by Tamara Scully
Peppers are highly susceptible to infection from Phytophthora capsici, a fungal-like pathogen, as are cucurbits such as watermelon, squash and cucumbers. The disease spreads rapidly in wet conditions, particularly if there is free moisture available, which allows the reproductive zoospores to swim to reach host plants and cause infection. While the fungus can also reproduce under drier conditions, infection of plants is slowed. Wet conditions on plants and moisture-laden soils are the primary ingredients for rapid proliferation of P. capsici.
“Infection of the plant would be slow and production of sporangia and zoospores on infected tissues would be limited” at normal moisture levels for crop growth, Dr. Mohammad Babadoost, professor of plant pathology and extension specialist, University of Illinois, said.
But if conditions are conducive — think of low spots in fields, poorly drained soils, humid conditions or wet crops — the pathogen can rapidly cause severe infection. Warmer temperatures, in the 75-91 degrees F, favor infection.
In recent years, growers in the Midwest have suffered large crop losses in cucurbits due to the P. capsici pathogen. And in Pennsylvania, the impact as more farms become infected with the pathogen has led to research on effective treatment strategies.
Disease dispersal
The pathogen causes problems in solanaceous crops, but it is not the causative organism of the dreaded late blight seen frequently in tomatoes or potatoes. That blight is caused by a related organism, Phytophthora infestans. Both of these Phytophthora species, as well as an array of others that cause concern in a variety of vegetables, aren’t true fungi but are classified as oomycetes.
Oomycetes can reproduce sexually, via oospores, or asexually by producing fruiting bodies, known as sporangia. These sporangia then produce zoospores, which along with oospores, are the causative agents of infection. Oospores can survive in soils, under adverse conditions, for many years.
Sporangia themselves can be moved via water, including wind-blown rain. Minuscule strands of mycelia are another form in which P. capsici can be transmitted, carried in transplants, on equipment or in soils. Although not considered a seed borne pathogen, saving of seeds from fields where P. capsici is known to be harbored should be avoided.
Because the pathogen can be transmitted readily in many forms, careful cultivation practices are essential. Equipment, clothing and plant debris can harbor and transmit the pathogen. Irrigation water can runoff a field and transport the oocmycete to other fields.
Good crop rotations with at least three years between susceptible crops is advisable, but in highly infected fields the pathogen can remain virulent for much longer periods. Some growers, particularly of cucurbits where no resistant cultivars have been developed, have had to abandon farms and find new, virgin ground in order to grow these crops.
Planting susceptible crops in raised beds, regular scouting and prompt removal of plants showing any signs of infection are recommended to curtail total crop loss. Monitoring moisture levels and avoiding over-irrigation of crops is necessary. Using brassicas as a cover crop can prevent the germination of P. capsici oospores, while mulching with straw or vetch can keep water from splashing onto susceptible crops, reducing the spread of infection.
Crop symptoms
Depending on when a crop is infected, the disease can cause a total crop loss. Infection prior to fruit set can prevent fruit production, while plant infection during later growth stages might still allow for a saleable crop.
Infected fruits will have a water-soaked lesion that expands, followed by a white mold-type growth. Asymptomatic fruits can become infected from hands or equipment that have previously handled an infected fruit or plant. Keep any crop harvested from an infected field dry and monitor for symptom development in storage, Dr. Babadoost said.
Although some specific symptoms are crop-dependent, P. capsici generally causes water rots to occur at the site of infection. If seedlings are infected, the young plants will show signs of damping-off, with watery rot developing at the soil line. Young plants will wilt suddenly, without recovery.
More mature plants can show stem symptoms, with soft, water-soaked stems which collapse and die. Many times, this is at the soil line and crown rot occurs. Leaf or vine infections are indicated by dark brown, watery spots. Lesions become necrotic, and plant death results. If infection occurs, but conditions become less favorable for the pathogen, spread of infection will be halted until conditions become favorable again.
Virulence of P. capsici can vary from strain to strain. Local research is valuable to best help growers understand the strains found in their immediate areas, Dr. Babadoost said.
Pennsylvania pepper study
A 2017 Pennsylvania research study involving P. capsici control in peppers was conducted under John Esslinger, Penn State Extension Horticultural Educator. The study arose from the increasing impact the disease is causing on farms in the state.
The study investigated the efficacy of four different approaches to P. capsici control, as well as the associated costs. The test plot was planted to two pepper varieties — one with poor resistance to the pathogen and a resistant variety. The crop was planted on black plastic mulch.
The four treatments compared were: Ornodis Gold fungicide; manure-based compost to promote competitive biological soil activity; Revus Nu-Cop HB rotated with Tanos Nu-Cop HB copper fungicide; and a control receiving no treatment. In late August 2017, the crops were inoculated with P. capsici.
Results showed that the copper fungicide treatment group, for both the resistant and non-resistant pepper cultivars, out-performed all other treatments, with only a few plants showing symptoms of infection by P. capsici. The Orondis Gold was the second most effective treatment, reducing disease incident in both cultivars, while the manure-compost proved to be more effective than the untreated control only with the resistant cultivar.
Because the resistant pepper variety was a large bell pepper, and no harvesting occurred, the plants leaned down to the ground and were therefore in contact with any P. capsici in the soil, while the smaller fruited, non-resistant cultivar did not have direct soil contact. In some instances, the resistant pepper showed more pathogen infection than the non-resistant one did.
The most effective copper fungicide regiment, applied over seven applications, was also the most expensive. However, the Orondis Gold was applied once only, at the lowest labeled application rates, which has now been doubled by the manufacturer. Additional applications or higher rates could prove more effective at disease control, but would also increase the cost.
“Phytophthora capsici is a destructive pathogen on cucurbit and peppers,” Dr. Babadoost said. “For peppers, in the field with a history of Phytophthora blight, applications of fungicides should begin immediately after transplanting.” For Cucurbita, “Seed treatment is very effective for preventing seedling death for about five weeks from the time of seeding, and with negligible cost. Fungicide application should be initiated immediately after observing the first symptoms or signs of disease.”