With the interest in healthier diets coupled with packaging innovations that facilitate trimmed and washed fresh baby and teen spinach in convenient bags, trendy consumers demand this nutrition superstar.

Growers, however, must contend with diseases. Downy mildew causes more damage to spinach than any other disease. Widespread and destructive worldwide, downy mildew starts with yellowish green, irregularly shaped lesions on cotyledons and true leaves of all ages. The lesions enlarge and turn bright yellow. Later, these blotchy spots become tan and dry. Older leaves typically succumb first. In very wet conditions, the diseased tissue can rot and become soft. In addition to the quality losses caused by the spotting, the infection can cause rotting in packed spinach.

The undersides of affected leaves exhibit the grayish-purple fuzzy growth of the fungus sporangia and sporangiospores. Winds and splashing rain disperse the spores from plant to plant and from field to field. In advanced stages of the disease, the leaves curl, distort and look blighted.

The fungus can overwinter on both seed and overwintered spinach plantings. Depending on the area, plant debris may harbor the pathogen over the winter.

Although downy mildew pathogens attack basil cultivars, beans, cucurbits and other vegetables, the one challenging spinach, Peronospora farinosa f. sp. spinaciae, infects only spinach. Damage to related crops in the Chenopodium family (i.e., beets and Swiss chard) escape such damage. An exception may be a few weeds such as goosefoot, lambsquarters and nettleleaf.

Pathogen characteristics

The pathogen requires cool, wet conditions for infection and development of the disease. Spinach, particularly when densely planted, retains the necessary moisture for the disease in the canopy. Even during seemingly dry situations, adequate moisture can be present. Downy mildew progresses rapidly when temperature and leaf wetness are favorable.

These early stages of downy mildew show the light green blotches.PHOTO BY DR. JIM CORRELL, U. OF ARKANSAS.

This pathogen has adapted quickly during the past decade. Prior to 2004, seven races were identified. Currently, there are 15 races. Plant breeders have been responding by developing resistant cultivars.

How to attack pathogen

Foliar fungicides can provide protection, but must be applied before infection occurs and symptoms develop. Forecast systems, which alert growers to the presence of spinach downy mildew in their area, and predict when favorable conditions are likely to occur, can save pesticide usage. When symptoms appear, fungicide application may be too late. Numerous fungicides are labeled for downy mildew on spinach; special needs labels are available in some regions. Most fungicide product labels post a range of restrictions for re-entry periods, days until harvest, and other data. Local extension educators can assist growers with this information. In addition, these educators can advise on the recommended fungicides for their area, hot water treatment procedure, plus the best resistant cultivars for their region.

For organic growers, several new biological controls are labeled. Resistant cultivars are usually recommended as part of an integrated pest management program. Many cultivars include multiple race resistances.

Consider cultural practices

In addition to resistant cultivars, numerous cultural practices can minimize or prevent damage. Purchase treated seed or hot-water treat the seed. Because airflow is particularly important, circulation to facilitate fast leaf drying may help curtail the pathogen’s harm. Avoid overhead irrigation. Space plants to control density. Site selection should also include good soil drainage. Avoid fields with a history of downy mildew or with proximity to spinach; a two-year rotation without spinach is considered a minimum. Control weeds and destroy plant debris.

California produces over two-thirds of this country’s fresh market spinach. Their high-density systems, along with year-round spinach plantings, aids the opportunity for carryover of the pathogen inoculum. University of Arkansas pathologists James C. Correll and Chunda Feng, as well as Steven T. Koike and Katherine E. Kammeijer, pathologists of the University of California Cooperative Extension at Salinas, have been identifying the new races and deviating strains of the spinach downy mildew pathogen.

The Correll-Koike team, with the European team in the International Working Group on Peronospora (IWGP) are monitoring the development of new races by collecting and testing suspected new isolates. IWGP includes the seed companies Advanseed, Bejo, Enza, Monsanto, Nunhems, Pop Vriend, Rijk Zwaan, Sakata, Syngenta, Takii and Vilmorin.

COVER PHOTO BY V_ZAITSEV/ISTOCKPHOTO.COM.