Diseases of Spinach

Glenn S. Pound.

Most of the large spinach-growing areas are in coastal States. The largest acreage for fresh-market spinach is in southern Texas. For fresh-market spinach, savoy (wrinkled leaf) types are generally grown; for processing, the smooth-leaved types are generally used. However, the important canning acreage in the Arkansas River Valley of Arkansas and Oklahoma is predominantly of savoy type. The production of spinach seed in the United States is centered in the Puget Sound area of Washington.

DOWNY MILDEW blue mold is one of the most serious diseases of spinach. It develops when the weather is cool and moist and generally is most severe in coastal areas. It is especially severe at times in southern localities that grow fresh-market spinach. Although downy mildews occur on closely related plants, the fungus (Peronospora effusa) does not affect any other host.

The disease appears on plants of any age. It is first noticed as large yellow blotches on the leaf, the under side of which becomes covered with a fuzzy growth that is white at first and bluish purple later. The growth contains a mass of spores (conidia), which are easily detached and are carried by air currents from plant to plant.

At night or in cloudy, rainy weather when temperatures are low and plants are covered with a film of water, the spores germinate and penetrate the plant. In about a week the newly infected leaves produce a crop of spores, which can start another cycle of the disease. Many such cycles occur within a growing season.

The conidia are vegetative spores and will not remain viable more than a few hours in a dry atmosphere. However, the fungus produces in the host tissue sexual spores (oospores), which are very resistant to unfavorable conditions and which serve to carry the fungus from one season to another. Their germination has not been observed, but it is generally believed that they are the important source of overwintering or oversummering inoculum, as the case may be.

The oospores may be carried as a surface contaminant of spinach seed. Seed may even be internally infected by the fungus. It has never been proved that contaminated or infected seed will give rise to infected seedlings, however. First infections probably occur from oospores germinating in the soil, or in areas of extremely mild climate the conidial stage may extend from one crop to another on volunteer host plants. The fungus is an obligate parasite, and cannot persist except on spinach.

Copper-containing fungicides are said to give successful control but have not been generally practical. A foreign introduction of spinach (P. I. 140467) carries a single dominant gene for immunity. The development of resistant varieties is the best hope of control.

WHITE RUST of spinach is caused by the fungus Albugo occidentalis, an obligate parasite. It was first reported on spinach in Virginia in 1910 but did not assume importance until it became established in the Winter Garden region of Texas about 1935. As a field disease it has been practically restricted to southern Texas, although it has occurred in the Arkansas River Valley of eastern Oklahoma and western Arkansas. Periodically it has been severe in the Winter Garden, Coastal Bend, and lower Rio Grande Valley areas of Texas.

The fungus has been reported on two or three wild plants closely related to spinach, but it does not attack any cultivated plant other than spinach.

Early symptoms consist of yellow spots, similar to those of blue mold, on the leaves. The under surfaces are distinct white, blisterlike, circular or elongate pustules develop, which may have a concentrically zonate pattern. The pustules (sort) break open to release a mass of conidia to the air. Severely affected leaves often die and turn brown and give a frosted or blighted appearance to a field.

The conidia are short-lived and do not germinate well until they lose some of their moisture usually during the day in the dry atmosphere of the Southwest. At night, if temperatures drop enough to cause dew, the conidia germinate to produce six to eight swimming spores, which in turn germinate and send infection threads into the host. Mature pustules are produced on these newly infected leaves in about a week.

The fact that conidia require a certain amount of dryness before germination probably explains why the disease has become severe only in the Southwest. In the Arkansas River Valley, spinach is growing during the rainy part of the year and only sporadic periods are favorable for its spread. The failure of the disease to spread during foggy or rainy periods probably explains its absence in other spinach-producing areas.

The fungus produces sexual spores in great abundance in the infected tissue, especially in seed plants and under warm temperatures. They are even produced abundantly on seeds, but such seeds have never given rise to infected seedlings. The spores in the soil are probably the means by which the fungus is carried from one season to the next although their germination has never been observed. No adequate control measures have been devised.

BLIGHT, caused by the cucumber mosaic virus, is the most widespread and serious virus disease of spinach. The virus is transmitted from a number of wild and cultivated hosts to spinach by aphids, and the disease is most severe on fall and winter crops because of a greater build-up of inoculum in other hosts.

Symptoms of the disease appear as a general yellowing. Plants ultimately become completely yellowed, twisted, and stunted. If warm temperatures prevail, death of the plants quickly follows, but if air temperatures are cool, death does not occur for several days.

Virginia Savoy, a resistant variety, was developed in 1920. It is a hybrid of Bloomsdale Savoy and an Asiatic variety. Old Dominion, a second resistant variety, was developed by selection following a cross of Virginia Savoy and King of Denmark. This was one of the first successful attempts to control plant virus diseases by breeding resistant varieties. Virginia Savoy and Old Dominion are both Savoy varieties and have been widely used for fresh-market spinach.

The resistance of Virginia Savoy is due to a single dominant gene that is dependent on certain air temperatures for expression of resistance. At temperatures below 80 F. resistant plants show no symptoms when inoculated, but at temperatures above 80 F. inoculated plants rapidly succumb with a systemic necrosis. Thus, in breeding programs the control of air temperature is of prime importance.

J. P. Fulton of Arkansas has isolated a strain of the virus from spinach to which Virginia Savoy and Old Dominion are readily susceptible.

GLENN S. POUND has specialized in the diseases of vegetable crops since 1943.