Young spinach plants can be very susceptible to root disease problems, including damping-off and root rot, and these diseases can result in substantial reductions in plant stands. Damping-off and root rots of spinach can be caused by pathogens, including the fungi Fusarium oxysporum and Rhizoctonia solani, and several species of the oomycete (water mold) Pythium.^1,2,3 Pythium species that commonly cause diseases on spinach include P. ultimum, P irregulare, P. aphanidermatum, and P. myriotylum.^1,4,5

Pythium pathogens can cause both pre- and post-emergence damping-off of spinach. With pre-emergence damping-off, spinach seed and seedlings are infected, wilt, collapse, and often rot before they emerge, leaving gaps in plant rows. In post-emergence damping-off, seedlings exhibit stunted growth and other symptoms that can result in plant death.^1,3

Symptoms of pre-emergence damping-off can only be seen by digging up seeds and non-emerged seedlings in the gap areas of the planted row. Seeds and seedling tissues are water-soaked and rotted, usually brown in color.² Seedlings with post-emergence damping-off will show stunted growth, yellowing of lower leaves, wilting, plant collapse, and eventually plant death. The roots of affected seedlings show water-soaking and brown to black discoloration. On older seedlings, the upper areas of tap roots can be girdled by necrotic lesions, and the tips of roots can become necrotic. Eventually, the entire root system may become rotted.^1,2 The symptoms of damping-off and root rot caused by Pythium spp. can be difficult to distinguish from the symptoms caused by Rhizoctonia solani and Fusarium oxysporum, so sending samples to a plant clinic for diagnosis may be required to determine the causal agent(s). Damping-off symptoms can also be similar to those caused by abiotic factors, such as overwatering.^1,2

Pythium species that affect spinach are usually soil inhabitants, able to survive in the absence of spinach plants by colonizing other plants, growing on organic matter, and producing persistent resting structures. Most Pythium are not highly hostspecific and have wide host ranges. Once present in a field, they can be difficult to eradicate.⁴

Young plants are most susceptible to infection by Pythium spp., and as plants age, infections may be limited to root tips and feeder roots.⁴

Some soil fumigation treatments can reduce soil populations of Pythium, and steam treatments can be used to lower inoculum levels in greenhouse systems. Disease severity can be affected by soil texture, irrigation practices, pathogen populations, and the varieties of spinach planted. Dampingoff is usually favored by warm conditions, soils that are heavy and poorly drained, low areas of the field where water collects, and consecutive spinach crops or short crop rotation sequences.^1,2 Soil pH, light intensity, and the presence of other microorganisms may also affect the levels of damping-off and root rot.⁴ Spinach is a cool-season crop that grows best when soil temperatures are between 59° and 64°F (15° to 18°C), and the severity of damping-off is usually higher at temperatures outside of this range, depending on the pathogen involved. Pythium ultimum grows best in culture at temperatures between 77° and 86°F (25° and 30°C), so spinach damping-off is usually more severe when soil temperatures are above 64°F when P. ultimum is involved.⁴

Although some differences in the severity of Pythium preemergence damping-off have been observed among spinach varieties, acceptable levels of resistance have not yet been identified in spinach. Studies evaluating damping-off resistance in spinach have found inconsistent results and differences among seed lots and trials using the same varieties.⁴ Research is being done to help standardize screening techniques to better identify and characterize resistance in spinach.

Practices that promote even, rapid germination and strong growth of spinach seedlings can help reduce the incidence and severity of damping-off and root rot. Growers are advised to plant into well-prepared seed beds in locations with welldrained and aerated soils and to plant when soil moisture and temperature levels favor rapid germination and growth. If applying compost, choose material that is thoroughly composted. Schedule irrigation frequency and duration to prevent excessively wet, saturated soils. Avoid consecutive cropping, and, if possible, rotate to non-host crops, which can be difficult with these species of Pythium because they have a very wide host range. ^1,2,3,4

Fungicides can also be used to help manage damping-off and root rot caused by Pythium species. Choose fungicides with active ingredients that are effective against water molds, such as Pythium, Phytophthora, and Aphanomyces. Active ingredients that are effective against true fungi, such as Rhizoctonia solani and Fusarium oxysporum, may not be effective against Pythium species. Fungicide seed treatments can help protect seed and young seedlings from damping-off. Active ingredients that are registered for use on spinach include mefenoxam. However, resistance to mefenoxam has been documented in some populations of Pythium. Fungicides that contain mefenoxam, metalaxyl, cyazofamid, and propamocarb HCL can also be used for pre- and post-plant in-furrow, in-row, or chemigation treatments to help manage damping-off and root rot of spinach caused by Pythium secies.^1,2,3,4,5

Several biological control products are also registered for use on spinach to help manage damping-off and root rot. Actinovate® AG Biological Fungicide (Streptomyces lydicus) can be applied as a soil drench at planting. Double Nickel® LC (Bacillus amyloliquefaciens strain D747) can be sprayed in a band or applied as a drench at 4- to 6-week intervals during the growing season. Mycostop® Biofungicide (Streptomyces strain K61) can be applied as a seed treatment, and Rootshield® Plus+ WP Biological Fungicide (Trichoderma harzianum strain T-22, Trichoderma virens strain G-41) can be applied through chemigation or as an in-furrow spray at planting.³