Phytophthora capsici is an oomycete organism and not a true fungus. Most oomycetes live in water and are often referred to as “water molds”. These organisms often do well in areas with poor drainage and proliferate after heavy rains or in areas with standing water. They can survive in bodies of water (ponds, rivers, streams) for extended periods. P. capsici can survive in soil and crop debris for more than ten years and can also survive on the roots of susceptible weed species including purslane, jimson weed, and nightshade.^1,2,3,6 The pathogen forms structures called sporangia, which can be disseminated by splashing rain and irrigation water. The sporangia germinate in the presence of water (films on plant surfaces), releasing 20 to 40 swimming zoospores, which can infect the plant. Sporangia can also germinate to form a germ-tube, which can infect the plant directly.¹ Infection is favored by wet conditions such as overhead irrigation, heavy rain, and poor soil drainage. Dispersal patterns are often associated with the directions of water flow within the field, such as down a row. Younger plants are usually more susceptible to Phytophthora blight infection and wounding can increase the likelihood of infection.^1,2,6,7 Infection is also favored by soil temperatures above 65°F and air temperatures from 75° to 85°F.^3,6

Because of the wide host range of Phytophthora capsici and the ability of the pathogen to survive in soil for over 10 years, Phytophthora blight of melon can be difficult to manage. No disease resistance against Phytophthora blight is currently available in melon, though there may be differences in susceptibility among melon types and varieties.^1,3 Crop rotation away from susceptible crops for at least four years is recommended. Rotation may not be effective in reducing inoculum of P. capsici in the soil, but avoiding the use of susceptible crops, including pepper, tomato, eggplant, green beans, and other cucurbits may help prevent further increases in inoculum levels.³ Integrating cultural control practices with the use of fungicides is recommended for managing Phytophthora blight on melon. Relying solely on fungicide applications will usually not result in sufficient levels of control.^1,3,6

Regularly scout fields for symptoms of Phytophthora blight beginning early in the season, especially following periods with wet weather and warm temperatures.⁴

Inoculum (sporangia) of P. capsici is spread by moving and splashing water. So, use drip irrigation when possible and avoid using overhead irrigation and planting in fields with poor drainage. If overhead irrigation can’t be avoided, water early in the day to give the plants time to dry before sunset, minimizing the time plants are exposed to free moisture. Avoid using surface sources of water (ponds, streams, rivers) for irrigation, as pathogen inoculum may be present in water from these sources.^1,2,3

Planting on raised beds covered with plastic mulch helps promote drainage and prevent the splashing of inoculum in the soil up onto stems, leaves, and fruit. However, plastic mulched raised beds will not protect from splashing inoculum once the vines grow into the non-mulched, between-row areas. Low areas of the mulched beds can also retain water, and fruit that develop in or near those areas may be more likely to become infected. Sub-soiling the field to disrupt hardpans can also help improve drainage.^2,3

To help prevent introduction of the pathogen into noninfested fields, practice good sanitation. Thoroughly clean all equipment between use in infested and non-infested fields, removing all soil and crop debris. Additionally, clean tools and workers shoes before entering non-infested fields. When possible, work in infested fields last.^1,2,4 If the disease develops in the field, remove the infected plants and the adjacent, healthy-looking plants to help reduce the buildup of inoculum. Do not dispose of any infected plant debris, including culled fruit, in the production field.^2,3,4

Applying fungicides can help slow disease spread and development; however, if used as the only management practice they may not provide acceptable control, especially under conditions of high disease pressure (extremely favorable conditions for disease).¹ Melons are mostly susceptible to leaf and fruit infections, so fungicide applications should focus on protecting those tissues.³ Because P. capsici is an oomycete organism and not a true fungus, fungicides that are effective against fungal pathogens (anthracnose, gummy stem blight, powdery mildew) may not be effective against Phytophthora blight. Select fungicide products that show activity against oomycetes. Fungicide products that contain active ingredients such as cyazofamid, cymoxanil, dimethomorph, fluopicolide, mandipropamid, mefenoxam, oxathiapiprolin, or zoxamide are usually recommended to help manage oomycete pathogens.²

Resistance to some fungicides, including cyazofamid and mefenoxam, have been observed in populations of P. capsici. Consult local and regional vegetable disease management guides for the most current recommendations on fungicides to use for managing Phytophthora blight on melons in your area. Alternate applications of fungicide products that contain active ingredients from different FRAC groups to help prevent the development of fungicide resistant strains of the pathogen.^1,2,4 Always consult the most current fungicide product label and follow the application instructions and restrictions listed on the label.