Submitted by: Jennie Mazzone, Penn State Extension Educator, Horticulture
No vegetable grower wants to live with any plant disease, especially one as destructive as Phytophthora blight. But despite our best efforts, disease happens, and the key to protecting yield is understanding the factors that cause disease and using the tools in your toolbox to keep it in check. Growers who have battled Phytophthora blight know it is a perennial problem once established and extremely wet weather, like many of us experienced in August, can quickly turn a manageable problem into a serious outbreak.
Phytophthora blight is caused by a soilborne, water mold pathogen (P. capsici) that can infect a wide range of crops, including solanaceous crops (pepper and tomato), cucurbits (pumpkin and watermelon), legumes (lima and snap bean), and weeds such as common purslane and nightshades. The disease is often introduced through contaminated irrigation water from streams or ponds, runoff from infested fields, or infested soil moved on tractors and other equipment. Once established, Phytophthora can make a thick-walled spore that is able to survive in the soil for about 4 years, although survival could be shorter or longer depending on conditions.
Disease begins when swimming spores called zoospores are released in saturated soils. These spores are chemically attracted to plant roots. After reaching the root or crown, the pathogen penetrates into the plant and colonizes the root system, causing a root and crown rot. The disease can spread quickly to the stems, leaves, and fruit, causing symptoms of wilt, blight, and fruit rot. The signs of this disease are especially obvious on the pumpkin. The sporulation of the pathogen looks like white powdered sugar sprinkled all over the rotting fruit (Fig. 1). This white material contains millions of spores of the pathogen that spread quickly in water and splashing rain, especially with storm events. The pathogen moves wherever water moves, so low-lying areas and sections of fields with standing water are especially vulnerable.
For those of you lucky enough not to have this disease, congratulations, keep it out! Use well water for irrigation whenever possible and remove soil from tractors and equipment before moving between fields to prevent Phytophthora blight from becoming established on your farm. For those of you with the disease, I’m sorry. This is a tough one to manage, but you do have options. Avoid planting susceptible crops in low-lying or poorly drained areas, as these locations are often the first and most severely affected. If you have a localized hot spot with Phytophthora blight, make a 20-foot buffer around the affected area and remove or disk under all plants within that zone, beginning with healthy-looking plants on the outside and working inward. Infected plants left in the field serve as a breeding ground for the pathogen, allowing it to make more spores and spread to neighboring plants.
Save field work in sites with known Phytophthora for last, and always clean and sanitize equipment before moving between fields. Even a small amount of infested soil carried on a tractor can introduce Phytophthora blight into a previously clean field. Do not put infected plant material in the compost or cull pile near your production fields. Discard infected plants in a landfill or bury them. Rotate to corn, small grains, forage grasses, or another non-host in contaminated fields for at least the next 4 years or longer, and the longer the better. Grow a biofumigant mustard cover crop just prior to planting a susceptible crop into the affected field. Improve drainage with raised beds and avoid overhead irrigation and standing water, which create ideal conditions for disease development. There are some Phytophthora-tolerant bell pepper cultivars, such as ‘Paladin’ and ‘Aristotle’.
Apply fungicides preventatively, especially when conditions are warm (75-85°F) and wet. Group 4 fungicides containing the active ingredients mefenoxam (Ridomil Gold 4SL) and metalaxyl (MetaStar 2E AG) can provide early-season protection but should not be used if insensitivity or fungicide resistance is known. Other effective active ingredients include oxathiapiprolin (Orondis products, FRAC 49), cyazofamid (Ranman 400SC, FRAC 21), fluopicolide (Presidio 4SC, FRAC 43), dimethomorph (Forum 4.17SC, FRAC 40), and propamocarb-containing products where labeled. Alternate products with different modes of action to reduce the chances of fungicide resistance development. Consult the Mid-Atlantic Commercial Vegetable Production Recommendations for additional, crop-specific recommendations, and always read and follow product label instructions.
Living with Phytophthora blight is not easy, but it doesn’t have to mean a career change out of farming. With careful water management, sanitation, rotation, and preventative fungicide programs, you can make life much more difficult for the pathogen and less stressful for yourself.
