper. The most effective control measures are to avoid planting susceptible crops in infested soil and to limit the spread of the pathogen to clean fields. Crop rotation has limited benefits due to the long term survival of P. capsici in the soil. Cultural control methods, such as properly constructed raised plant beds, can be helpful by reducing saturated soil conditions. Foliar applications of fungicides directed at the base of the pepper plant applied prior to disease development can be helpful. Several resistant or tolerant bell pepper cultivars are available to Michigan producers. A combination of host resistance, effective fungicides, and cultural control strategies may be used to reduce significant yield losses from P. capsici.

Fungicide trial

Currently registered fungicides were evaluated for control of Phytophthora crown, root and fruit rot of bell pepper (Table 1). The trial was conducted at the Michigan State University Muck Soils Research Farm in Laingsburg, Michigan on clay loam soil previously planted to pickling cucumber. According to the seed producers, the bell peppers ‘Red Knight’ (Seminis, Inc., St. Louis, MO) and ‘Paladin’ (Syngenta Seeds Inc., Boise, ID) are susceptible and intermediately resistant to P. capsici, respectively.

On June 9, six-week-old pepper plants were transplanted by hand into raised beds six-inches-high and covered in black plastic mulch with a single drip irrigation tape under the plastic. In each bed, peppers were planted into double staggered rows; one row of ‘Red Knight’ and the other of ‘Paladin.’ The spacing between rows was 12 inches. Drenches were applied to pepper seedlings in transplant trays just prior to planting. Applications of foliar fungicides were initiated at planting, and continued on a seven-day schedule. The plants were inoculated with a virulent isolate of P. capsici from Michigan that is insensitive to the fungicide mefenoxam, trade name Ridomil (Syngenta, Greensboro, NC). Plants exhibiting symptoms of Phytophthora crown and root rot, including wilting and death, were counted weekly. The pepper fruits were harvested four times, sorted by cultivar, and graded by size and incidence of Phytophthora fruit rot.

Table 1. Products tested.

Disease pressure was high at the test site with 96.3 percent of the susceptible ‘Red Knight’ and 30.6 percent of the tolerant ‘Paladin’ peppers showing Phytophthora root and crown rot symptoms by September 9 (Table 2). Although plants treated with the drench/spray program of a Prophyt 4.2EC drench, followed by Presidio 4FL alternated with Acrobat 50WP alternated with Prophyt 4.2EC significantly limited death of ‘Red Knight’ plants and had significantly higher yields than the untreated inoculated control, over 83 percent of treated plants died. ‘Paladin’ plants treated with Revus 2.08SC and Presidio had significantly less plant death (greater than 10 percent) than the untreated inoculated control (30.6 percent). No statistical differences in yield were observed among treatments applied to ‘Paladin.’

Table 2. Evaluation of fungicides for management of Phytophthora crown and root rot of pepper.

*Column means with a letter in common or with no letter are not significantly different (Fisher’s Method, P=0.05)
**Foliar application began 14 days after transplanting

Variety trial

Pepper cultivars and breeding lines were evaluated in the greenhouse for tolerance to Phytophthora root and crown rot. Twenty-seven breeding lines and cultivars with three to four true leaves were transplanted into individual pots. Pots were inoculated using millet seed inoculum with one of four P. capsici isolates from Michigan; 12889, OP97, SP98 and SFF3. Eight plants of each breeding line or cultivar were used per isolate. Eight plants per cultivar or breeding line were not inoculated. The plants were evaluated for wilting and plant death every two days. The fruits were harvested from the remaining plants at the end of the trial. The experiment was replicated twice.

Pepper plants exhibited symptoms of infection seven days after inoculation. Infected plants appeared wilted, and in some cases, had a dark brown stem lesion girdling the base of the plant. A week after initial disease symptoms, signs of infection appeared, including mycelia and pathogen sporulation within the stem lesions. The uninoculated control pepper plants never presented symptoms in either of the experiments (data not shown).

Differences in virulence among P. capsici isolates were observed across all cultivars and breeding lines screened (Table 3). Inoculation with isolate 12889 resulted in a significantly higher plant death than the other isolates. OP97 was less virulent than 12889, but significantly more virulent than SFF3 and SP98. Differences in cultivar susceptibility to crown and root rot were observed. ‘Paladin’ had the lowest average plant death (percentage) among all cultivars across all isolates. The bell pepper PRO3-15x16R-5 and the Poblano pepper XPP2548 had the lowest average plant death (percentage) among all breeding lines screened across all isolates. All cultivars and breeding lines in the trial were tolerant to the isolates SP98 and SFF3, including those cultivars considered susceptible.

Cultivar selection plays an important role in disease management. Different pepper cultivars will not provide an equal level of tolerance to local isolates of P. capsici. Growers should evaluate several pepper cultivars on-site, selecting those which provide the greatest level of tolerance to the isolates at their location.

Table 3. Plant death (%) among bell pepper lines inoculated with four isolates of P. capsici.